164 History of Mechanical Inventions, and 



difficult parts of the theory of steam engines, and at the same time one of 

 the most interesting. 



I have before observed, that the resistance being the same, the quantities 

 of water and caloric are in ordinary cases equal, whatever be the dimen- 

 sions of the piston. It is of course understood that I mean when acting 

 against a vacuum. The loss of heat by the conducting power of the air, and 

 by radiation, may be in a great measure prevented by the application of 

 thick coatings of non-conducting matter. But there is yet another cause, 

 which, although of importance, seems to have been hitherto overlooked ; — 

 at all events it has not been submitted to calculation. 1 allude to the large- 

 er quantity of heat which must of necessity pass to the chimney, when the 

 steam is of great elastic force, than when only a little above the atmo- 

 spheric pressure ; — supposing it in both cases to pass off when at the same 

 temperature as the steam in the boiler, below which point it is obvious it 

 cannot be. * 



In a former part of our investigation we have seen that the quantity of 

 hydrogen from a bushel of coal is 1.7237 lbs. which by combustion yields 

 of aqueous vapour, - 14,5133 lbs. 



The oxygen required for combustion being derived from the 

 atmosphere, the quantity of azote with which it was 

 mixed, and is also heated, is, - - 55,1584 



75,5 lbs. of coke by combustion gives carbonic acid, 276,82 



The azote mixed with the oxygen here combining, 805,28 



3,2206885 lbs of carburetted hydrogen by combustion is con- 

 verted into carbonic acid, - 8,8568933 



-aqueous vapour, - - 7,2465491 



The azote mixed with the oxygen entering into combina- 

 tion is, - - - - ' 51,529116 

 3,2206885 lbs. of carbonic oxide by combustion become 



carbonic acid, - - - 4,9610777 



and bringing azote, ... 6,9615568 



The whole weight of aqueous vapour being, 21,759849 lbs. 

 h carbonic acid, - - 290,637971 



-azote, - - - 918,929072 



But assuming as data for our calculation the before-mentioned experi- 

 ments of De la Roche and Berard, we obtain the following quantities re- 

 presenting the pressures and capacities : — 



* Under ordinary circumstances, it seems probable that the air passing into the 

 chimney is of a temperature much above that of the steam in the boiler. Mr Sims 

 informs me that at one of the engines in Huel Vor Mine, the air in the chimney 

 was 318° — The steam at that time was probably not above 270°, and the dampei 

 open about half way. 



