848 Transactions of the American Institute. ^ 



degrees, and multiply the quotient b}'^ the total number of pounds of 

 water evaporated at the observed pressure ; the result will be the total 

 evaporation from our supposed temperature of 212 degrees, and at 

 atmospheric pressure. The latter divided by the total amount of 

 coal burned, or, if desired, by the combustible, gives the final result, 

 in the usual comparative terms, viz.: The mimher of lyounds of toater 

 evaporated i)er pound of coal (or combustilde). The coal may be 

 corrected to a uniform rate of ten per cent refuse, as has been before 

 explained. 



We have reason to suppose that in many experiments abroad, the 

 ashes were " weighed back," and credited on the coal account ; in 

 other words, that what is reported as coal was really only the com- 

 bustible portion thereof. In purchasing coal, we pay as much for 

 the ashes as for the combustible, and ships must carry both, in a 

 combined state; tlierefore the report of every experiment should 

 clearly state what is meant by the word coal, if that be the term 

 employed, whether the weight of the coal as actually purchased, that 

 of its combustible, or a weight proportioned to the combustible, on 

 our plan of correcting to a standard of ten per cent refuse. 



TestixCt Engines. 

 We will examine, first, a.sim])le mode, wliich may be practically 

 applied, in any case, to test the econom}"^ of steam machinery, in the 

 actual performance of its regular duty. In ordinary trials, when but 

 little care and expense can be afforded, the engine and boilers must be 

 tested as a whole, the comparison being made by the " riumhei' of 

 pounds of coal consumed per indicated Tiorse-power per hour!''' The 

 indicator is used to measure the power, because, as has been before 

 explained, it is the simplest device we have for this purpose, and most 

 generally applicable. AVe first desire to give, from our experience, 

 some directions about the use of the indicator, and the manner of 

 attaching it to the engine. Since the invention of the " Richards " 

 or " Porter indicator," the direct-acting instrument known as the 

 " McNaught indicator," has fallen into disuse, except on engines 

 working very slowly. We will make our remarks more especially 

 applicable, then, to the first instrument, often called the " parallel 

 motion indicator." Before using the instrument, see that it is cor- 

 rectly made and in good order. See that the spring screws down 

 squarely on the piston, and does not tend to one side, and thus make 

 friction in the guide of the piston rod. Examine every joint, and see 



