HEAT. 



cylinder, by which the steam was con- 

 densed, and a vacuum produced; the 

 air pressing upon the piston above 

 forced it down into the cylinder, raising 

 the weights at the other end of the 

 beam ; the steam was then permitted to 

 enter again to elevate the piston, as 

 before. 



In this operation it will be obvious 

 that immense quantities of steam 

 must be lost in raising the tempera- 

 ture of the cylinder to the boiling 

 point, every time after being cooled 

 with cold water, and exposure to the 

 atmosphere, and consequently corres- 

 ponding portions of heat wasted, 

 amounting, according to the calculations 

 of Mr. Watt, to half the quantity used 

 for working the engine. 



It was "seen by him that the cool- 

 ing of the working cylinder must de- 

 prive the steam of " its latent heat, 

 and occasion its condensation, until the 

 cylinder was again heated to 212. To 

 remedy this, after many experiments, 

 Mr. Watt contrived to condense the 

 steam in a separate vessel without cool- 

 ing the working cylinder. A com- 

 munication being established between 

 the cylinder and a separate vessel, called 

 a condenser, exhausted of air and im- 

 mersed in cold water : the moment the 

 steam had performed its office, a valve 

 was opened, by the working of the en- 

 gine, which permitted the steam to rush 

 into the exhausted vessel, where it was 

 condensed by the cold temperature of 

 the vessel. 



This is decidedly the most important 

 of the numerous inventions of Mr. 

 Watt. 



To prevent the cooling effect of the 

 atmosphere, and still further to eco- 

 nomize heat, he excluded air altogether 

 from the cylinder, by making the piston- 

 rod work through a collar, steam being 

 introduced above the piston, and after- 

 wards both above and below it : when 

 the steam had pressed the piston to the 

 top it was allowed to escape to the con- 

 denser ; and steam was admitted above 

 to press it down into the vacuum pro- 

 duced below by condensation, in the same 

 way the steam above escaped into the 

 condenser ; and the steam entering below 

 forced the piston up into the vacuum, so 

 produced, thus enabling the engine to 

 exert great power, without the aid of at- 

 mospheric pressure. 



It is affirmed that Mr. Watt, when he 

 first began to operate on steam, had 

 neither means nor leisure to permit the 

 use of a complicated apparatus, and, 

 therefore, made some of his most im- 

 portant researches with apothecaries' 

 phials. In this way he discovered that 

 a cubical inch of water forms a cubical 

 foot of steam, or 1 728 inches ; and that 

 the heat evolved by the condensation of 

 that quantity of steam would be suffi- 

 cient to heat six cubical inches of 

 water from the atmospherical tempera- 

 ture to 212 degrees. It is now usually 

 considered that steam, arising from 

 boiling water, occupies 1800 times as 

 much space as the water from which it 

 was produced. 



The apparatus used by Dr. tire, in 

 his researches on the latent heat of 

 vapours, and which he considered well 

 adapted to the purpose, although ex- 

 ceedingly simple, consisted of a small 

 glass retort, with a short neck, inserted 

 into a globular receiver of very thin 

 glass, and about three inches in diame- 

 ter. The globe was surrounded witli 

 a certain quantity of water, at a known 

 temperature, in a glass basin. A quan- 

 tity of the liquid,the vapour of which was 

 to be examined, amounting to 200 grains, 

 was put into the retort, and rapidly dis- 

 tilled into the globe, by the heat of an 

 Argand lamp. His experiments were 

 performed when the temperature of the 

 air was 45, that of the water in the 

 basin being from 42 to 43 ; the heat 

 imparted by the condensation of steam 

 to this water, never raised its tempera- 

 ture higher than about four degrees 

 above that of the atmosphere, and each 

 operation generally lasted about five or 

 six minutes. A very delicate thermo- 

 meter was constantly moved through 

 the water, and its indications were read 

 off, to small fractions of a degree, by 

 the aid of a lens. 



The elevation of temperature pro- 

 duced in these experiments being so 

 little above that of the atmosphere, Dr. 

 Ure was of opinion that the influence of 

 the air did not affect the results. The 

 water in the basin weighed 32,340 grains, 

 and the globe was held steadily in its 

 centre by a slender ring round its neck. 

 The experiments were repeated a num- 

 ber of times with corresponding results, 

 which are stated in the following 

 table : 



