of some Hocks at High Temperatures. 47 



which represents a limestone cylinder, is then cut in one 

 face, and a multiple couple with four pairs of junctions is 

 sunk in them, one set of junctions being equally spaced 

 round the circumference of the inner circular groove and 

 the other set round the outer circle. In the figure, A is one 

 of the inner junctions and B one of the outer. The grooves 

 are V-shaped, the couple being sunk to the bottom of the V 

 and the groove filled in with some kind of cement. The 

 junctions are situated in the circular grooves at points 

 approximately midway between the radial grooves so as to 

 minimise errors due to the latter. The free ends of the 

 couple are cemented into grooves C cut in the outer surface 

 of the cylinder parallel to the axis. This couple measures 

 the average differences of temperature between points on 

 the inner and outer circles whose radii are known. The 

 average temperature of the rock is measured by a single 

 couple cemented into a straight groove cut in the ground 

 face of the other half of the cylinder, so that the junction is 

 at a known distance from the centre. The two halves are 

 then carefully cemented together, the layer of cement between 

 the two faces being as thin as possible. 



Limestone. 



The first determination was made on limestone, it being an 

 easy rock to work, and the results obtained for it being of 

 less importance than those for the igneous rocks. The total 

 length of the complete cylinder is 13'67 cm. and the diameter 

 7*89 cm., the diameters of the circular grooves being 1*94 cm. 

 and 6'36 cm. respectively. The distance of the single couple 

 from the axis is 1'93 cm. The grooves are V-shaped, about 

 2*5 mm. deep, and 1 mm. wide at the top. The wires of the 

 couple are O'l mm. in diameter, the junctions being twisted 

 together and fused in a Meker burner. The cement used 

 with the limestone and also with the granite is a mixture of 

 very finely powdered fireclay with water-glass. This cement, 

 though satisfactory when set, is very difficult to work, and 

 great difficulty was experienced in cementing in the fine 

 wires with it, so that in some experiments at present being 

 carried out on basalt, other cements have been tried. The 

 central hole is about 1*5 mm. in diameter and the platinum 

 wire 0'3 mm. diameter, the space round the wire being filled 

 with fine carborundum powder. This material was not a 

 very happy choice, as it is liable to attack platinum at high 

 temperatures, but in this case no harm was experienced 

 with it. 



