the Heat-conductivity of Stone. 245 



the object of taking the mean of a number of observations 

 being, of course, to calibrate the scale near the zero-point with 

 considerable accuracy. This determination of the sensibility 

 of the galvanometer was made before every experiment; and it 

 was usually found that one division of the scale corresponded 

 to rather less than one fiftieth of a degree Centigrade differ- 

 ence of temperature between the junctions when the junctions 

 themselves had a temperature of about 23° C. This amount 

 of delicacy was really more than was absolutely necessary, 

 since the thermometer in the compensating-bath could only be 

 read to the twentieth of a degree. 



The baths D D and B were now heated up to about 70° C, 

 and kept at that temperature until the temperature of all parts 

 of the stone ball had become uniform — that is, until there was 

 no current when the two thermometers S and H indicated ex- 

 actly the same temperature, the small scale-error in H being, 

 of course, allowed for. At this moment the cork at was 

 removed, the tap R opened, and a quantity of cold water poured 

 into B by means of the tube M M so as to flood the bath B ; 

 the whole of the warm water in B was therefore suddenly dis- 

 placed by cold cistern-water. was now closed but R left 

 open, so that there was a continual stream of cold water flow- 

 ing upwards and overflowing the bath at the top. Constant 

 readings of the thermometer in the compensating-bath, com- 

 bined with simultaneous readings of the galvanometer (the 

 latter being kept as small as possible), were now taken for about 

 80 minutes, by which time the whole ball had cooled down 

 nearly to the temperature of the cold water. By opening the 

 key K the zero of the galvanometer was frequently taken, to 

 detect any slight change. It was the duty of one observer 

 solely to observe the galvanometer, of another to cool the com- 

 pensating-bath D D at the right rate and to take readings of 

 the thermometer suspended in it, and of a third to record the 

 time together with the readings made by the other two, as well 

 as to take occasional readings of the thermometer H. 



IV. Reduction of the Readings. — Fourier's equation for the 

 temperature of a point at a distance x centimetres from the 

 centre of a homogeneous globe, when the globe has been 

 initially all at constant temperature and when it is cooling by 

 a constant external temperature being maintained, is 



!r * hl (%) 



2Er_ \ »•/ sina _**< 



