Tables 248, 249. 

 COOLING BY RADIATION AND CONVECTION. 



239 



TABLE 248. — At Ordinary Pressures. 



According to McFarlane* the rate of loss of heat by a sphere 

 placed in the centre of a spherical enclosure which has a 

 blackened surface, and is kept at a constant temperature of 

 about 14° C, can be expressed by the equations 



t r= .000238 + 3.06 X 10 — ^t — 2.6 X 10—'/', 



when the surface of the sphere is blackened, or 



e = .000168 + 1. 98 X io-«< — 1.7 X io-«/2, 



when the surface is that of polished copper. In these equa- 

 tions, e is the amount of heat lost in c. g. s. units, that is, 

 the quantity of heat, small calories, radiated per second per 

 square centimetre of surface of the sphere, per degree differ- 

 ence of temperature t, and t is the difference of temperature 

 between the sphere and the enclosure. The medium through 

 which the heat passed was moist air. The following table 

 gives the results. 



TABLE 249. — At Different Pressures. 



Experiments made by J. P. Nicol in Tail's Labo- 

 ratory show the effect of pressure of the en- 

 closed air on the rate of loss of heat. In this 

 case the air was dry and the enclosure kept at 

 about 80 C. 



Polished surface. 



Blackened surface. 



Pressure 76 cms. of Mercury. 



63.8 



57-1 

 50.5 

 44.8 

 40.5 

 34-2 

 29.6 



23-3 

 18.6 



.00987 

 .00S62 

 .00736 

 .00628 

 .00562 

 .00438 

 .00378 

 .00278 

 .00210 



6l.2 



50.2 

 41.6 

 34-4 



27-3 

 20.5 



.01746 

 .01360 

 .01078 

 .00860 

 .00640 

 .00455 



Pressure 10.2 cms. of Mercury. 



67.8 

 61. 1 



55 

 497 

 44.9 

 40.8 



.00492 



•00433 

 .00383 

 .00340 

 .00302 

 .00268 



62.5 



57-5 

 53-2 

 47-5 

 43-0 

 28.5 



.01298 

 .01158 

 .0104S 

 .00898 

 .00791 

 .00490 



Pressure i cm. of Mercury. 



65 

 60 



50 

 40 



30 

 23-5 



.00388 



•00355 

 .00286 

 .00219 

 .00157 

 .00124 



62.5 



57^5 



54-2 



41.7 



37^5 

 34^o 



27-5 

 24.2 



01 182 

 01074 

 01003 

 00726 

 00639 

 00569 

 00446 

 00391 



Smithsonian Tables. 



* " Proc. Roy. Soc." 1872. 

 t " Eroc. Roy. Soc." Edinb. 1869. 

 See also Compan, AnnaL de chi. et phys. 26, p. 526. 



