170 MR. J. H. GRAY ON A METHOD OF DETERMINING 
quantity of water in contact with the other side. The result was that Ciiment’s 
value was 200 times too small, while PrcLet, who tried to avoid CLEMENT'S error by 
having the water violently stirred, succeeded in obtaining a value six times too 
small. With the large surface to carry away the heat, PEcLET’s method is useless 
for any substance having so high a conductivity as even the worst of the metals, 
since the stirring could never be rapid enough without introducing fresh complications. 
The direct tests applied to determine this error in the present investigation showed 
that the temperatures on the inside and outside of the bottom of the box were the 
same. 
The following approximate calculation serves to show the order of magnitude of 
the error due to the assumption that the thermometer in the ball indicates the 
temperature of the end of the wire. 
It is clear that the difference of temperatures between the wire where it enters the 
ball, and the thermometer will be much less than that found as follows :— 
Take the case of a wire carrying heat to an infinite block of metal whose surface is 
Fie. 5. 
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a ee 
plane, and remark that the isothermals in the block would be hemispherical surface 
(fig. 5) having for their common centre the point where the axis of the wire enters 
the ball, and suppose the flow of heat to be steady. 
Let 
K = conductivity of the wire, 
i .s is ball, 
a = radius of the wire, 
beh ies j)eaballll: 
