[62 History qf Mechanical Inventions, and 



of a bushel of coal would increase the temperature of a pound of water, 

 provided it could be retained in the liquid state, and without increasing its 

 capacity. But 588878°,36 =M813 cubic feet of water converted int0 



960 X 62,6 

 steam, by the combustion of one bushel of coal- * This is much below the re- 

 sult obtained by Mr Grose at Huel Towan. Mr Dalton's experiments, 

 which we have assumed as data in the preceding calculation, were conducted 

 in such a manner as to give the lowest possible results ; the ignited matter 

 under experiment being placed in contact with the vessel containing the 

 ice, without the application of any contrivance to prevent the radiation or 

 conduction of heat through the surrounding medium. The vessel itself 

 was of tinned iron, and it is to be presumed its surface was bright or polish- 

 ed. Certainly the quantity of heat absorbed by it must have been very 

 small in comparison with the whole given off* by the combustible ; bright 

 metal being known to absorb a very small portion of the heat impinging 

 against it. But one portion of the heated air must have been displaced by 

 its successor, before it could possibly have parted with its superabundance 

 to the vessel. Much of the difference between Mr Dalton's* experiments 

 and Mr Grose's results may thus be accounted for. 



In Mr Watt's time, the evaporation was from 8 to 12 cubic feet of water 

 with each bushel of coal ; and since that period some highly important 

 improvements have been made in the construction of steam boilers. 



3. On the Steam Case. 



The cylinders of steam engines are frequently surrounded with cases, 

 containing steam of the same elasticity as that in the boiler. The water 

 proceeding from the condensation which takes place in the case was for a 

 considerable time allowed to pass off without further use ; — but it has 

 subsequently been the practice to return it to the boiler. 



The object of the case was to prevent condensation taking place within 

 the cylinder. But we shall see that the application of heat in this manner 

 is by no means advantageous. As far as experiments hitherto published 

 afford us a clue, we will endeavour to investigate it in the case of the 

 engine at Huel Towan. 



MM. De la Roche and Berard found that the capacities of equal volumes 

 of air under the pressures of 29.2 and 41.7 inches of mercury were nearly 

 as 1 : 1,2396 differing from the ratio of the pressures or densities which 

 is 1 : 1,358. 



If we suppose the densities and capacities to be in this ratio to each other ; 

 then under a pressure of 24.9 inches of mercury .8983. 



29,2 1,000 



39,6 1,215 



71,15 i j9 i 



* I am aware of having omitted to notice the ammonia and tar, but the former 

 as probably not decomposed, and the quantity of heat from the second greater than 

 an equal weight of carbon would have given, is fully required to volatilize the other 

 combustible substances, which assume the elastic state. 



3 



