A SHORT TREATISE ON THE Co IPOUND STEAM ENGINE, WITt A NEW METHOD OF FINDING THE RELATIVE AREAS OF THE TWO CYLINDERS. ILLUSTRATED WITH DIAGRAMS, TABLES, ETC. BY JOHIN TURNBULL, JR. NEW YORK: D. VAN NOSTRAND, PUBLISHER, 23 MURRAY AND 27 WARREN STREET. 1 8 7 4. THE COMPOUND ENGINE. The compound engine-whatever diversity of opinion may be held by engineers and others as to its merits as an economical expansive engine-has attracted towards itself a very considerable share of attention, from the superior results that have been obtained.by it in many instances; and it is reasonable to suppose that, when a certain degree of perfection has once been attained in the manufacture of any machine, or economy secured by any new arrangement of its parts, similar machines can be so constructed as to give out the same results, if proper care is taken that the same arrangement and construction is faithfully carried out as in that of the more perfect machine. And when that degree of economy has not been obtained from a compound engine which had reasonably been 4 expected, it would, no doubt, be found, if proper inquiry were made, that the fault lay, not in theprinciple that had been adopted, but that sufficient skill had not been exercised in properly proportioning the different parts through which the steam had to pass or come in contact on its way from the boiler to the condenser, and that sufficient means had not been employed to prevent or replace any waste of heat from condensation and other causes. As the compound engine is being now so universally adopted in the Mercantile Marine Service, and a knowledge of its principles absolutely necessary by those engaged in attending it, we will, in the following remarks, explain these principles in as simple a manner as possible, and institute a comparison between the respective merits of the single-cylinder expansive condensing engine and the compound engine: The compound engine is a high and lowpressure condensing engine, having two ordinary steam cylinders, the smaller or high-pressure cylinder communicating direct with the boiler, the larger or low 5 pressure condensing cylinder direct with the condenser, and both with each other. The steam is admitted freely from the boiler into the high-pressure cylinder until the piston has been moved through a certain distance where the valve is so regulated that the communication with the boiler is entirely shut off, and the remainder of the space to be passed through by the piston is performed by the expansion of the steam now shut up in the cylinder, and which, after doing its work in this cylinder, passes on to the condensing cylinder, where it does an equal or proportionate quantity of work, and then passes into the condenser. It has been found from modern practice that when the length of stroke of both cylinders is the same, it is necessary that the condensing cylinder be about three times greater in area than the high-pressure one, and this proportion is best suited when the steam employed is from 45 to 50 lbs pressure above the atmosphere, and cutting off the steam after being admitted during ~ of the stroke in the high-pressure cylinder. When the steam to be employed is of a less pressure, but the point of cut-off the same, then the relative proportions of the cylinders must be nearer to each other, and the reverse when steam of a greater pressure is to be used. To get the maximum of economy out of any class of expansive condensing engine, the pressure of steam and point of cut-off must be so regulated that the steam passes into the condenser at the end of the stroke at a pressure not exceeding 5 lbs. above a perfect vacuum, and with steam at 45 lbs. pressure above the atmosphere, which is equal to 60 lbs. pressure above a perfect vacuum (the pressure of the atmosphere being considered as equal to 15 lbs. on the square inch), and a terminal pressure of 5 lbs., we get 12 expansions, because the pressure at the end of the stroke is 12 times less than what it was at the point off cutoff, and is expressed by the formulaP=R. Where P - pressure at point of cut-off, t= terminal pressure, and R = ratio or number of expansions, and as the pressure of 7 steam, according to Marriotte's law, varies inversely as the space it occupies, the steam will now fill 12 times the space it originally occupied at a pressure equal to M-ath of the original pressure, that is, supposing there had been no loss of heat during the process of expansion, and this we must suppose to simplify this inquiry. On reference to the annexed table of average pressures, it will be seen that steam admitted at 60 lbs. pressure, and cut-off at Math part of the stroke, exerts an average pressure = 17.32 lbs. per sq. in. on the piston throughout the whole stroke, and, although this is 34 times less work than would have been done had the steam been used at the full pressure of 60 lbs. throughout the whole length of the stroke, still only a 12th part of the cylinder's contents had been filled from the boiler, and the power required is thus got by working the steam expansively, at a saving equal to about 34 to 1. (See Table A.) TABLF, A.-.-AVERAGE PRESSURE FOR ANY RATE Pressure in Lbs. at ComSteam _ nttoffat 30 35 40 45 50 55 60 21 241 28 311 35 381 42: 284 321 371 42 461 511 564 1 171 204 231 261 291 321 3.5 251 29.2 334 38 42L 461 501 284 33 — 381 431 484 53 571 151 18 203 231 26 281 31f 23 261 301 341 381 42 46 27 311 361 403 45 L 491 541 29- 344 39 44 49 531.58i 14 16- 18'1 201 23-1 251 273 4 294 34L 394 44L 491 54 59 7 121 141 161 181 2L 231 251 4 191 224 251 284 32 35, 381 23 271 311 3 51 391 431 A47~ 7 261 3t1 351 40 441 49 53, * 281 332 381 421 471 521 574 7 29 341 394 444 494 544 594 L 11 13. 151 17L 191 211 23 3 22L 26 294 33L- 37 4o4 441 8 27' 3A 364 414 45L 50. 554 29E1 341 394 44 49. 541 591 10 12L 14 154 4 174 19 L 21g 161 19L 221 25 271 304 331 A 24 28 32 36 401 44L 481 4 26L 304 351 39 44 48 t- 523 29 34 383 431 48L. 53 584 291 341 39 44L 491 541 591 _-1T 91 - 104 12L 1133 151 161 18f - 141 17 1 1- a22 241 27 291 ii 181 21 2.5 28 31L 341 372 1 211 25. 294 324 361z 4(4 434'- 244 281 321 36B- 40- 441 484 A 264- 30L 35 39L 434 48 52&.I- 274 324 361 411L 46 504 551 -,,- 281 334 388 43 474 524 571 1- 29L 34L4 39L 44 49 54 584 8 10 11{ 13 14.L 15 17 UPON PISTON DURING ONE STROKE. OF EXPANSION. mencement of Stroke. 65 70 75 80 85 90 95 100 451 49 521 56 591 63 661 170 61 651 704 75 791 841 89 93* 381 411 444 47I 504 534 56* 594 55 594 631 67* 72 76* 801 84 624 671 724 774 82 87 91* 96 34 364 39 414 444 47 491 52* 493 534 571 614 65 69 72* 761 584 683 674 721 77 811 86 901 631 681 731 784 83 88 92T 974 304 321 344 374 391 41 44* 461 64 69 734 78* 83 884 93 9812 274 291 311 33L 354 37* 40 42 414 45 48* 511 541 57* 61 64* p 511 554 591 63* 67* 71* 75* 79 ) 573 624 664 71* 751 80 841 89 62 664- 714 761 81 853 904 951 6 633 694L 744 79 84 89 93* 98 25- 27 283- 30 321 341* 61 381 484 52 554 594 63 66* 701 74* f 591 64L 681 73z 78 821 87* 914 23 241 264 284 304 31* 33* 351 36 384 414 44 -474 50 52* 551 524 56t 60t 64t 684 72' 76* 80 2. 561 61.a 66 70i 744 79* 83* 88' 634 68 724 774 8241 87 92* 97 641 694 741 79t 844 891 94* 993 20 214 23 242 264 27* 293 30* 314 344 364 394 414 444 4 6 49 401 434 47 50 534 564 59i 622 471 51 544 58l 62 65* 691 73 524 561 601 65 69 73 77 81T 561 61 651a 70 744 78* 83 871 60 641 691 734 781 83 87* 92* 624 67 714 763 81 1 86* 91 954 631 68a 701 78L 83.1 88* 931 98* 181 20L 21t 234 24i 26 271 29 TABLE A.-Continued. Pressure in Lbs. at Commencement of Stroketeam _ cutoffat 110 120 180 140 150 3 774 84 91 98 105 2 103 111 121 131 1404 4 65 714 774 834 89} I 4 938 1014 110 1184 127 4 106f 115{ 1254 135* 1444 574 62I 67* 73 784 84~ 91{ 991 1071 115 o 99 108T 1174 1264 135 107L 1174 1271 1361 146 - 4 51f 55 (I04 65 693 c 108 1184 128 1874 147 o 46f 501 544 584 63 704 771 834 90 964 } 87 944 1021 1104 1183 7 98 106* 115* 1244 1833 a -T 105 114f 124 1334 1438 7 108] 1184 128 1384 1484 421 46* 50 544 574 4 I 81* 89 961 104 11141. 101 110* 1194 128 1374 p 109 119 128* 1381 1481 0 9 89 424 46 494 53 61 661 724 774 83 1 i 4 884 961 10442 1124 1204 ID 9 97 105* 1144 123* 132* s 107 116* 1264 136f 145 109* 119* 129 139 149 8- 33{ 37 40 431 46 it 54 59 63* 683 738 - 684 75i 814 873 94 ~h- 80( 87* 95 102* 109 I 89* 97 1054 1131 1214 fi- 96* 105 1134 1224 131 - 1011 110* 120 129* 138i 5-l 105t 115 1244 1341 1438 f% 108 117t 1274 1374 147*'~ 81* 834* 373 404 43 11 As the point of cut-off may be different from any of those shown in the table, it is as well that the student should be in possession of a simple formula for ascertaining the average pressure for himself at any time, and the following is given to find out the mean pressure during a stroke in lbs. per sq. in. LetL = Whole length of stroke in inches. I = Distance travelled by piston before the steam is cut off, in inches. R = Ratio or number of expansions =H = Hyperbolic logarithm of R. P = Initial pressure of steam in lbs. per sq. in. p = Mean pressure during the stroke in lbs. per square inch. Then, p - P + A table of hyperbolic logarithms is also annexed so that H may be found without any difficulty. TABLE OF HYPERBOLIC LOGARITHMS. The Hyperbolic Logarithm of a number is found by multiplying the common Logarithm of the number by 2.30259. No. Logarithm. 1-1...................................0953102 1-2................................1823215 1-3...................................2623642 1-4.................................. 3364722 12 No. Logarithm. 1-5.................................4054652 1-6,.............4700036 1-7.................................5306282 1-8....................5877866 1-9...................................6418538 2-0...................................6931472 2-]......................7419373 2-2.......................,7884573 2-3.................................. 8329090 2-4..................................8754686 2-5..................................,9162907 2-6................................9555113 2-7...............................9932518 2 -8.......................... 1.0296913 2-9.................................. 1.0647107 3-0.................................. 1.0986124 3-1.................................. 1.1314021 3-2............................. 1.1631508 3-3........................ 1.19392S4 3-4.................................. 1.2237754 3-5............................... 1.2527629 3-6.................................. 1.2809338 3-7............................. 1.3083328 3-8................................ 1.3350010 3-9................................. 1.3609765 4-0..................... 1.3F62943 4-1.................................. 1.4109869 4-2.......................... 1.4350845 4-3.............................. 1.4586149 4-4 4.............................. 1.4816045 4-5.......................... 1.5040773 4-6........................... 1.5260563 4-7............................. 1.5475625 4-8............................... 1.5686159 4-9.................................. 1.5892352 5-0.................................. 1.6094379 5-1.................................. 1.6292405 5-2.................................. 1.6486586 5-3................................. 1 6677068 5-4.................................. 1.6863989 5 -5................................. 1.7047481 5-6.....1..7227665 5-7......................... 1.7404661 13 No. Logarithm. 5-8................................ 1.7578579 5-9................................ 1 7749523 6-0.................................. 1.7917595 6-i.................................. 1.8082887 6-2.................................. 1.8245493 6-3........................... 1.8405496 6-4.......................... 1.8562979 6-5................................. 1.8718021 6-6.................................. 1.8870697 6-7.................................. 1.9021075 6-8.................................. 1.9169226 6-9................................. 1.9315214 -0................................ 1.9459100 7-1........................... 1.9600947 7-2............................... 1.9740810 7-3.................................. 1.9878743 7-4.................................. 2.0014800 7-5....................... 2.0149030 7-6................................. 2.0281482 7-7............................... 2.C412203 7-8................................. 2.0541237 7-9.............................. 2.0668627 8-0.................................. 2.0794414 8-1.................................. 2.0918640 8-2.................-............ 2.1041341 8-3.................................. 2 162555 8-4................. 2 1282317 8-5............. 2.1400661 8-6.............................. 2.1517622 8-7......................... 2.1633230 8-8.................................. 2 1747517 8-9.................... 2.1860512 9-0............................... 2.1972245 9-1............................... 2.2082744 9-2............................... 2.2192034 9-3.................................. 2.2300144 9-4.................................. 2.2407096 9-5.................................. 2.2512907 9-6....................... 2.2617631 9-7.................................. 2.2721258 9-8.............................. 2 2823823 9-9.................... 2.2925347 10-0.................................. 2.3025851 14 No. Logalithm. 11-0................................. 2.3978953 12-0.................................. 2.484906o 13-0................................. 2.5649494 14-0.................................. 2 6390572 15-0.................................. 2.7080502 16 O -0........................... 2.7726067 17-0.................................. 2.8332341 18-0.................................. 2.8903847 19-0............... 2.9444497 20-0.................................. 2.9957322 21-0.................................. 3.0445437 22-0.................................. 3.0910562 23-0.................................. 3.1354964 24-0................................... 3.1780715 25-0.................................. 3.2188757 26-0....................... 3.2581099 27-0.................................. 3.2958495 28-0.................................. 3.3322306 29-0................................. 3.3672992 30-0.................................. 3.4011974 In order to arrive at the merits and capabilities of the compound engine, let us first see what are the results got from a single-cylinder condensing engine of given dimensions and cutting off the steam to work with a certain number of expansions. LetD = Diameter of cylinder in inches, L - Length of stroke in feet, N = Number of revolutions of crank per minute, p = Mean or average pressure on piston, then, for arriving at the horse power we use the following formula: 15 D2 X.7854 X 2L X N)v x p 3 horse-power. 33. 000 But as D2 X.7854 = area of piston, and 2 L X N = speed of piston in feet per minute, we will make D2 X.7854 =A, and 2 L X N = S, the formula then becomes AXSXp -H.P A X S X p =H. P. 33.000 and supposing the cylinder to be 24 in. diameter, length of stroke=4 ft.; number of revolutions per minute=50; pressure of steam at beginning of stroke=60 lbs. (all pressures here mentioned are above a perfect vacuum), point of cut-off, =-.lth part of the stroke, or, after the piston has travelled 4 in., so thatD = 24 inches. 2 L = 8 feet. N = 50 revolutions. P 60 lbs. 1 XH p =P' = 17.32, we get452 4 X 400 X 17 32 33.000 = 95 horse-power; To distribute this power equally over the working parts of a compound engine, it is 16 desirable that both cylinders be so proportioned that they will each give out nearly the same power, and that the thrust caused by the entrance of the steam at the beginning of each stroke be the same in both cylinders. To attain this with an accuracy sufficient for all practical purposes, it is necessary that the condensing cylinder be larger than the high-pressure cylinder in area, by the ratio of expansion that takes place in the high-pressure cylinder; that is to say, if a = area of piston in high-pressure cylinder, r = ratio of expansion in high-pressure cylinder, A = area of piston in condensing cylinder, then A=a r. So that if P = initial pressure in small cylinder, and P'=initial pressure in large cylinder, the area of large piston, multiplied by P', will be equal to the area of small piston multiplied by P, then P' A= P a. But as the ratio of expansion is the same in both cylinders, and the whole ratio of expansion equal to the initial pressure in small cylinder, divided by the terminal 17 pressure in large cylinder, we get / ratio of expansion in each cylinder, and as we have already taken P-60 lbs., and t5 lbs., we have V 5L/ 3.46 = differences of area of the two pistons, and also ratio oi expansion in each cylinder, and consequently=r. From the nature of the compound engine, the area opened up for the steam by the movement of the large piston is at all times decreased by a proportionate part a, 1, by the advancing area of small piston, so that the space actually occupied by the expanding steam is = A - 1, and from this we get the formula for ascertaining the average pressure in the condensing cylinder of a compound engine. H 18 A table showing the relative areas of the two cylinders of a compound engine, with the average pressure in each cylinder, etc.: P R p/ H S pt p 30 2.449 12.25.896 23.23 7.52 15.71 35 2.645 13.22 972 26.10 7.83 18.27 40 2 828 14.14 1.040 28.85 8.04 20 81 45 3 000 15.00 1.098 31.47 8.22 23 25 50 3.162 15.86 1.150 34.00 8.40 25.60 56 3.316 16.58 1.197 36.44 8.58 27.86 60 3.464 17.32 1.242 38.83 8.74 30.09 65 3.605 18.02 1.281 41.13 8.85 32.28 70 3.741 18.70 1.319 43.39 9.08 34.31 75 3.872 19.36 1.353 45.57 9.11 36.46 80 4.000 20.00 1.386 47.72 9.24 38.48 85 4.123 20.61 1.415 49.78 9.34 40.44 90 4.242 21.21 1.444 51.85 9 44 42.41 95 4.358 21.80 1.470' 53.84 9.56 44.28 100 4.472 22.36 1 497 55.84 9.64 46.20 105 4.582 22.91 1.521 57.77 9.73 48.04 110 4.690 23 45 1.545 59.69 9.82 49.87 115 4.795 23.98 1.567 61.57 9.89 51.68 120 4.898 24.45 1.589 63.43 9.96 63.47 125 5.000 25.00 1.609 65.32. 10.05 55.27 130 5.099 25.50 1.629 67.02 10.13 56.89 135 5.196 26 00 1.647 68.77 10.21 58.66 140 5.291 26.46 1.665 70.51 10.26 60.25 145 5.385 26 93 1.693 72.25 10.32 61.93 1.50 5 477 27.38 1.700 73.95 10.38 63.57 155 5.567 27.84 1.716 75.67 10.46 65.21 160 5.656 28.32 1.732 77.28 10.52 66.76 165 5.744 28.72 1.748 78.93 10.58 68 35 170 5.830 29.15 1.763 80.56 10 64 69.92 175 5.916 29.58 1.777 82.14 10.70 71.44 180 6.000 30.00 1.791 83.73 10.75 72.98 185 6.082 30.41 1 805 85.32 10.80 74.52 190 6.164 30.82 1.818 86.86 10.85 76.91..... - 19 The accompanying table has been drawn out for easy reference in conformity with this rule. The first column = — P -- the initial pressure of the steam above a perfect vacuum on entering the small cylinder; the second = R /- shows the relative areas of the two cylinders, and also the number of expansions in high-pressure cylinder; the third column, -P - the terminal pressure in high-pressure cylinder, gives the pressure at beginning of stroke in condensing cylinder; the fourth column, = I, contains the hyperbolic logarithms of R; the fifth, = S, gives the average pressure during a stroke in a single cylinder, for the different values of R and -P the sixth column,=p', gives the average pressure during a stroke in the condensing cylinder of a compound engine, and = P' R-1 and the last column, p, gives the average pressure during a stroke in the high-pressure cylinder P=- - + P R-H 20 Now, as the power to be given out by both cylinders is to be the same; the power that is required to be given out by A 95 60 2 -47.5 horse-power, and as V 3.464, t X 3.464 = 17.32 - P', and from the above formula we get p'= — 17.32 B1 8.74 lbs. average pressure per square inch on piston. So that we can now get what area of piston is required to give out this power by 47.05 X 33.000 = 450 -= 24"- diameter. 400 X 8.74 and as the area of the two pistons are to each other as 1 to 3.464, we get the area of small piston = 130 sq. in. = 13" diameter. 21 Table of the pressure, temperature, volume, and mechanical effect of steam. Mechanical Total Volume of effect of pressure steam a cubic inch of in lbs. Corresponding compared water per temperature. with evaporated, in square volume of lbs. raised 1 ft. inch. water. high. 1...... 102 9 20868 1739 2...... 126.1 10874 1812 3...... 141.0 7437 1859 4..... 152.3 5685 1895 5...... 161.4 4617 1924 6.... 169.2 3897 1948 7... 175.9 3376 1969 8...... 182.0 2983 1989 9...... 187.4 2674 2006 10...... 192.4 2426 2022 11..... 197.0 2221 2036 12...... 201.3 2050 2050' 13...... 205.3 1904 2063 14...... 209.1 1778 2074 15...... 212.8 1669 2086 16.. 216.3 1573 2097 17...... 219.6 1488 2107 18.... 222.7 1411 2117 19...... 225.6 1343 2126 20...... 228.5 1281 2135 21...... 231 2 1225 2144 2...... 233.8 1174 2152 23...... 236.3 1127 2160 24..... 238.7 1084 2168 25.... 241.06 1044 2175 26..... 243.3 1007 2182 27...... 245.5 973 2189 28...... 247.6 941 2196 29...... 249 6 911 2202 30..... 251.6 883 2209 22 Mechanical Total Volume of effect of pressure steam a cubic inch of in lbs. Corresponding compared water per temperature. with evaporated, in square volume of lbs. raised 1 ft. inch. water. high. 31...... 253.6 857 2215 32...... 255.5 833 2221 33...... 257.3 810 2226 34...... 259. 1 788 2232 35..... 260.9 767 2238 36...... 262.6 748 2243 37...... 264.3 729 2248 38...... 365.9 712 2253 39...... 367.5 695 22c9 40...... 269.1 679 2264 41...... 270.6 664 2268 42...... 272.1 649 2273 43...... 273.6 635 2278 44...... 275.0 622 2282 45...... 276.4 610 2287 46..... 277.8 598 2291 47..... 279.2 5R6 2296 48...... 280.5 575 2300 49...... 281.9 564 2304 50...... 283.2 554 2308 51...... 284.4 544 2312 52...... 285.7 534 2316 53.... 286.9 525 2320 54.... 288.1 516 2324 55...... 289.3 508 2327 56.... 290.5 500 2331 57...... 291.7 492 2335 58.. 292.9 484 2339 59...... 294.2 477 2343 60.,.... 295.6 470 2347 61...... 296.9 463 2351 62...... 298.1 456 2355 63...... 299.2 449 2359 23 Mechanical Total Volume of effect of pressure steam a cubic inch of in lbs. Corresponding compared water per temperature. with evaporated, in square volume of lbs. raised 1 ft. inch. water. high. 64...... 300.3 443 2362 65.... 301.3 437 2365 66...... 302.4 431 2369 67..... 303.4 425 2372 68..... 304.4 419 2675 69..... 305.4 414 2378 70...... 306.4 408 2382 71...... 307.4 403 2385 72..... 308.4 398 2;86 73...... 309 3 393 2391 74...... 310.3 388 2394 75...... 311.2 383 2397 76...... 312.2 379 2400 77...... 313.1 374 2403 78..... 314.0 370 2405 79...... 314.9 366 2408 80..... 315.8 362 2411 81..... 316.7 338 2414 82.... 317.6 354 2417 83...... 318.4 350 2419 84..... 319.3 346 2422 85...... 320.1 342 2425 86...... 321.0 339 2427 87...... 321.8 335 2430 88...... 322.6 332 2432 89...... 323.5 328 2435 90...... 324.3 325 2438 91...... 325.1 322 2440 92...... 325.9 319 2443 93...... 326.7 316 2445 94...... 327.5 313 2448 95...... 3282 310 2450 96...... 329.0 304 2453 24 Mechanical Total Volume of effect of pressure steam a cubic inch of in lbs. Corresponding compared water per temperature. with evaporated, in square volume of lbs. raised 1 ft. inch. water. high. 97...... 329.8 304 2455 98...... 330.5 301 2457 99...... 331.3 298 2460 100...... 332.0 295 2462 110..... 339.2 271 2486 120..... 345.8 251 2507 130...... 352.1 233 2527 140...... 357.9 218 2545 150..... 363.4 205 2561 160...... 368.7 193 2577 70...... 373.6 183 2593 180...... 78.4 174 2608 From this we can see that for a compound ertgine to exert the same power'as a single-cylinder condensing engine with the same number of expansions in both cases, the condensing cylinder of the compound engine requires to be equal in diameter to the single condensing cylinder; and from this being the case, it is quite reasonable to say, that if the power exerted can be got from one cylinder with the steam expanded a certain number of times, it 25 would be unwise to add to the expense of the engine by expanding the same number of times in two cylinders instead of one. But as the source of the power obtained is -the heat passed into the cylinder from the boiler, and as the economical working of the engine depends on the greatest quantity of this heat that can be converted into power, it is herein where the compound,engine has the advantage over any other class of engine, and we will compare the single and the compound engine from this point of view. The steam enters the single-cylinder engine at a pressure=60 lbs. per sq. in., the temperature of which, on reference to the annexed table, will be found to be equal to 295.6 deg. After doing its work it terminates with a pressure = 5 lbs.=161.4 deg. iii temperature, and consequently has'cooled down the cylinder to the same temperature, so that the fresh steam on entering to perform the next stroke can only be effective at a temperature corresponding to its pressure, and it has to part,with its heat until it brings the cylinder up 26 to its own temperature, and has consequently to be supplied with new steamfrom the boiler to do its work. Now the pressure at the beginning of th&e stroke of the high pressure cylinder of thecompound engine is the same==60 lbs. per sq. in.; but owing to the fewer number of expansions carried out in this one cylinder, it terminates this stroke with a pressure17.32 lbs. per sq. in., the temperature of which will be found on reference to be 220 deg., being a difference of only 76 deg. in — stead of 135 deg., or just about one-half.. A great part of this waste of the heat that is passed into the cylinder can be prevented by having a space round about the cyl — inder, and at both ends, filled with steam. at the boiler pressure; but this steam jacket,. as it is called, is much more effective in the compound engine than in the single cyl — inder engine, for this reason: It has been found from experiment that the rapidity with which two volumes of steam of different temperatures seek to equalize themselves. is as the square of their difference in tem — perature, that is to say —that if you mix 27 asteam of 200 deg. with steam of 100 deg., and steam of 400 deg. with steam of 100 deg., the difference of temperatures of the former being as 2 to 1, and of the latter as -4 to 1, and as 22 —4, and 42 —16; the latter -temperatures will seek to equalize them-.selves four times quicker than the former, -and, as the variation of temperature is much greater in the single cylinder than in either of the cylinders of a compound engine, the heat from the steam jacket must pass through the metal with great rapidity to -replace that wasted by condensation, and -this it cannot do so effectively as when the -temperatures are not so widely varied; and this is one of the great advantages possess-ed by the compound or double-cylinder -engine. Another feature in which the,compound engine bears favorable comparison with the single-cylinder engine is in the difference of the thrust caused by the -entrance of the steam at the beginning of each stroke, and consequently on the amount of pressure or friction thrown on the crank pin and crank shaft journals, compared with the power to be exerted. If we 28 multiply the area of piston in single cylinder, by the initial pressure, we get 452X6027,120 lbs. total pressure, or equal to a blow of fully 12 tons at the beginning of the stroke. If, in like manner, we multiply the areas of both pistons of the compound engine by their respective initial pressures we get 131X60=7,860 and 452 X17.32=7,828, which being added together, gives a total pressure at beginning of stroke when both pistons are movingsimultaneously=15,688 lbs.=about 7 tons. or little more than one-half of that in the single cylinder, and from this it can be easily seen that as a less shock is given to the working parts by about one-half, the dimensions of these parts can be made proportionately less, and a gentler, steaddier, but equally effective motion is imparted. The compound engine, both for marine and stationary purposes, has had the position of its cylinders and the combinations of its parts arranged in many different ways, in some cases to suit the space available for its erection, and in others, accord 29 ing to the different ideas of the different manufacturers; but the principle being the same in all cases, an equal economy should be got if care is taken in so proportioning the passages for the steam that no undue obstruction is caused, and that proper and efficient means are employed to prevent any waste of heat as far as possible. The principle of the compound engine was known as early as 1781, when a patent was taken out by Jonathan Hornblower for ": Employing the steam after it has acted on the first vessel to operate a second time in the other by permitting it to expand itself." But Hornblower was never able to carry out the principle to be of much practical use, owing to Watt's patents being in existence at the time. The earliest compound engine in which the principle was practically carried out was patented in 1804 by Arthur Woolf, and his style of engine has been in use almost ever since that date in France and the Continent generally, and is still constructed by many engineers in this country, and is known as "Woolf's Engine," and employed for sta 30 tionary purposes only. Both the cylinders are placed together at one end of the walking beam, the condensing cylinder being at the outer end, and the high-pressure cylinder close up to it with a proportionately less stroke. This arrangement is perfectly capable of carrying out the principle equal to any other. But the great pressure it exerts on the main centre of the walking beam, owing to all the power requiring to pass from the one end of the beam to the other, has caused it to be less extensively used than otherwise might have been the case; but more especially since 1845, when a patent was taken out by Wm. McNaught, wherein this great pressure is removed from the main centre by having the condensing cylinder only at the outer end of beam, and the high-pressure cylinder between the main centre and the crank, thus having the power equal on both sides of the main centre, and the pressure consequently merely nominal at that part. This arrangement is by far the best when the engine has to be of the walking-beam class. Another good arrangement is carried out 31 in- horizontal engines, with both cylinders lying side by side (generally cast together in one piece), and secured to a single sole plate; both piston rods are attached to one crosshead, so that one connecting rod conveys the whole power to the crank, nothing being in duplicate but the cylinders. In the compound engine at present so largely employed in the British Navy, the cylinders, outwardly, are the same diameter, but the high-pressure cylinder, as previously mentioned, is generally made about 3 less in area than the condensing cylinder. The space round the actual high-pressure cylinder being used to receive its exhaust steam until the valve of condensing cylinder opens to admit it, as the pistons do not move simultaneously, owing to the cranks being at right angles to each other. It has been explained in the first part of this work that the nominal horse power of an engine is ascertained by assuming the mean pressure on the piston to be equal to 7 lbs. on the square inch, and the speed of piston equal to 220 ft. per minute. But as 32 both the working pressure and speed of piston have been greatly increased since the above rule was first adopted, it fails to convey any adequate idea of the actual capabilities of the engine. Still, in all negotialions connected with the purchase of a steam engine, it is, as a rule, the nominal horse power alone that is referred to, although it is understood that with a pressure, say of about 60 lbs., and a piston speed of about 400 ft. per minute, fully six times the nominal power is got from a condensing engine.'As the term "Nominal Horse-Power" is only used when speaking of the steam engine as a marketable commodity, a particular size of cylinder may be called a certain nominal power by one maker, and a different nominal power by another, and unfair competition often takes place by two manufacturers offering for sale say an 80 horse-power condensing engine, one of whom means to give a cylinder 50 in. diameter, whilst the other calls a 40 in. cylinder the same nominal power. The rules now generally adopted in this country to deter 33 mine the nominal power of the different kinds of steam engines are as follows:Rule to find the nominal horse-power of a high-pressure non-condensing steam engine: Square the diameter of cylinder in inches, and divide by 12, that is to say, a non-condensing engine with a cylinder=30 in. diameter, is called a 75 horse-power engine nominal, although it is capable of giving out at least three times the power when a pressure of say 60 lbs. is employed, and piston speed-400 ft. per minute. Rule to find the nominal horse-power of a single cylinder condensing engine: Square the diameter of cylinder in inches, and divide by 24, that is to say, that a condensing steam engine with a cylinder=30 in. diameter, is called a 371 horse-power engine nominal, but is capable of working to at least six times its nominal power with 60 lbs. pressure and speed of piston=400 ft. per minute. The rule now generally adopted by marine engineers for the nominal power of a compound engine is: Add the square of the 34 diameter of each cylinder in inches together, and divide the sum by 30, that is with a compound engine whose condensing cylinder is 30 in. diameter, and high-pressure cylinder 17 in. diameter, is called a 40 horse-power compound engine nominal, and is also capable of working to at least six times that power with 60 lbs. pressure and speed of piston=400 ft. per minute. Some diagrams are herewith given, the first two of which are theoretical, and the shape that would actually be got was there no loss of heat during the stroke from condensation or other causes. In the theoretical diagram, showing the expansion curve when the steam is expanded 12 times in a single cylinder condensing engine, A B represents the total initial pressure of 60 lbs., B C the constant supply of steam from the boiler at that pressure, C the point where the steam is entirely shut off==1th part of the stroke, C D the expansion curve formed by the decreasing pressure of the steam in the ratio that the space it occupies is increased by the advance of the piston, D E represents the terminal pressure, and E A 35 the line of perfect vacuum. In the compound theoretical diagram, C D is the expansion curve formed from the high-pressure cylinder, and D A the expansion curve formed from the condensing cylinder, the line F B representing the initial pressure of 60 lbs., D E the terminal pressure in high-pressure cylinder, and initial pressurein low-pressure cylinder, and equal to 17.32; lbs., and F A the terminal pressure in lowpressure cylinder, and equal to 5 lbs. A Theoretical Diagram showing the Exparision Curve in a Single Cylinder Condensing Engine, with,Steam at 60 lbs. pressure above a perfect vacuum, expanded 12 times. c B 45 40 35 30 25 20 15 ATMO0SPHERIC LINE / - 5 5 ED~~~~~~~~ A.~~10 [F —-~~~..... ~~~~~~~~15 E A Theoretical Diagrams showing the Expansive Curve in both Cylinders of a Compound Engine, oith Steam at 60 lbs. pressure above a perfect vacuum. Total number of expaneions = 12. CB 45 40 85 30 25 20 * 15 10 ATMOSPHERIC LINE 0 5 A 10 -- -,...... 15 E F -Gn rRESSu:RE. —Diagrams taken from a Compovnd Engine with Steam cut-off after the Piston has travelled one-eighth of the stroke in high-pressure Cylinder. ScATMOSP HERIC L1 lbNE Scale-l-'6 inch-1 lb. Low PREssURE. — Diagrams taken from a Compound Engine with Steam cut-off after the Piaton has travelled one-~ighth of the stroke in high-pressure Cylinder. ATMOSPHERIC LINE Scale -1-8th inch = 1 lb. HIGH PRESSURE. —Diagrams from the Compound Engine of the S. S. 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Io oo 12 i). VAN.NOSTRANI)'S P;1';1,ICA )NSS.. SI13/iMS. A Treatise on the Principles and Practice of Levelling, showing its application to purposes of Railway Engineering and the Construction of Roads, &c. By Frederick W. Simms. C. E. From the 5th London edition, revised and corrected, with the addition of Mr. Laws's Practical Examples for setting out Railway Curves. Illustrated with three Lithographic plates and numerous wood cuts. 8vo, cloth. $5 so BURT. Key to the Solar Corfipass, and Surveyor's Companion; comprising all the rules necessary for use in the field; also description of the Linear Surveys and Public Land System of the United States, Notes on the B3arometer, suggestions for an outfit for a survey of four months, etc By XV. A. Kurt, U. S. Deputy Surveyor. Second edition. l'ocket book form, tuck........................... 2 50 THE PLANE TABLIE. Its uses ill Topographical Surveying, front the Papers of the IU. S. 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A Practical Treatise on Pure Fertilizers, and the chemical conversipn of Rock Guano, Marlstones, Coprolites and the Crude Phosphates of Lime and Alumina generally, inte various valuable products. By Campbell Morfit, AM.D., with 28 illustrative plates, 8vo, cloth..................................... 20 Oa, BARNARD. The Metric System of Weights and Mleasures. An address delivered before the convocation ot the University of the State of New York, at Albany, August, 187i. By F. A P. Barnard, LL.D., President of Columbia College, New York. Second edition from the revised edition, printed for the Trustees of Columbia College. Tinted paper, 8vo, cloth 3 oo Report on Machinery and Processes on the Industrial Arts and Apparatus of the Exact Sciences. By F. A. 1'. Barnard, LL.D. Paris Universal Exposition, i867. Illustrated, 8vo, cloth.............. o00 BARLOW. Tables of Squares, Cubes, Square Roots, Cube Roots, Reciprocals of all integer numbers up tm io,ooo. New edition, I2mo, cloth................. 2 o I4 D. VAN NOSTRAND'S PUBLICATIONS. MYER. Manual of Signals, for the use of Signal officers in the Field, and for Military and Naval Students, Military Schools, etc. A new edition enlarged and illustrated By Brig. General Albert J. Myer, Chief Signal Officer of the army, Colonel of the Signal Corps during the War of the Rebellion. 12mo, 48 plates, full Roan................................. $ oo WILLIAISUON. Practical Tables in Meteorology and Hypsometry, in connection with the use of the Barometer. By Col. R. S. Williamson, U. S. A. 4to, cloth............................................ 2 50 THE YOUNG MECHANIC. Containing directions for the use of all kinds of tools and fbr the construction of Steam Engines and Mechanical Models, including the Art of Turning in Wood and Metal. By the author "The Lathe and its Uses," etc. From the English edition with corrections. Illustrated, I2mo, cloth............................... I 75 PICKERT AND METCALF. The Art of Graining, How Acquired and How Produced, with description of colors, and their application. By Charles Pickert and Abraham Metcalf: Beautifully illustrated with 42 tinted plates of the various woods used in interior finishing. Tinted paper, 4to, cloth................ Io oo HUNT. Designs for the Gateways of the Southern Entrances to the Central Park. By Richard M. Hunt. With a description of the designs. 4to. cloth...... 5 oo LAZELLE. One Law in Nature. By Capt. H. M. Lazelle, U. S. A. A new Corpuscular Theory, comprehending Unity of Force, Identity of Matter, and its Multiple Atom Constitution, applied to the Physical Affections or Modes of Energy. I2mo, cloth... I 50 PETERS. Notes on the Origin, Nature, Prevention, and Treatment of Asiatic Cholera. By John C. Peters, M. D. Second edition, with an Appendix. x2mo, cloth................... 50 I5 ID. V'sAN NOSTRIAND'S PIUBLICATIONS. BOYNTON. History of West Point, its Military Importance during the American Revolution. and the Origin and History of the U. S. Military Academy. By Bvt. Miajor C. E. Boynton, A.M., Adjutant of the Military Academy. Second edition, 416 pp. 8vo, printed on tinted paper, beautifully illustrated with 36 maps and fine engravings, chiefly from photographs taken on the spot by the author. Extra cloth.......................................... $3 50 WOOD. West Point Scrap Book, being a collection of Legends, Stories, Songs, etc., of the U S. Military Academy. B;y Iieut. O E. Wood, U. S. A. Illustrated by 69 engravings and a copperplate map. Beautifuilly printed on tinted paper. 8vo, cloth..... 5 no WEST POINT LIFE. A Poem read before the Dialectic Society of the United States Military Academy. Illustrated with Pen-and-Ink Sketches. Bv a Cadet. To which is added the song,' Benny Havens, oh 1" oblong 8vo, 21 full page illustrations, cloth.......... 2 50 GUIDE TO WEST POINT and the U. S. 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