OBSERVATIONS OF UNDERGROUND TEMPERATURE. 427 
cubic feet of rock, in their calorimetric specifications, to be applicable similarly to 
define and illustrate the meaning of the conductivity denoted by &. The fluidity 
of the water allows a modified and somewhat simpler explanation, equivalent to 
that of § 36, to be now given, as follows :— 
44. If a long rectangular plate of rock, one foot thick, in a position slightly 
inclined to the horizontal, have water one foot deep flowing over it in a direction 
_ parallel to its length, and if the lower surface of the plate be everywhere kept 1° 
higher in temperature than the upper, the water must flow at the rate of & times 
the length of the plate per unit of time, in order that the heat conducted through 
the plate may raise it just 1° in temperature in its flow over the whole length. 
_ [it must be understood here, that the plate becomes warmer, on the whole, under 
the lower parts of the stream of water, its upper surface being everywhere at the 
same temperature as the water in contact with it, while its lower surface is, by 
hypothesis, at a temperature 1° higher.| If, for instance, the plate be of Calton 
Hill trap-rock, the water must, according to the result we have found, flow at the 
rate of 141-1 times its length in a year, or of -3863 of its length in twenty-four 
hours, to be raised just 1° in temperature in flowing over it. Thus water, one 
French foot deep, flowing over a plane bed of such rock at the rate of -3863 of a 
mile in twenty-four hours, will, in flowing one mile, have its temperature raised 1° 
by heat conducted through the plate. The rates required to fulfil similar condi- 
tions for the sand of the Experimental Gardens and the sandstone of Craigleith 
Quarry are similarly found to be ‘2435 of the length and -9929 of the length, in 
twenty-four hours. 
