146 OF THE GENERATION AND [SECT. in. 



of coal per hour, for each horse power, of an engine above 10 horse power. But 

 where 14 or 16 Ibs. of coal per hour is necessary, the flue should be increased 

 in direct proportion to the quantities to be consumed. See the mode of finding 

 the rule in art. 168. 



276. When wood is used for fuel, it affords a much larger quantity of smoke, 

 but it is also much lighter, and about one and a half times the area necessary for 

 coals will be sufficient. 



277. The same rules may be applied to high pressure engines ; taking the 

 cubic feet of water per hour, or the one-eleventh part of the pounds of coal per 

 hour, instead of the number of horse power. 



278. The engine chimneys for steam boats and steam carriages are circular, 

 and should not be larger than is absolutely required to give effect to the fuel. 

 This will be about obtained when the square of the diameter is equal to 90 

 multiplied by the horse power, and divided by the square root of the height 

 in feet. 



But here it must be remarked, that where a chimney is less than about 40 or 50 

 feet in height, the smoke must be allowed to rise at a much higher temperature. 

 It must not therefore be allowed to cool too much by giving its heat to the boiler, 

 otherwise there will be a want of draught. Hence in low chimneys the fuel will 

 not produce its full effect. 



Different modes of finishing chimney tops are shown in Plate i. The least 

 expensive is one of the form of an Egyptian obelisk, and it offers least obstruction 

 to the wind. 



OF THE CONDENSATION OF STEAM. 



279. When any substance or body colder than steam itself is put in contact 

 with it, the steam condenses till the temperature of the cold body becomes the 

 same as that of the steam, or till the whole mass of steam be condensed to a 

 degree of elasticity corresponding to the temperature to which the cold body is 

 raised by the heat of the steam. The greater the quantity of the cold body, the less 

 its temperature will be raised ; and also the colder it is, the more the elastic force 

 will be reduced. Hence, to reduce the elastic force of steam as low as possible, the 

 coldness and the quantity of the cooling body should be as great as possible. 



280. Any cold body condenses steam ; but that it may be effectively done, the 

 body should be capable of presenting a large quantity of surface, and be a good 

 conductor of heat ; as when power is to be obtained by condensation, the more 

 rapid the condensation is, the more power is obtained. It may be easily proved 

 that if steam were so condensed as to lose only equal degrees of elastic force in 

 equal times during the action, half the power would be lost. (See art. 294.) 



