696 Mr. A. H. Davis on 



we have 



c 2 gl z a6\k 2 = constant, . . . . . (7) 



and then for models so chosen in size (/) 



h oc kO/l (8) 



If only the temperature and size of the model are varied, 

 c, g, a, and k, the gaseous constants, remaining- the same, 

 (7) becomes 6l z = constant (7a) 



Data are available for testing whether hoc kd/l when 

 Ol 3 = const. B. Peclet has made, perhaps, the most 

 complete experiments on heat-losses from surfaces (Trait d 

 de Chaleur, 3rd ed., 1860), and he measured the rate of 

 cooling of metal cylinders and spheres filled with water. 

 He used bodies ranging from 5 to 30 cm. in diameter 

 and 5 to 50 cm. in length, placed in a large water-jacketed 

 metal cylinder about 80 cm in diameter and 100 cm. high. 

 He worked at various values of temperature excess up 

 to 65°. He found for these bodies 



h a A0 1 ' 233 , 

 where A has the value 



for spheres A = " 1-778 + 0"13/r, 



for horizontal cylinders . A = 2*058 + 0"0382/r, 

 " r " being radius in metres. 



Also a formula is given for vertical cylinders from which 

 the values of A can be calculated for the similar bodies we 

 require of this shape. All spheres are similar: and in a 

 long horizontal cylinder it is obvious that the length does 

 not affect the heat-loss per unit area, so the formula is 

 applicable to models, even although in the actual expe- 

 riments the ratio of length to diameter may not have been 

 constant. 



From Peclet's formulae heat-losses have, therefore, been 

 calculated for similar bodies in the cases mentioned above. 

 Following (1 a) the models have been so chosen that 

 0/ 3 = const. = 6 x 10 4 . The result is given in Table I. 

 This choice is such that the first seven models roughly 

 range in temperature excess and size over the region in 

 which Peclet's experiments were confined. The eighth 

 model, working at 480° C. excess, is somewhat theoretical. 



