of the most effective Condensation of Steam. 61 



Similar remarks from the same respectable authority occur 

 in the Proceedings of the British Association at the Glasgow 

 Meeting (p. 186). I fully concur in the statement that a 

 vacuum may be made so good as to be a drawback on the 

 performance of the engine in which it is obtained, but I beg 

 respectfully to represent that this, instead of being now a 

 novelty, has been long known and acted on. 



Mr. Farey (Treatise on the Steam-engine, p. 375) says, 

 " Mr. Watt ascertained that if the temperature of the steam 

 and water contained in the condenser of his engines is re- 

 duced to 100 degrees, then as the steam remaining in the 

 exhausted space will only have an elastic force equal to a 

 pressure of not quite one pound on the square inch, it is better 

 to leave this weak steam always in the condenser, and also 

 in the cylinder, to oppose the descent of the piston, than to 

 throw in any more injection water, which would be requisite 

 to cool it more, and render it more rare and feeble." 



So long ago as the year 1828, I had remarked that the 

 duty of Wilson's engine at Huel Towan, was improved bv 

 raising the temperature of the hot well within certain limits : 

 and I then calculated, for intervals of five degrees extending 

 from 80° to 100°, the resistance which the residual vapour 

 would oppose to the descent of the piston, the accelerating 

 influence it would exercise on the ascent of the air-pump 

 bucket, and also the greater or less atmospheric action to 

 which that bucket was exposed by the period of discharging 

 the hot water being lengthened or shortened. 



The results of these computations appeared in the Edin- 

 burgh Journal of Science for 1829 (x. p. 41, O. S.), and I 

 beg permission to quote the remarks by which they were ac- 

 companied. 



" The quantity of water (to be discharged by the air- 

 pump) should be as small as possible, not so much on ac- 

 count of its weight, as of the greater period during which the 

 piston of the air-pump will be exposed to the atmospheric 

 pressure. On the other hand, the smaller the quantity of 

 water injected, the higher will be the temperature of the hot- 

 well, and consequently the less perfect the vacuum. It is ob- 

 vious that the smaller the quantity obtained by adding the dif- 

 ference between the impeding influence of the steam remaining 

 in the hot-well on the piston, and its accelerating action on 

 the air-pump, to the whole resistance experienced by the latter 

 during its exposure to the atmosphere, the better will be the 

 operation of the machine." 



I have also mentioned the same subject in your valuable 

 pages (Third Series, xir. p. 491), and in the Transactions of 



