Proceedings of the P olytechntc Association. 839 



construction. The majority of the practical loss has, however, never 

 been satisfactorily explained. The "vvi'iter, like others, has his own 

 theories on the subject, but he has no desire to present them publicly 

 till they have been tested ; for if they be correct, the principal 

 difficulties can be removed. Few appreciate the extent of the losses 

 in the steam engine. It is only the best examples that utilize even 

 one-tenth of the heat. In such cases, one-tenth is condensed for the 

 work, and. about four-tenths is wasted in the clearances and. the 

 exhausting steam, even when expansion is carried on, until the 

 terminal equals the back pressure. The remaining five-tenths are 

 imperfectly accounted for. Cases are not unfrequent where only 

 three to five per cent of the heat taken from the boiler is utilized in 

 work. The discrepancies occur chiefly at the higher grades of expan- 

 sion. Without expansion, it is easy to understand that most of the 

 heat must go away with the exhaust. 



When steam is generated by the application of heat in the boiler, 

 to water only, the water, in becoming steam, always takes up a 

 certain fixed quantity of heat ; in other words, becomes saturated 

 with if, and forms saturated steam. Hence, if we can measure the 

 water evaporated, to produce the power of an engine, we can easily 

 estimate the quantity of heat used. The feed water is therefore a 

 perfect measure of the comparative cost of the power, when evapo- 

 rated in a good boiler, having no superheating surface. The economy 

 of steam machinery is, however, generally measured by the amount 

 of coal or other fuel consumed to perform a certain quantity of work. 

 The conventional standard of comparison between all kinds of engines 

 is, the number of pounds of coal burned per indicated horse-power 

 per hour. The indicated power can be obtained with comparative 

 ease, as has been explained ; so also can the -coal per hour. Hence 

 the above standard has the merit of great simplicity, and consequently 

 is used by all nations. We must therefore adopt it, or at least use it, 

 in order to be able to compare our results with those of others ; still 

 the method is liable to very considerable errors, which we will 

 examine with a view of correcting them. 



It has been shown that the indicated power is not always propor- 

 tioned to the useful work. The qualities of coal var}^ so much, also, 

 in different localities, that the amount consumed does not furnish an 

 accurate comparative measure of the cost of the power. When the coal 

 measure alone is used, too, the engines and boilers are both tested 

 together, which gives no opportunity to ascertain which of the two is 



