858 Transactions of tee American Institute. 



Newcomen used steam of barely more than atmospheric pressure, 

 and raised 105,000 pounds of water one foot high with a pound ot 

 coal consumed. Smeaton raised the pressure somewhat, and increased 

 the duty considerably ; Watt started with a duty double that of New- 

 comen and raised it to 320,000 foot-pounds per pound of coal, with 

 steam at ten pounds pressure. 



To-day, Cornish engines of the same general plan as those of Watt, 

 but worked with forty to sixty pounds of steam and expanding three 

 or four times, do a duty probably averaging, with the better class of 

 engines, 600,000 foot-pounds per pound of coal. The compound 

 pumping engine runs the figure up to about 1,000,000. 



The increase in steam pressure and in expansion since Watt's time 

 have been accompanied by a very great improvement in workman- 

 ship ; a consequence, very largely, of the rapid increase in perfection, 

 and in the wide range of adaptation of machine tools, by higher skill 

 and intelligence in designing engines and boilers, by increased piston 

 speed, greater care in obtaining dry steam, and in keeping it dry, 

 until thrown out of the cylinder, either by steam jacketing or by 

 •superheating, or both combined ; and it has further been accompanied 

 by greater attention to the important matter of providing carefully 

 against losses by radiation and-jconduction of heat. 



The use, finally, of the compound or double-cylinder engine, for 

 the purpose of saving some of the heat usually lost in consequence of 

 condensation from great expansion, has already been alluded to when 

 treating of the marine engine. 



It is evident that, although there is a limit, tolerably well defined, 

 in the scale of temperature, below which we cannot expect to pass, a 

 degree gained in approaching this lower limit is more remunerative 

 than a degree gained in the range of temperature available by 

 increasing temperatures.* 



Hence, the attempt made by the French inventor, Dr. Trembly, 

 twenty-five years ago, and by other inventors since, to utilize a larger 

 proportion of heat by approaching more closely the lower limit, was 

 in accordance with known scientific principles. The form of engine 



* The fact here referred to is easily seen, if it is supposed that an engine is supplied 

 with steam at a temperature of four hundred degrees above absolute zero, and works 

 it, without waste, down to a temperature of two hundred degrees. Suppose one 

 inventor to adapt the engine to the use of steam of a range from five hundred down 

 to two hundred degrees, while another works his engine, with equally effective pro- 

 vision against losses, between the limits of four hundred and one hundred degrees — 

 an equal range with a lower mean. The first case gives an efficiency of one-half, the 

 second three-fifths, and the third three-fourths, the latter giving the highest effect. 



