Fig. 2. 



168-5 Jbs. 



ST't Mr. Joule on the Chatiges of Temperature produced by 



20i lbs. of brass and copper, and 6 lbs. of tinned iron. It was 

 therefore equivalent to 13°-628 per lb. avoirdupois of water. 



The force necessary to effect the above condensation ma}' 

 be easily deduced from the law of Boyle and Mariotte, which 

 has been proved by the French academicians to hold good as 

 far as the twenty-fifth 

 atmosphere of press- 

 ure. Let fig. 2 re- 

 present a cylinder 

 closed at one end, the 

 length of which is 

 21-654 feet, and the 

 sectional area ir3?6 

 square inches. Then 

 one foot of it will have 

 exactly the same ca- 

 pacity as the copper 

 receiver used in the 

 experiments, and its 

 whole capacity will be 

 2956 cubic inches. It 

 is evident, therefore, — 

 that the force used in 

 pumping (considered to be without friction) was exactly equal 

 to that which would push the piston p to the distance of a toot 

 from the bottom of the cylinder. Excluding exterior atmo- 

 spheric pressure, the force upon the piston, when at the top 

 of the cylinder, will be 168*5 lbs., the weight of a column ot 

 mercury 30'2 inches long and of 11 "376 square inches section ; 

 and at a foot from the bottom it will be 21*654 times as much, 

 or 3648*7 lbs. The hyperbolic area, abed, will therefore 

 represent the force employed in the condensation, including 

 the assistance of the atmospheric pressure. Applying the for- 

 mula for hyperbolic spaces, we have, 



s = 3648*7 X 2*302585 X log 21*654 = 11220-2. 



The force expended in condensation was therefore equiva- 

 lent to that which can raise 11220*2 lbs. to the perpendicular 

 height of one foot. 



Comparing this with the quantity of heat evolved, we have 



11220*2 823 „ , , . 1 r ui f • • 



o = — -Q-. oo that a mechanical force capable oi raismg 



13 *ozo 1 



823 lbs. to the height of one foot must be applied in the con- 

 densation of air, in order to increase the temperature of a 

 pound of water by one degree of Fahrenheit's scale. 



The following Table contains the results of experiments si- 



3648-7 lb8. 



