274 Mr. A. W. Porter on the Influence of the 



Peclet's results were derived from experiments on five 

 cylinders : only two of them are available for my purpose, 

 since the others were either of different material or different 

 surface. They were short cylinders of unpolished brass, 

 terminated by hemispherical ends, allowance for which was 

 made from the experiments on spheres. Peclet, therefore, 

 considered his results to apply to infinitely long cylinders. 

 Their radii were 5*1 and 6*9 cm. respectively, and measure- 

 ments were made on them in a cylindrical enclosure 40 cm. 

 radius and 100 cm. long. His results are : — 



Experimental 

 Radius. results. 



5-1 -0000867 

 6-9 -0000803 



and the formula which I derive from these is 



•00022 



£ = •0000383 + 



a(log 10 40-log 10 a)' 



In large cylinders like these radiation plays a more con- 

 spicuous part, and the value of the first term in my formula 

 which represents it is in very good accordance with values 

 obtained by direct methods. The value of " b " is, however, 

 very different from that obtained from Ayrton and Kilgour's 

 experiments ; and the conclusion is forced upon one that 

 although to consider " b " a constant may serve as long as 

 the enclosure remains the same, it will by no means suffice 

 to take it the same constant under such widely different 

 conditions as obtain in these two sets of experiments. The 

 character of the convective flow is evidently totally different 

 in the two cases. 



The same conclusion is arrived at by an examination of 

 results obtained from experiments on spheres. The formula 

 for a sphere, found by means of the same assumption as 

 before, is 



e = h+— r ^ -• 



Peclet experimented on three spheres in the same enclosure; 

 and assuming the enclosure to behave like a spherical one of 

 45 centim. radius, the formula becomes 



•000417 x 45 



e= -00003845 + 



a (45 — a) ' 

 The experimental and calculated values are here given: — 



