ber of teeth in the driver, so is the velocity of the driver t'i the number of teeth in the driven. Note.—To find the proportion that the velocitics of the wheels in a train should bear to one another, subtract the less velocity from the greater, and divide the remainder by the number of one less than the wheels in the train; the quotient will be the number rising in arithmetical progression, from the least to the greatest velocity of the train of wheels. EXAMPLE I. What is the number of teeth in each of three wheels to produce 17 revolutions per minute, the driver having 107 teeth, and making three revolutions per minute ? 17—3=14 =7, therefore 3 10 17 are the velocities of the three wheels. 107x3 10 : 107 :: 3:32= =32 teeth. By the rule 32 +10 17 : 32 :: 10:19= =19 teeth. 10 17 EXAMPLE II. : : What is the number of teeth in each of 7 wheels, to produce one revolution per minute, the driver having 25 teeth, and making 56 revolutions per minute ? 56—1=55 =9, therefore 56 46 37 28 19 10 1, are the 7—1=6 progressional velocities. 46 30 Teeth. 37 49 19 49 28 72 10 72 19 137 1 10 1370 It will be observed that the last wheel, in the foregoing example, is of a size too great for application; to obviate this difficulty, which frequently arises in this kind of train : : :: : : : : : 137 : ing, wheels and pinions are used, which give a great command of velocity.-Suppose the velocities of last example, and the train only of 2 wheels and 2 pinions. 56-1=55 =18, therefore 56 19 1, are the progres4-1=3 sional velocities. 10 : 25 :: 56 : 74 = teeth in the wheel driven by the first driver, and 1:10 :: 19 : 190 = teeth, in the second driven wheel, 10 teeth being in the driving pinion. 25 drivers 74 driven. 10– 190– STEAM ENGINE. BOILERS—are of various forms, but the most general is proportioned as follows, viz. width 1, depth 1.1, length 2.5; their capacity being, for the most part, two horse more than the power of the engine for which they are intended. Boulton and Watt allow 25 cubic feet of space for each horse power; some of the other engineers allow 5 feet of surface of water. STEAM-arising from water at the boiling point, is equal to the pressure of the atmosphere, which is, in round numbers, 15 lbs. on the square inch; but to allow for a constant and uniform supply of steam to the engine, the safety valve of the boiler is loaded with three lbs. on each square inch. The following table exhibits the expansive force of steam, expressing the degrees of heat at each lb. of pressure on the safety valve. 24 25 48 49 26 50 212° 216 219 222 225 229 232 234 236 239 241 244 246 248 250 252 254 256 258 260 261 263 265 267 0 268° 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 2989 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 44 45 46 47 23 By the following rule the quantity of steam required to raise a given quantity of water to any given temperature is found. RULE.-Multiply the water to be warmed by the difference of temperature between the cold water and that to wbich it is to be raised, for a dividend, then to the temperature of the steam add 900 degrees, and from that sum take the required temperature of the water: this last remainder being made a divisor to the above dividend, the quotient will be the quantity of steam in the same terms as the water. EXAMPLE What quantity of steam at 212° will raise 100 gallons of water of 60° up to 212° ? 212°—60° x 100 = 17 gallons of water formed into 212° + 900°—212 steam. Now steam, at the temperature of 212°, is 1800 times its bulk in water; or 1 cubic foot of steam, when its elasticity is equal to 30 inches of mercury, contains 1 cubic inch of water. Therefore, 17 gallons of water converted into steam, occupies a space of 4090} cubic feet, having a pressure of 15 lbs. on the square inch. In boiling by steam, using a jacket instead of blowing the steam into the water, about 10.5 square feet of surface are allowed for each horse capacity of boiler; that is, a 14 horse boiler will boil water in a pan set in a jacket, exposing a surface of 10.5 x 14=147 square feet. HORSE POWER.-Boulton and Watt suppose a horse able to raise 32,000 lbs. avoirdupois 1 foot high in a minute. Desaguliers makes it 27,500 lbs. Smeaton do. 22,916 do. It is common in calculating the power of engines, to suppose a horse to draw 200 lbs. at the rate of 2] miles in an hour, or 220 feet per minute, with a continuance, drawing the weight over a pulley—now, 200 x 220=44000, i. e. 44000 lbs. at 1 foot per minute, or 1 lb. at 44000 feet çer minute. In the following table 32,000 is used. One borse power is equal to raise gallops or lbs. feet high per minute. Feet high Ale Lbs. Ale Lbs. Avoirdupois. per Minute. Gallong. Avoirdupois. per Minute. 20 25 30 35 40 45 50 55 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 3123 15611 1041 780 624 520 446 390 347 312 284 260 240 223 208 195 183 173 164 32000 156 60 65 70 75 80 85 90 95 100 110 120 1600 1280 1066 914 800 711 640 582 533 492 457 426 400 376 355 337 320 291 267 31 28 18 19 26 LENGTH OF STROKE.—The stroke of an engine is equal to one revolution of the crank shaft, therefore double the length of the cylinder. When stating the length of stroke, the length of cylinder is only given, that is, an engine with a 3 feet stroke, has its cylinder 3 feet long, besides an allowance for the piston. The following table shows the length of stroke, (or length of cylinder,) and the number of feet the piston travels in a minute, according to the number of strokes the engine makes when working at a maximum. When calculating the power of engines, the feet per minute are generally taken at 220. |