Steam-Engines in Cornwall. 41 



rienced by the latter during its exposure to the atmosphere, 

 the better will be the operation of the machine. Mr Watt 

 thought 102^ to 110° to be the temperature of his hot well 

 when the engine performed the best duty. 



The following table will exhibit an approximation to the 

 resistance that is opposed by the air-pump at Huel Towan at 

 various temperatures of the hot well. 



It appears that about ^^ of the first quantity in the first co- 

 lumn should be added to it and to each of the succeeding num- 

 bers in that column, the quantity of water arising from the 

 steam itself being omitted. Mr Watt's idea seems to have been, 

 that, whatever might have been the temperature of the injected 

 water, that of the hot well should be invariable. 



Let a — latent heat (960°) of vapour under ordinary circumstances. 

 b z= diiFerence between temp, of injected water and 212°. 

 c = diiFerence between temp, of hot well and injection. 

 d = quantity of water in steam used at each stroke. 

 e = area of air-pump piston. 



./= total pressure of atmosphere on air-pump in lbs. 

 h ■=. area of steam-piston. 



