48 Mr Kenwood's account of the 



Let i ■=. pressure of steam from water in hot well. 



"' ^ ' ^J -\-h.i — e.i = effective resistance opposed to 

 ce 



the operation of the engine, by atmospheric pressure on air-pump, 



imperfection of vacuum, &c. 



Tiiis leads us to a different conclusion from that at which Mr 

 Watt arrived, and shows us that the temperature of the injec- 

 tion water directly determines the temperature at which the re- 

 sistance is a minimum. The whole of the preceding investiga- 

 tion has been conducted on the idea that the steam whilst in the 

 cylinder absorbed no heat from the steam case, provided one be 

 used. But we have no correct data for calculating the increase 

 which must thus obtain. On the other hand, the higher the tem- 

 perature of the hot well, the less the quantity thus abstracted 

 from the case ; so that in practice the water in the hot well may, 

 with economy, be worked at between 95° and 100°. It may not 

 be altogether out of place to remark, that Mr Grose found his 

 engine at Huel Hope performed rather better when working J 

 than when at | expansive. The water passing into the con- 

 densing cistern may with economy be first passed over the 

 eduction pipe. The saw-dust placed around the cylinder and 

 steam-pipes is quickly charred, and, if not frequently removed, 

 will act on the iron, especially if it be not quite free from mois- 

 ture. It has been well observed that the only improvement 

 in the steam-engine since that of Mr Watt is in the dimen- 

 sions of the valves. At Huel Towan the valve which admits the 

 steam into the cylinder is 8 inches in diameter ; the equilibrium 

 valve 12 ; the exhausting valve 16. As the steam is usually 

 worked at about the pressure of 20 ft)S. on the square inch, the 

 weight on valves of such dimensions is very considerable. Many 

 contrivances have been made for obviating this inconvenience, 

 but the best yet invented is that of Mr Hornblower's, called 

 the skeleton valve, and described in Gregory's Mechanics. But 

 there is another recently invented by Mr Sims, a Cornish en- 

 jrineer, and extensively used by him in large engines. In 

 Plate I. Fig. 1, a a a^ a a, a" a!' is the seat, which at a a a 

 is solid. At & a apertures are cut in its sides for the pas- 

 sage of the steam ; and at a" a" is the beat, into which it is 

 ground with emery. The valve 6 6 is a plain cylinder, bored 



