60 Mr. Henwood on the Temperature 



as is the end of the working piston, so that when the latter is 

 forced down, there is no residuum of air left in the barrel at 

 the end of the stroke, between the pistons. There is also a 

 spring to keep the pistons in close connexion till the upper 

 one has returned a little past the openings, by which the air 

 is forced out, a little above which openings the lower plug is 

 retained by a stay coming against the end of the cylinder. 



It has been objected that, though the model works well at 

 first, the metallic piston will soon wear, and then the instru- 

 ment will become inefficient. This may be obviated, I con- 

 ceive, by making the apertures very small, and by using pis- 

 tons packed or covered with some elastic material, as India- 

 rubber, which would easily pass and repass the openings 

 without tearing or injuring the packing. But in order to 

 dispense with the apertures, and to avoid any objection that 

 might be taken to the use of elastic packing on account of 

 them, it has occurred to me, that the upper and lower ends 

 of the cylinder might be made a little wider than the rest of 

 the barrel, for the inlet and escape of the air. In this case, 

 however, the piston rods would require longer collars to serve 

 as guides, that the pistons might come truly into that part 

 where they fit tight to the cylinder. 



I am, Gentlemen, your obedient servant, 



J. Park. 



Ulverstone, Lancashire, March 13, 1841. 



XI. On Mr. J. Scott Russell's Remarks on the Temperature 

 of most effective Condensation (of Steam). By W.J. Hen- 

 wood, Esq., C.E., F.R.S., F.G.S., Secretary of the Royal 

 Geological Society of Cornwall, fyc. 



To Richard Taylor, Esq., F.L.S., fyc. 

 Sir, 



ri^HE article " Steam Navigation" in the Encyclopaedia 

 A Britannica (xx. p. 697), from the able hand of Mr. J. Scott 

 Russell, contains the following remark : — 



" It does not appear to be known that a vacuum may be 

 too good. We hear it boasted every day by rival engineers 

 that their engines have the best vacuum. Some boast their 

 vacuum at 27 inches, others at 28, others at 29, some at 30, 

 and at last an engineer appears who boasts a vacuum of 80| 

 inches. It is to be regretted that time and talent should be 

 thus wasted. It is a fact of great importance, confirmed by 

 experiment and by practice, that a vacuum may be too good, 

 and become a loss instead of a gain." 



