1819.] P ro f- Leslie on Heat and Climate. 15 



of an air-pump a fine loose bag of silk, including the bulb of a 

 delicate thermometer. When the air contained within the re- 

 ceiver is partly exhausted, the bag will become distended, and, 

 after some time, its rarefied air will attain the temperature of the 

 room. If the cock be now opened, the bag will collapse, and 

 the included air, suffering a diminution of its attraction for heat, 

 will indicate a high temperature. But before the external air is 

 completely admitted, or the thermometer has acquired its just 

 temperature, a great part of the heat extricated is lost on the 

 bag, or conducted off through its substance. As these circum- 

 stances are accidental and irregular, the rise of the thermometer 

 will indicate not the exact change of the air's temperature, nor 

 even the proportional change. 1 had, therefore, recourse to a 

 different method for the solution of this interesting problem. I 

 reflected that, if warm air be admitted into a glass vessel, the 

 internal surface will almost instantly be heated, and the air 

 cooled down to the same standard. The subsequent cooling is 

 extremely slow, for the rate at which the heat is communicated 

 is inversely as the depth it has penetrated into the glass, which 

 is besides a very bad conductor. But as the surface of the 

 vessel and its contents are constant, the excess of temperature 

 retained by the air after admission will in every case bear the 

 same ratio to what it possessed previously. I suspended an 

 exceedingly delicate thermometer in a large receiver 900 cubic 

 inches in capacity, and extracted one-fifth of the contained air : 

 after the general temperature was exactly diffused, I suddenly 

 admitted the external air, and observed the mercury of the ther- 



tering the same medium, and consequently produce perfect vision. As during the 

 emission of light from a body each particle is succeeded by another, it will be ex- 

 posed during the imperceptible interval of space to the general repulsion of the, 

 combined luminous fluid. Let A he situated on the surface of the body, and B at 



that of the fluid, which will protrude a little farther; let B C denote the mutual 

 distance between the particles of the fluid, B E the general repulsion, and the 

 ordinates G F, I K the attractions of the body at the distances A F, A D, and sup- 

 pose the curve to coincide with the straight line A C, which appears very proba- 

 ble. Then, if by any cause, the attraction B E is in the smallest degree diminished, 

 or the repulsion B E increased, the particles will be propelled, and will after- 

 wards be carried forwards by the excesses G H, and L K of the repulsive above 

 the attractive forces. Hence, from the principles of dynamics, the square of the 

 final velocity will be as the curve space E I) C K G. 



But the curve is given in species, and consequently proportional to the circum- 

 scribing rectangle B£DC, which is given in magnitude, tince the repulsion B E 

 is inversely as the distance of the particles of the fluid B C. Wherefore the final 

 velocity will in every case be the same. 



