J^eto gorfe £>tate College of Agriculture &t Cornell SBnt'bersfttp Stbaca, JJ. §. Htfirarp Cornell University Library QC 915.S64 Indoor humidity, 3 1924 002 966 871 Indoor Humidity The original of this book is in the Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://www.archive.org/details/cu31924002966871 INDOOR HUMIDITY* By Henry Mitchell Smith, M.D. The demands of our variable winter climate, especially along the Atlantic Coast, have led to the development of devices for heating buildings by steam, hot water and hot air that are beyond criticism if considered from a standpoint of mechanical perfection and efficiency as heat producers. A study of the conditions actually created by such seemingly perfect systems shows, however, that they are not entitled in their present form to an unqualified expression of ap- proval. Viewed from a scientific, or from a hygienic standpoint the aver- age heating system of to-day is an anomaly productive of conditions that. can be explained only upon the theory that the subject of artificial heating has not been broadly consid- ered, and, therefore, certain funda- mental and vital principles have been overlooked. The principle upon which modern heating systems are based is er- roneous in that the idea is simply to heat the air to a desired temperature, as indicated by the thermometer. Here is the great error. The air tem- perature alone is an insufficient index of our sensations of heat and cold. The human body in health maintains a constant temperature, irrespective of the temperature of its surround- ings, and there is always a loss of body heat toward the atmosphere. It would seem, therefore, that in heating buildings we had failed to ap- preciate the full importance of the fact that our sensations of tempera- ture depend, not upon the heat that ♦Read in abstract before the Brooklyn Medical Society, May 15, 1903. is received from outside sources, but upon the rapidity with which the body heat is lost. Admitting this, we are at once confronted with the fact that loss of body heat is gov- erned by other factors besides the temperature of the surrounding at- mosphere, notably its relative hu- midity. This is a factor that has been over- looked, and in this omission lies the anomaly of modern heating. So long as we continue to neglect the indoor relative humidity we shall continue to live in the unhygienic surroundings created by any method of heating that is not supplied with means for properly moistening the- air. To do this should be as much the purpose of a scientifically constructed heating system as to furnish sufficient heat. All methods of heating that neglect to provide that important atmo- spheric element, watery vapor, are almost certain to bring about over- heating or uncomfortable chilling. The so-called overheating of the average house is so common that it has come to be accepted as an in- evitable accompaniment of indoor life. The key to this uncomfortable and unhygienic feature is the low relative humidity of the indoor at- mosphere which is bound to exist unless provision is made for control- ling it. • ( It is a fundamental principle in climatology that the relative hu- midity of any location is of the great- est importance, and yet in regulating the "climate" of our homes in winter this factor is entirely overlooked. It is startling to hear that the average ^ heated room is as dry as a desert, that this dryness forces us to heat to a most unhygienic degree, that it causes a waste of fuel and that dis- eases of the air passages are trace- able to this source. Actual experi- ments show, however, that such statements are not exaggerated. Ref- erences to the dryness of heated rooms have, as a rule, been so indefi- nite that they have failed to carry due weight and the importance of the sub- ject has not been fully appreciated. Some of us have endeavored to fulfil the requirement by placing pans of water in the air-ducts of furnaces, but still the problem of sufficient moisture has remained unsolved. There are many reasons why water pans in the air-ducts will not give satisfactory and controllable condi- tions of humidity in the rooms, but it is not within the province of this paper to discuss that phase of the subject. At the outset of this investigation it was necessary to know exactly the meaning of the phrase "overheated houses." In order to obtain data from which a starting point for im- provement could be made records of the temperature in a large number of houses have been taken by the writer. It was found that it commonly ranged from 72 to 76°, and at times, in very cold weather, 78 F. was re- corded. Startling as these figures are in themselves and contrary to all ac- cepted ideas of the proper conditions for living rooms, yet a puzzling fact was that the sensations of the occu- pants were frequently not in accord with the readings' of the thermom- eter. Rooms felt chilly when the re- corded temperature indicated that they were far too hot. It was often hard to believe that the temperature was above 68°, when it was actually 72 ° to 74 . It was at once apparent that some unrecognized factor was responsible for this discrepancy between the tem- perature recorded by the thermom- eter and one's sensations. Moreover, it was found that the colder the weather the higher was the average temperature maintained indoors, an observation which any one may veri- fy and which will show that rooms may feel warm at 70 to-day and de- cidedly chilly at 72 ° or 74 to-morrow. This fact should arrest our atten- tion at once; if there be no draughts in the rooms the outdoor tempera- ture, however low, should not neces- sitate an indoor temperature higher than usual. If we assume that 68° or 70 is as warm as a room should be allowed to become, from the standpoint of health, and that such a temperature should be comfortably warm for any healthy person, an out- door temperature of o° should not affect the indoor comfort at 70 . We shall find, however, that we usually do heat above 70° when the ther- mometer out of doors is below 20°. The reason for this is the insufficient amount of moisture in our rooms in proportion to the temperature (low relative humidity). The colder the weather the lower will be the indoor relative humidity, because of the greater difference between the out- door and indoor temperature, as will . be further explained. During the winter of 1901 repeat- ed records of the relative humidity taken in many different buildings ac- cumulated a mass of figures which established the fact that 30 per cent, is the average in rooms heated as de- scribed. This statement is in accord with the findings of other investigators. "The Proceedings of the Convention of Weather Bureau Officials," held at Omaha in 1898, contains a paper en- titled "Atmospheric Moisture and Artificial Heating," by W. M. Wil- son, of Milwaukee, who found that in buildings heated by steam and hot water, with an average temperature of 72 °, the relative humidity was 28 per cent., while with furnace heating it was as low as 24 per cent. Wilson refers to experiments made by Barnes, of Boston, who found the relative humidity at a temperature of 72 to be 31 per cent. He also re- fers to investigations of Mr. Henry in the Weather Bureau building at Washington during March, 1896, where it was found that at 72° the relative humidity was 31 per cent. Now the United States Weather Bureau reports show that such a low average relative humidity naturally occurs only in the most arid regions.. The reports contain tables of the mean monthly and yearly relative humidity observed at official stations through- out the United States and cover pe- riods of from five to fourteen con- secutive years. These tables show that from the total, 130 stations, in only 10 is the mean annual relative humidity as low as 50 per cent. Denver, Colo., is a place one usual- ly associates with the term "dry cli- mate;" There the mean annual rela- tive humidity is 48.7 per cent. — near- ly 20 per cent, higher than that of the winter atmosphere of our homes. The Weather Bureau maps, showing the normal annual relative humidity in the different sections of the United States disclose the startling fact that the area where the average is 45 per cent, comprises the section of country generally designated "the arid region of the Southwest,'' or "the Great Southwestern Desert." We are principally interested, how- ever, in the average outdoor relative humidity of New York city, where indoor records were taken, and I shall give only that of the months of October to April, inclusive: Octo- ber, 73.9 per cent. ; November, 74.8 per cent.; December, 73.6 per cent.; January, 75.2 per cent; February, 73.6 per cent.; March, 71. 1 per cent.; April, 67.8 per cent. Comparison of these figures with 30 per cent, ob- served indoors shows at once the great contrast between the natural and the artificial conditions, and fur- ther elaboration of this point will re- veal the fact that we are spending a great part of our lives in an environ- ment the effect of which cannot but be detrimental. The comparison will also explain many of the discomforts experienced in artificially heated rooms. Objections may be raised because the above figures of the outdoor hu- midity have been given without stat- ing the accompanying air tempera- ture, but omission of the temperature in this connection is allowable. Within extreme limits, the effect upon the human organism is depend- ent upon the percentage of saturation of the aqueous vapor and not upon the amount of vapor present. Hann says in his "Handbook of Climat- ology:" "All organic substances are more or less hygroscopic, and this condition, so far as it depends upon the humidity of the air, is determined by the relative and not by the abso- lute humidity." The point to be emphasized is that every time we step out of our houses during the winter season we pass from an atmosphere with a relative humidity of about 30 per cent, into one with a relative humidity of, on an average, 70 per cent. Such a sharp and violent contrast must be productive of harm, particularly to the delicate mucous membranes of the upper air passages. In seeking for the cause of the low indoor humidity we have to consider certain laws of physics and of meteor- ology. In the first place, the idea is so prevalent that air in some mysterious way absorbs moisture that a protest should be entered against all expres- sions that tend to perpetuate this er- roneous impression. Watery vapor, what we term moisture, is as much a part of the air as is oxygen; abso- lutely dry air does not exist in na- ture. It is with certain conditions of this vapor that we have to deal. A vapor may be defined as an aeri- form state of liquid. Every volatile liquid under favorable conditions changes to a vapor, which either be- comes saturated or remains unsatu- rated, according to circumstances. These terms refer to the state of the vapor itself. The higher the temperature, the more of a given vapor is required per unit of space before the vapor be- comes saturated. The weight of saturated vapors at different temperatures has been de- termined with accuracy. A cubic foot of saturated watery vapor at a temperature of 30° F. weighs 1.935 grains; at 70° F. it' weighs 7.980 grains whether the cubic foot con- tains air or not. The presence of an- other gas only retards the diffusion of the vapor particles and does not affect the amount required for satu- ration, and all such expressions as "air saturated with watery vapor," "the capacity of the air for watery va- por," etc., are unscientific and should be avoided. A vapor is said to be saturated when in a unit of space at a given temperature the maximum amount by weight exists and, the temperature remaining the same, no more vapor can form from the liquid with which the vapor is in contact. There will be no evaporation. If the tempera- ture is reduced some of the vapor will be changed back to a liquid. Now, if the temperature be raised the previously saturated vapor be- comes an unsaturated vapor, because with increase of temperature more vapor is required for saturation. If the vapor is still in contact with the liquid from which it is was formed, vaporization, or evaporation, at once begins and continues until as much vapor again exists as can at this high- er temperature. There is then more saturated vapor by weight in the same space than when the vapor was saturated at the lower temperature. If, on the other hand, the tempera- ture of the saturated vapor is raised, but the vapor be not in contact with the liquid from which it was formed, or if it is in contact with an insuffi- cient amount of liquid the vapor will remain unsaturated and its degree of saturation is expressed by the per- centage that its existing weight is of its weight were it saturated at the same temperature. Thus, at a given temperature, if the weight required for saturation is double that of the existing weight the vapor is said to be "50 per cent, saturated," and in speaking of such a condition in the atmosphere the correct expression is "the aqueous vapor in the atmos- phere is 50 per cent, saturated," not "the atmosphere, or the air, is 50 per pent, saturated with aqueous vapor." It is the tendency of all volatile liquids to form saturated vapors; this is spontaneous evaporation, and it fol- lows that our sensations of "moist" and "dry" are wholly relative, de- pending upon the rapidity with which the tissues of the body, particularly the skin and mucous membranes, are called upon to give up water. This is governed by the percentage of satura- tion of the watery vapor in the atmos- phere that surrounds us; not upon the amount of vapor that exists. A given amount of watery vapor in the atmosphere to-day, at a cer- tain temperature, may be t nearly saturated and demand little moisture, and the air feels damp because our bodies give up little water to the at- mosphere. The same amount of va- por at a higher temperature will be farther from saturation' and the air will then feel much dryer, because, although the same amount of vapor is present in each case, in the latter instance the lower percentage of satu- ration demands more moisture and causes the human body to give up more through the skin and the mu- cous membranes. Let us suppose that at a tempera- ture of 40° F. the weight per cubic foot of the existing watery vapor be 70 per cent, of that required for satu- ration. We express this atmospheric condition as a "relative humidity of 70 per cent." If now the tempera- ture falls below 40° and the amount of vapor remains the same then the percentage of saturation will be high- er than 70 per cent., since at a lower temperature less vapor. is required for saturation. If the temperature con- tinues to fall a point will be reached at_ which the existing vapor will be saturated; water will no long- er be evaporated, the relative hu- midity has increased to 100 per cent, (saturation), with the same amount of moisture present that at 40° was enough to give a relative humidity of only 70 per cent. ; on the other hand, if the temperature becomes higher than 40,° the relative humidity will be less than 70 per cent., because more vapor is required for saturation at the higher temperature. • This latter cgnditon jg exactly what 4 occurs in every artificially heated room, unless an increased amount of watery vapor is produced by artificial means. Whatever be the percentage of saturation of the vapor that enters from out of doors a lower relative hu- midity is created because of the high- er room temperature, and the human body is exposed to the enormous moisture-extracting properties of an atmosphere that is very dry — not be- cause it contains less moisture, but because the existing vapor is so much further from the saturation point; for spontaneous evaporation is a con- stant phenomenon and is, most rapid when the aqueous vapor is furtherest from saturation. The effect of different conditions of humidity is best shown by determin- ing what the aqueous vapor lacks of saturation, for this measures not only its capacity but its demand for mois- ture. There is a factor for this called the "saturation deficit," which many meteorologists have proposed to substitute for the relative humidity in the psychrometric tables. The "saturation deficit" is the dif- ference at any given temperature be- tween the pressure, or tension, ex- pressed in millimeters of the baro- meter, of the existing aqueous vapor and that of saturated aqueous vapor at the same temperature. The prac- tical difficulty with the "saturation deficit" expressed in this way is that it means little or nothing to any one save the trained meteorologist. Now, if we should interpret the saturation deficit to be the difference in grains per cubic foot between the weight of the existing watery vapor and the weight of saturated watery vapor at the same temperature — in other words, the lack of aqueous va- por per cubic foot of space — we shall have a factor which, I believe, will show more graphically than any other the bearing that different con- ditions of atmospheric humidity have upon the human organism. Let us compare in grains per cubic foot the saturation deficit of the aver- age winter outdoor conditions with tbat of the artificially heated room. The mean relative humidity of New York city for the months of October to April, inclusive, is approximately 7$ per cent., as has been seen. The mean average temperature for the same months is 44°. The tables give the following values for the absolute humidity in grains, from which we obtain the saturation deficit as fol- lows: Average Outdoor Conditions. Temper- ature. Average rel. hum . Average ab. hum. 44° 73S* 2.404 (grs. per cu.ft.) 44° ioo# (Saturation) 3-294 " " " " Saturation Deficit=3.294— 2.404=0.890 (grs. per cu. ft.) Average Indoor Conditions. Temper- ature. Average rel. hum. Average ab. hum. 72 30$ 2.552 (grs. percu. ft.) 72 ico# (Saturation) 8.508 " " " " Saturation Dencit=8.5o8— 2.552=5.956 (grs. percu. ft.) This shows that each cubic foot of aqueous vapor out of doors is 0.890 grains short of saturation, while each cubic foot of aqueous vapor indoors is 5.965 grains short of saturation. The skin and the mucous mem- branes of the respiratory passages are the principal sufferers, since these tissues are always kept moist with their own secretions and from them water is freely abstracted to satisfy this large saturation deficit, such air passing with every inspiration over the moistening surfaces nature has provided in the mucous mem- branes calling for an enormous out- put of the fluid elements of these tis- sues. This leads to glandular over-ac- tivity and its consequent evils, the elaboration of which subject the scope of this paper does not permit. The overheating of our houses has been accepted as a prominent cause of "catarrh," but I am confident that the low relative humidity and con- sequently the large saturation deficit of the aqueous vapor in the atmo- sphere of our rooms in winter is much more important than is the overheating in itself, and it may be doubted whether the so-called damp climate of the sea coast or the shores of large inland lakes is in itself so re- sponsible for the above diseases, as has been generally supposed. It seems much more likely that the great contrast between the indoor and the outdoor relative humidity in those regions is the real factor. For, if we eliminate the class who lead a sedentary life and remember that the coast fishermen and sailors habitually live in an atmosphere in which the aqueous vapor is about 80 per cent, saturated, the question pre- sents itself: "Are such persons more prone to catarrhal inflammation than are the inhabitants of a dryer cli- mate, other factors in their lives be- ing equal?" We know that all per- sons suffer more in winter from exist- ing "catarrh" than they do in the summer, when even if their lives are sedentary the doors and windows of their homes and offices being opened the indoor and the outdoor humidity are practically the same and they are living under conditions of higher hu- midity, both relative and absolute, or in a damper climate, so to speak, than in winter, when so much of the time is spent in the dry atmosphere in- doors. It is self-evident that at any given temperature the less the relative hu- midity is the larger is the saturation deficit, and this undoubtedly governs most closely the rapidity of evapora- tion and is a direct measure of the de- gree of damage that dry atmosphere causes to pictures, furniture, books, etc., for if there were sufficient mois- ture to maintain a smaller value for the saturation deficit these articles would withstand a temperature much higher than that ever attained in living rooms. The temperature in itself is not the destructive agent. It may be of special interest to compare the saturation deficit in ar- tificially heated rooms with the aver- age at Denver, Colo., which is as fol- lows: Average temper- Average Average ature relative humidity ab. hum. 51 50$ (Approximately) 2.111 (grs. per. cu. ft.) 51° ioo# (Saturation) . 4.222 " " '■ ,l Saturation Deficit=4.222 — 2.111 = 2.111 grs. percu. ft. The previous computation showed that in artificially heated rooms this was 5.956 grains, or more than twice as large as the average at Denver, Colo., and nothing could express more graphically the dryness pro- duced by artificial heating without ar- tificial moistening. In addition to the excessive evaporation caused bv such conditions the loss of body heat is greatly accelerated. If our rooms contained more moisture we could live more comfortably at a lower temperature. The overheating is re- quired because of the low relative humidity. The atmosphere of our planet re- tains heat almost wholly by virtue of the aqueous vapor. Tyndall stated that "the removal for a single summer night of the aqueous vapor from the atmosphere which covers England would be attended by the destruction of every plant that 'a freezing tem- perature would kill." The importance of aqueous vapor in the atmosphere is thus stated by Prof. Warren S. Johnston, who says: "It is a curious fact that it is only through the moisture in the air that it retains heat. Heat naturally ra- diates from all bodies that are warm- er than their surroundings; if the air has little or no moisture in it the ra- diated/heat goes right through _ it without warming it, but if it is moist it stops the radiated heat, and heat warms it. If it were not for the moisture in the air it would be too cold to live in. Humidity in the air is nature's great bed blanket for her children without which they would all perish; so, likewise, moisture in the living room acts as clothing and helps to keep us warm." A moment's consideration, there- fore, shows that the prevailing prac- tice of depending upon the thermom- eter as the sole o guide in the heating of buildings is not only inadequate and unscientific, but it is often mis- leading. It is not sufficient to know only the temperature if we desire either comfort or health, for the same temperature produces varying sensations of warmth or cold, de- pending upon the relative humidity at the time existing. It is unscientific and arbitrary to lay down a fixed temperature as a standard for living or sleeping rooms unless the rela- tive humidity is indicated as well. The same air temperature is not the same in its effects at different times, because the relative humidity varies, and unless more moisture is supplied to the air which we obtain from outside by various methods of ventilation the relative humidity in- doors becomes 'unusually low in cold weather, because of the great differ- ence between the indoor and the out- door temperature, and even though the relative humidity be fairly high out of doors,' the absolute humidity is low, on account of the low tem- perature of the air. Moreover, at New York and along the Atlantic coast the prevailing winds during cold periods are usu- ally from the north or northwest, having passed over a dry, frozen area which has presented little opportun- ity for the air to take up moisture. At such times the temperature of the air indoors is allowed to become as high as 76° to 78° in order to feel comfortably warm. Records from steam heated apartments show that the relative humidity was sometimes as low as 25 per cent., with a tem- perature of 78°, during a period of very cool weather in January, 1902. The high temperature is necessitated by the chilling of the body by the in- creased evaporation, evaporation be- ing essentially a cooling process. It is needless to say how un- hygienic as well as uncomfortable is such a distortion of the proper rela- tionship between temperature and relative humidity. By regulating the indoor relative humidity we could keep our room temperature much more nearly stationary, irrespective of the temperature outside. But no improvement in indoor atmospheric conditions can be expected until heating engineers and the people they serve realize that with the ever- varying absolute humidity out of doors no system of heating can be made satisfactory if the indoor rela- tive humidity is disregarded. Even thermostatic temperature control will not fill the requirement, for a . con- stant temperature is constant in its effects only if accompanied by a con- stant relative humidity. The winter of 1902 and 1903 was occupied in taking records and ob- servations -in rooms in which an ex- perimental moistening apparatus was attached to a radiator. The mechanism was such that the control of the temperature and of the mois- ture were independent, a most im- portant point, and observations were made at different temperatures and with varying percentages of humid- ity. These tests were most instructive. In the first place it was observed that with a proper percentage of moisture 70° F. was uncomfortably hot, 68° F. warm and 65° comfortable. By prop- er percentage of moisture is meant one which is never below 50 per cent, or above 70 per cent. — average about 60 per cent. It was determined by repeated ex- periments that a temperature of from 65° to 68° and a relative humdiity of 60 per cent, produced the most com- fortable conditions, which were in marked contrast to a temperature of 72° F., with a relative humidity of 30 per cent. The former felt warm and balmy, the latter, notwithstanding the higher temperature, chilly and dry, and the slightest motion of the air suggested a search for the source of suspected draughts. Moreover, properly moistened in- door atmosphere lacks all the op- pressive dry feeling so characteristic of the average artificially heated room, seeming more like the corridor of a well ventilated hotel. The quiet- ing effect of such an atmosphere is striking. There is an indescribable sense of relaxation and "poise," con- trasting strongly with the feeling of nervous tension so frequently experi- enced in overheated dry rooms. It was satisfactorily proven that one may live during the coldest weather with perfect comfort in a room at 65° F. where the relative hu- midity is kept at about 60 per cent. During the experiments upon the sensations produced by different per- centages of saturation and in order to obtain the opinion of persons having no knowledge of the existing condi- tions one room was equipped with a moistening apparatus and the tem- perature kept at 65° to 68°, with a relative humidity of about 60 per cent. An adjoining room, without a moistening apparatus and heated by an ordinary steam radiator, had an average temperature of 72" to 74°, with a relative humidity of 30 per cent. In every instance, and without at all knowing what the tempera- tures were in the ' two rooms, the opinion was unhesitatingly expressed that the first room- was several de- grees warmer than the second. It has been mentioned that the feeling of comfort was greatest in a room at 65° with a relative humidity of 60 per cent, produced by artificial means. It was most interesting and instructive in connection with this matter to find, on the perfect days in May and early June, with all the windows opened, admitting free- ly the outside air, that the thermom- eter stood at 65° to 68° and the hygrometer registered about 60 per cent, relative humidity. A moment's thought recalls the fact that we often sit out of doors with perfect comfort at a temperature that would cause u's to shiver in our rooms in the winter. The relative humidity is the balance- wheel that regulates our comfort at different temperatures in still air. So repeatedly was this demon- strated that it may be accepted as a cardinal rule that if a room at 68° is not warm enough for any healthy person it is because the relative hu- midity is too low, in which case the procedure is to raise the relative hu- midity and not raise the tempera- ture. By raising the temperature we shall simply still further distort an already unnatural relation between the temperature and the moisture, which »is the real source of the dis- comfort. It seems well at this point to di- gress from our immediate subject and attempt to analyze, certain out- door conditions that upon hasty con- sideration appear to conflict with the statement that "indoors we may be more comfortable at 65° than at 72 °, provided the relative humidity is suf- ficiently high in the former instance." Tfie argument that is immediately brought up in refutation of the above statement is "that it is well known that cold weather is much more eas- ily borne if the humidity is, low than if it is high," which one must admit; but such sweeping, unqualified state- ments are scientifically meaningless, for it is equally true, using the same loose form of expression, that hot weather can be borne better if the hu- midity is low than if it is high. We should clearly define in each instance what we mean by the terms damp and dry, hot and cold. The outdoor atmospheric condi- tions to which it is desired to draw special attention are of frequent oc- currence along the North Atlantic coast in the winter. They are the so- called "raw" days, which the records show have not only higher air tem- perature, but also a higher relative humidity than the bracing clear and dry days which usually precede them. Yet on such days the cold seems much more penetrating. For exam- ple, in the vicinity of New York a temperature of 20° to 24° with a rela- tive humidity of about 65 per cent., little or no perceptible wind and a clear sky, are conditions under which one does not feel the cold nearly so much as when the weather moderates, the temperature mounts to about the freezing point and the relative humid- ity is usually between 80 per cent, and full saturation. The important fact to note here is that the relative humidity is higher and at the same time the temperature has risen, indicating a much greater absolute humidity; for if there were not more moisture in the air by weight (absolute humidity) the rela- tive humidity must have fallen when the air temperature rose. This higher absolute humidity is usually occa- sioned by winds that blow from the sea, for almost invariably on these raw days there is a strong east or northeast wind. - To analyze the problem of our sen- sations under the above conditions we have several factors to consider, the most' important of which is the great difference between the effect- ol air which is in rapid motion and that of air which is relatively still. A higher absolute humidity in connec- tion with greater air motion gives a condition that is most favorable for the rapid abstraction of heat from the body by convection. In most text books on climatology the term "conduction of heat" is used where "convection" is really meant. In the strict scientific sense of the term conduction of heat through gases is almost nil, gases being the poorest conductors of heat, liquids better and solids the best. But aqueous vapor is one of the best gaseous absorbents of heat ; therefore the loss of the body heat by convection is very great if the abso- lute humidity is high and the air is in motion. This accounts in the main for the greater feeling of cold on these raw days. A practical point here is that on such days the warmest outer garment one could wear would be of some waterproof material, in. preference to the ordinary woolen overcoat, of greater weight, because we should interpose a more perfect barrier be- tween our bodies and the vapor- charged air, with its heat abstracting powers. Body heat is lost through the at- mosphere principally in two ways — by convection and by surface evapo- ration. Out of doors in winter loss by evaporation is of relatively little importance because of the protection afforded by our heavy clothing ; but it becomes of greater importance in summer, when because of the higher air temperature we wear lighter clothing of such texture that the skin of the entire body is practically exposed to the air, and we endeavor as far as possible to take advantage of the loss of heat caused by surface evaporation. The heat absorbing properties of moist air in rooms operates directly to the advantage of the occupftnts, because the air is relatively still. We furnish, by artificial means, heat to keep the temperature of the air much nearer the temperature of the body than is the air out of doors, and a reasonable percentage of moisture (60 per cent) in a room at ordinary temperature conserves the heat sup- plied by the heat source. During my experiments it was very noticeable how much more uniformly heated seemed all parts of the room in which there was sufficient moisture than when no moisture was artificially supplied. Under ordinary condi- tions of heating the uncomfortable difference in temperature between the parts of the roorn that are near and those that are remote from a radiator is a familiar experience. It is inconceivable that with other- wise perfect means of heating, pro- vision for producing sufficient mois- ture to maintain a higher relative hu- midity should have been so disre- garded in all but those elaborate systems applicable only to large halls and public buildings, where with forced ventilation the air may be moistened in the main air ducts. But these systems, from the expense of installing and operating, are imprac- tical in the ordinary home, thus leav- ing the great majority of us to live in an atmosphere of desert dryness, and depriving us of an element almost as necessary for health as a proper- de- gree of heat. That we are wasting fuel is evident at once from the unnecessarily high temperature we maintain. The waste is probably greater than is generally supposed. Wilson says in the paper to which I have referred that "about 25 per cent, of the cost of heating is expended in raising the temperature from 60° to 70°, so if we can keep comfortable at a temperature of 65 we shall have saved at least I2| per cent, of the total cost of heating." Various instruments may be used to determine the relative humidity, the wet and dry bulb thermometers being perhaps the most accurate ; but for household purposes this form of hygrometer has the disadvantage that the relative humidity is not in- dicated directly, but reference to tables or else a somewhat complicat- ed calculation is required. It has one point of considerable scientific interest, however, for the reading of the wet bulb thermometer is a very fair measure of the temperature that is felt by the moist surface of the body; that is to say, the temperature produced by evaporation (which is always lower than the air tempera- ture, unless the aqueous vapor is saturated). In an interesting paper on this subject Prof. Harrington, of the Weather Bureau, has termed the temperature produced by evapora- tion "sensible temperature," because it is the temperature actually felt by the surface of the body. Now, given a room at.72 with a relative humidity of 30 per cent, and another room at 65° with a relative humidity of 60 per cent, the sensible temperature in each case can be de- termined from the psychrometric tables, and we find that in the first instance the sensible temperature is 55° (wet bulb) and in the second in- stance the sensible temperature is 57 , showing that the body surface temperature is 2° higher, at 65°, with a relative humidity of 60 per cent., than at 72°, with a relative humidity at 30 per cent. A careful study of the fluctuations in the wet bulb thermometer, having in mind this term, "sensible tempera- ture," shows that sensations of heat and cold conform much more closely to the. readings of the wet bulb than of the dry bulb, as the following in- stance will illustrate. On a very hot day in May, 1903, at 4.45 P. M., the dry bulb (air temperature) stood at 87 , and the wet bulb (sensible tem- perature) at 70°. The relative hu- midity indicated by these readings was 45 per cent, (unusually low for this locality). At 5 P. M. there ap- peared indications of an approaching thunder storm, which, however, passed around to the north and only a few drops of rain fell ; at 5:15 the im- pression was that it had become very much warmer, but the thermometer showed 86° dry bulb, and 73° wet bulb (relative humidity 53 per cent.). The air temperature was actually 1° lower, but the sensible temperature was 3° higher, which exactly coin- cides with one's sensations. The above is so common in sum- mer that every one will have experi- enced the feeling of greater heat whenever a thunder shower fails to clear the air. We say in explanation that the humidity is higher, which is true, but the above figures show how much more graphically the actual conditions are represented by the. readings of the dry and wet bulb thermometers than by expressing the relative humidity in per cent. The surface of the body is analogous to a wet bulb thermometer and the term "sensible temperature" deserves a prominent place in our nomenclature. The lowering of the "sensible tem- perature" by a low relative humjdity is of special importance in children's nurseries — a fact that has not been sufficiently recognized. Children and babies habitually uncover themselves in bed, thus exposing their bodies to the chill produced by excessive evap- oration from the skin. This would seem to account for many of the colds and bronchial affections that children and infants contract in rooms where the temperature has been kept sufficiently high. If a proper relative humidity were main- tained uncovering the body would be attended with less risk. It is easier to "take cold" in a room.at 72° with a relative humidity of 30 per cent, than in a room at 65° with a relative hu- midity at 60 per cent. A reliable hygrometer should be in every household and should be watched to see that proper conditions are maintained. An instrument for registering the percentage of mois- ture is as important as an instrument for registering the temperature ; per- haps more so, for with a proper per- centage of saturation of the aqueous vapor we may more safely trust to our feelings of comfort regarding the temperature. For practical purposes the hair hygrometer, or Lambrecht's poly- meter, is a most satisfactory instru- ment, combining as it does a ther- mometer and a dial from which the percentage of relative humidity can be easily read. Comparison of one of these instruments for over a year with the calculations made from the readings of the wet and dry bulb thermometers has shown it to be very accurate, 10 In sanataria there is a wide field for the investigation of the subject of in- door humidity. Certain classes of cases, particularly those of nervous exhaustion, we know do better in a comparatively moist climate, such for instance as that of Atlantic City, while others do better in a drier cli- mate like that of Denver. If the in- door relative humidity could be con- trolled and its relationship to tem- perature varied we might in these two factors approximate different cli- matic conditions greatly to the ad- vantage of properly selected cases. The importance of indoor humid- ity from a hygienic and therapeutic standpoint can hardly be overesti- mated and a paper could be written upon this phase of the subject alone. Generally speaking, dry air is an ex- citant often causing sleeplessness and irritability, accompanied with a drier skin and quickened pulse. Moist air is more of a depressant, producing quiet sleep and slower circulation of the blood. Moistening apparatus have been in the main unsatisfactory, because their control has not been independ- ent of the control of the heat source in the room; but instances frequently occur, owing to rapid atmospheric fluctuations, which make it neces- sary to shut off the production of moisture but at the same time to have full heat in the system. On the other hand, we may wish to shut off the heat, but not the production of vapor. It is absolutely necessary for practical use to have the heating and the moistening apparatus capable of being independently controlled. It is amusing to find that in spite of the advantages of a moister at- mosphere indoors objections have been raised against the use of devices for this purpose on the ground that when the weather is very cold mois- ture condenses upon window panes. It is true that this will happen; it is a natural and logical result of proper conditions. We can all remember that before the days of heating by direct radiation it was usual to see frost on the window panes in winter, a condition we now never see with the extreme dryness that our present di- rect heating systems produce. The fact that condensation does not occur is sufficient evidence of the indoor "dryness.'' However, this objection could be obviated by having double windows, which indeed are common in houses whose situation is at all ex- posed. This paper will have failed in its purpose if the impression is given that in any sense it advocates "moist climates." The deleterious effects of residence in the humid tropics are too well known to need more than pass- ing mention; but from the climate of the tropics to that of the desert is a wide difference; one shuns the latter as much as the former. When we consider how many are compelled by their occupations to spend a greater part of their lives in- doors the desirability of reducing as far as possible the contrasts between natural and artificial conditions is at once apparent. The relative dryness of the average indoor atmosphere in winter is so unnatural and so unhy- gienic that its correction should re- ceive the serious consideration that the subject merits. We who live in houses heated in the ordinary manner are responsible for the conditions that prevail, be- cause we have not demanded more hygienic methods. If the subject does not appeal to us upon the ground of health it should upon the ground of economy and comfort. If we would preserve our furniture, books, pictures, etc., from the rav- ages of artificial heat we must con- trive some means of controlling this low relative humidity. After living in rooms with a lower temperature and proper relative humidity no one will be satisfied with the other con- ditions. 64 Montague street, Brooklyn, N. Y.