398 
MESSRS. W. E. AYRTON AND IT. KILO OUR ON THE 
e = X(0-069 y + 0*038 y), 
hence, substituting the values given in the last table for y and y, we have 
Values of 
, 
ejX. 
0 
0069 
10 
0-137 
100 
0-222 
200 
0-291 
300 
0-389 
Consequently, if the law connecting the loss of heat from thermometer bulbs in air 
with the difference of temperature between the bulb and the enclosure is of the same 
nature as that connecting the loss of heat from very fine horizontal wires in air with 
the difference of temperature, we should expect to find that the curve connecting the 
values of e/X and t in the last table would, when plotted, be everywhere concave to the 
axis of t. But this is not the case, for we find on plotting this curve that wdiile it 
resembles our curves in being concave to the axis of t, for values of t less than about 
200° C. it changes its curvature at about this point and becomes distinctly convex. 
It is, of course, to be remembered the maximum value of t in the experiments of 
MM. Dulong and Petit was 240° C., while in some of ours t exceeded 300° C. We 
are, however, inclined to attribute the inability of the formulae of MM. Dulong and 
Petit to give even the general shape of the curves which we have obtained to the 
fact that the convection which occurs with thermometer bulbs hardly suggests the 
very great convective cooling that experiments show to occur with very fine wires at 
high temperatures.—May 31, 1892.] 
Y. Calculation of the Distribution of Temperature along a Platinum Wire Heated 
by an Electric Current. 
Let cl be the diameter of the wire in centimetres. 
t ,, temperature in degrees centigrade at any point of the wire distant 
x centimetres from the nearer of the two supports to which the 
ends of the wire are attached. 
t 0 ., temperature of the supports. 
f(t ) ,, electrical resistance, in ohms, of a cubic centimetre of the wire at a 
temperature t. 
4> ( t ) ,, thermal resistance of a cubic centimetre of the wire at a temperature t. 
