181*5.] Variation of Thermal Conductivity of Rocks. 



105 



Iron wires are marked t. 

 Platinoid wires are marked p. 



B. M, T. Thermoelectric junctions in slab. 



X. oil bath. 



A. Bath of molten tin. 



C. Tank of cold water. 



D. Oil bath. 



E. Thermometer. 



F. Junctions of platinoid and copper wires. The wires are insulated from one 



another, and wrapt all together in cotton wool at this part, to secure 

 equality of temperature between these four junctions, in order that the 

 current through the galvanometer shall depend solely on differences of 

 temperature between whatever two of the four junctions, X, T, M, B, is put 

 in circuit with the galvanometer. 



GK Galvanometer. 



H. Four mercury cups, for convenience in connecting the galvanometer to any 

 pair of thermoelectric junctions. 



x, b, in, t, are connected, through copper and platinoid, with X, B, M, T, respec- 

 tively. 



9. The temperatures, v(B), r(M), u(T) of B, M. T, the hot, inter- 

 mediate, and cool points in the stone, were determined by equalising 

 to them successively the temperature of the mercury thermometer 

 placed in the oil-tank, by aid of thermoelectric circuits and a galva- 

 nometer used to test equality of temperature by nullity of current 

 through its coil when placed in the proper circuit, all as shown in the 

 diagram. The steadiness of temperature in the stone was tested by 

 keeping the temperature of the thermometer constant, and observing 

 the galvanometer reading for current when the junction in the oil- 

 tank and one or other of the three junctions in the stone were placed 

 in circuit. We also helped ourselves to attaining constancy of 



