228 Report on the Progress and Slate of Applied Mechanics. 



of the energy exerted by the engine on the machinery which it drives, 

 to the work performed by the steam on the piston. 



83. The EFFICIENCY or the fues^ace and boileu has from time 

 to time been improved by a great number of inventors, sometimes by 

 contrivances for insm'ing a thorough combustion of the fuel, by means 

 of a regular supply of air, neither too much nor too little, and some- 

 times by giving a greater extent or a more favourable form to the heat- 

 receiving surface of the boiler. Inventions for improving the combustion 

 of fuel are so numerous, and involve so many controverted points, 

 that we cannot venture to enter upon their details. An important 

 .series of experiments on the evaporating powers of the coal of the North 

 of England has just been published by a committee who have for 

 some time been at work. It is very desirable that similar inquiries 

 should be carried out with reference to other coal-fields. 



84. The production of heat from mechanical power, or of mechanical 

 power from heat, depends on the Law of Joule, that heat requires for 

 its production, and produces by its disappearance, mechanical energy 

 in the proportion of 772 foot-pounds for so much heat as raises the 

 temperature of one pound of water by one degree of Fahrenheit's scale. 

 Some such law as this was anticipated by all who considered heat to be 

 a state and not a substance, but its exact numerical determination, 

 though approximated to by Seguin and Mayer, was accomplished by 

 Dr. Joule. 



85. The efficiency of the elastic vehicle in a heat-engine is 

 regulated by a law which is a case of the general law of the transformation 

 of energy, and which is as follows : — That the proportion of the heat con- 

 verted into mechanical energy/, to the whole heat received hy the elastic 

 svhstaiice, is that of the difference between tlie absolute temperatures at 

 ichich it alternately receives and gives out heat to tJie absolute temperat^ire 

 at ivhich it receives heat. 



86. The discovery of this law in its general and exact form is of 

 recent date ; but in its special application to steam, Watt knew its 

 practical effect sufficiently to be aware, that the efficiency of the steam 

 was promoted by keeping the temperature at which it enters the cylinder 

 as high, and the temperature at which it is condensed as low, as is con- 

 sistent with the circumstances of the case ; and this is the principle of 

 his separate condenser, his clothing for the cylinder, and his expansive 

 working, to lower the temperature of the steam by exerting energy 

 against the piston, and not by mere abstraction of heat by means of 

 additional water of condensation. 



87. All engines in which saturated steam is used are in fact Watt's 

 engine, modified and improved in the details. It is chiefly hy expansive 



