1825.] Advantages of High Pressure Steam, 433 



From these premises it follows, that the force of steam is 

 directly as its density, multiplied by -ifi for each degree of 

 increased temperature, the caloric corresponding with the density 

 alone. 



For instance : steam at 212° has an elastic force = 30 inches 

 of mercury; and at 300° = nearly 140 inches, neglecting frac- 

 tions. 



By the second law, steam of the density due to 212° raised 88° 

 with a geometrical increase of 4-|i for each degree, shall gain 

 about b'Q inches ; or possess at 300° a force = 35*6 inches of 

 mercury. 



And by the third, the density due to 300° shall be as 35*6 

 inches to the force found = 140 inches, or about 3*9 times 

 greater than at 212°. 



But the caloric being constant is in simple proportion to this 

 density ; and the fuel consumed must be expected to correspond 

 with the caloric. 



Then 30 inches x 39 = 117 inches, the force due to the 

 density at 300°, deducted from 140 inches, the force found by 

 experiment, gives 23 inches, the profit by working at 300°. 



If this example be just, the weight of steam employed having 

 its caloric constant, shall be a measure of the fuel consumed ; 

 and there is a direct profit in the ratio of -f^fi for each ascending 

 degree of Fahrenheit, as above stated. 



It is plain from the nature of geometrical progression, that this 

 profit shall increase as the temperature is more elevated: if we 

 work at COO°the force of each powwc? of steam shall be double of 

 ^hat at 212° ; and if we go up to 960° or 980°, it shall be quad- 

 ruple ; the caloric, and consequently the fuel, remaining a con- 

 stant quantity. 



It is easy to illustrate this from the reports of working engines, 

 but the effects in these cases are dependent on such mixed 

 causes, that no uniform conclusion can be drawn from them. 

 Can you refer me to Watt's experiments on the density of 

 steam ? 



In taking the caloric contained in the steam as a measure of 

 the fuel consumed, there is not exact precision : radiation will 

 of course increase with temperature ; but I thought this might 

 probably be more than compensated by the diminished surface 

 of the vessels ; and that, in the rapid action of a steam engine, 

 it could hardly make an appreciable difference. 



A collateral advantage of not less importance, well known to 

 engineers, and which did not escape the sagacity of Mr. Watt, 

 is gained, in allow^ing high pressure steam to expand in the 

 cylinder. Mr. Watt has given a formula for calculating the 

 profit on this proceeding ; but for a much more perspicuous 

 demonstration, I am indebted to a conversation with Mr. 

 Perkins. 



New Series, vol. x. 2 f 



