456 Relations of Air, Heat, and Cold. 



a painted and a metallic surface, and which in atmospheric 

 air was denoted by 20 and 1 1, will in hydrosjcn gas be re- 

 presented by 50 and 41. Those opposite surfaces are thug 

 less contrasted in a medium of hydrogen gas, their.difl'crent 

 rates of discharging heat being nearly in the proportion of 

 5 to 4. The silver ball cased with cambric, cools 2} times 

 faster, if immersed in hydrogen gas ; but when exposed 

 naked in the same fluid, it loses its heat almost four limes 

 as fast as in common air. 



*' The superior mobility of hydrooen gas accelerates re- 

 inarkablv the dispersion of heat, by the process of abduc- 

 tion. But the exposing of a heated body to the action of 

 any current of a fluid substance, will occasion a similar ex- 

 penditure of heat, and which is exactly proportioned to the 

 celerity of the stream. If a very larae bulb of a thermo- 

 jneier be suddenly plunged into water flowing at the rate of 

 one-third of a mile in the hour, it will be found to lose its 

 heat twice as fast, as when immersed in the stagnant pool ; 

 and a current of two miles in the hour would, therefore, 

 cause through the liquid a dissipation of heat no less than 

 seven times more rapid than usual. A similar acceleration 

 ofeflTectis produced, by the impulse of a stream of air. 

 With a velocity of about four miles in the hour, the su- 

 peradded influence of a current equals the ordinary power 

 of abduction. Hence the play of a breeze of eight miles 

 an hour will double the rate of cooling from a painted, and 

 will triple that from a metallic, surface ; but a wind sweep- 

 ing with a velocity of forty miles in the hour, would ac- 

 celerate the cooling of the painted surface six limes, and 

 that of the metallic one no less than eleven times, thus 

 bringing them both near an actual equalitv of , performance. 

 In general, tlie hourly velocity of wind might he computed, 

 bv multiplyinsr eight miles irito the proportional surplus^ 

 cliect exerted in tlie refrigeration of a vitreous or painted 

 surlace. 



*' But even in still air, if the body exposed to its action 

 have a very consideraljle elevation of temperature, the pro- 

 gress of cooling will be sensibly quickened, by the continual 

 ascentof the heated portions of the medium, and which ibrni 

 in fact a stream, varying in force according to the intensity 

 of excitement. Supposing the excess of temperature to he 

 30 centesimal degrees, or 54 by Fahrenheit's scale, this 

 gentle perpendicular flow of healed air will conjoin an in- 

 fluence equal to the ordinary abduclive dispersion of heat, 

 and therefore corresponding to that of a current whick 

 moves at the rate of four miles an hour. Plence, if the 



silver 



