Additional Notes. ACA 



nig easily up and down in it, and made tight by oakum or 

 hemp, and covered with water. This piston is suspended by 

 chains from one end of a beam, moveable upon an axis in 

 the middle of its length : to the other end of this beam are 

 suspended the pump-rods. 



" The danger of bursting the vessels was avoided in this 

 machine; as, however high the water was to be raised, it 

 was not necessary to increase the density of the steam, but 

 only to enlarge the diameter of the cylinder. 



*' Another advantage was, that the cylinder, not being 

 made so cold as in Savery's method, much less steam was 

 lost in filling it after each condensation. 



** The machine, however, still remained imperfect, for 

 the cold water thrown into the cylinder acquired heat from 

 the steam it condensed, and being in a vessel exhausted of 

 air, it produced steam itself, which, in part, resisted the ac- 

 tion of the atmosphere on the piston ; were this remedied by 

 throwing in more cold water, the destruction of steam in the 

 next filling of the cylinder would be proportionally increased. 

 It has, therefore, in practice, been found advisable not to 

 load these engines with columns of water weighing more 

 than seven pounds for each square inch of the area of the 

 piston. The bulk of water, when converted into steam, re- 

 mained unknown, until Mr. J. Watt, then of Glasgow, 

 in 1764, determined it to be about 1800 times more rare than 

 water. It soon occurred to Mr. Watt, that a perfect en- 

 gine would be tliat in which no steam should be condensed 

 in filling the cylinder, and in wliich the steam should be so 

 perfectly cooled as to produce nearly a perfect vacuum. 



" Mr. Watt having ascertained the degree of heat in 

 which water boiled in vacuo, and under progressive degrees 

 of pressure, and instructed by Dr. Black's discovery of la- 

 tent heat, having calculated the quantity of cold water ne- 

 cessary to condense certain quantities of steam so far as to 

 produce the exhaustion required, he made a communication 

 from the cylinder to a cold vessel previously exhausted of air 

 and water, into which the steam rushed, by its elasticitv, 

 and became immediately condensed. He then adapted a co- 

 ver to the cylinder, and admitted steam above the piston to 

 press it down instead of air, and instead o^ 'H^p'y'^'g water, 

 he used oil or grease to fill the pores of the oakum, and to 

 lubricate the cylinder. 



*' He next applied a pump to extract the injection water, 

 the condensed steam, and the air, from the conUcnsing vessel, 

 f very stroke of the engine. 



