﻿80 M. 0. E. Meyer on the Heating of a 



dilation k is therefore inversely proportional to the thickness, 

 and it follows that the heating of the disk must also increase 

 inversely as the thickness. 



All the magnitudes occurring in the formula are known from 

 Stewart and Tait's measurements, or may easily be calculated 

 from them, including the constant h which defines the thermal 

 radiation. No direct statements have been published ; so much 

 the more interesting, therefore, does it appear to deduce their 

 value from the observations in question. 



If we introduce into the above formula the numerical values 



£ = 0015 inch = 0'38 millim., 

 M = 10 ounces =310 grms., 

 R = 6'5 inches =165 millims., 



T = i£r= "- 013 ' 



Q = 423-5 metre, 



and for g and it their well-known values ; and if, finally, we 

 assume for the heating the mean value 



* = c -84 F, =0°-46 C, 



which holds for the disk coated by lampblack, we get the ther- 

 mal radiation 



£ = 00017. 



This number contains no arbitrary unit of heat, but is con- 

 nected solely with the so-called absolute units (that is, the milli- 

 metre and the second of time) , as well as the density of water as 

 unit of specific gravity. It stands as S for a surface blackened 

 by lampblack in a rarefied space in which there is a tension of 

 03 inch or 7'6 millims. of mercury. 



An idea is obtained of the meaning of the number thus 

 found by considering that a blackened surface of 1 square metre, 

 which has been heated 1 degree above the surrounding rarefied 

 air, loses in a second a quantity of heat which would raise a kilo- 

 gramme through 072 metre. 



The value found for the thermal radiation h is in remarkable 

 agreement with the result which, with the kind aid of Professor 

 Neumann of Konigsberg, I was able to deduce from the obser- 

 vations of Dulong and Petit. 1 take this opportunity of thank- 

 ing him publicly. 



Those philosophers have combined the results of their obser- 

 vations on the cooling of a heated body in a rarefied space, in 

 the law that the quantity of heat emitted in the unit of time by 

 the unit of surface is expressed by the formula 

 ma$(a' — l) + np c t b . 



