242 Professors W. E. Ayrton and J. Perry on 



somewhat doubtful. We are not, therefore, surprised to find 

 the conductivity of marble to be 0*0048 (gramme, centimetre, 

 second) as given by M. Peclet in 1841, to be 0*0097 for fine- 

 grained and 0*0077 for coarse-grained marble, as given by 

 M. Despretz in 1853, and as 0*0017 as given by Prof. G. 

 Forbes in 1873. 



The method employed by Principal Forbes, and Sir W. 

 Thomson, in 1860, of deducing the conductivity of rock from 

 observations of underground temperature is, of course, sus- 

 ceptible of much greater accuracy than the method referred 

 to above ; but it has the disadvantage that a considerable period 

 of time is necessary for the completion of one experiment, and 

 it can only be performed on a rather large depth of rock form- 

 ing part of the earth's crust. 



The following method which we have employed for deter- 

 mining the conductivity of heat in stone, and which was 

 suggested by some remarks made by Sir W. Thomson when 

 lecturing to the Higher Natural-Philosophy Class, in Glasgow, 

 in 1874, has the great advantage of perfect certainty in the 

 results ; and it can be used with comparatively little difficulty 

 for all bad conductors. The principle consists simply in 

 keeping a ball of the material to be experimented on in a 

 water (or other) bath at a constant temperature for a sufficient 

 length of time for the whole ball to acquire the temperature 

 of the bath ; then suddenly removing the warm water and 

 allowing a continuous rapid stream of cold w r ater of constant 

 temperature to flow round the outside of the ball, while time- 

 readings of the temperature of some fixed point in the ball (for 

 example, the centre) are taken as the ball slowly cools. Under 

 these circumstances one of Fourier's well-known equations 

 enables us to determine the internal conductivity of the ball, 

 and the emissivity of the surface. 



The obvious difficulty in this method of experimenting is 

 to determine the temperature, say, at the centre of the ball, at 

 successive intervals of time, without disturbing the flow of 

 heat in the sphere. The comparatively small size of our balls 

 of stone would make this difficulty very considerable if an 

 ordinary thermometer were used; but those who have worked 

 numerical illustrations of Fourier's results will see that the 

 introduction into the ball of a therm ometric junction attached 

 to very fine leading wires cannot appreciably affect the general 

 conditions. For absolute correctness it would be necessary 

 to have a conical tubulure space of which the sides were coated 

 with a substance impermeable to heat, extending from the 

 surface of the ball to the centre, and terminating in a small 

 spherical cavity at the centre, and to employ a thermometric 



