C. E. Dutton on the Contractional Hypothesis, 117 
cooling, and its value must be hypothetical. We are concerned, 
owever, only with such values of it as may have undergone 
change, and we may take it to be the melting point of the more 
refractory materials which constitute the chief bulk of the 
nucleus. Presuming these to be anhydrous silicates for at 
least 500 to 800 miles in depth, and paying due regard to the 
effect of pressure upon the congealing point, we may accept Sir 
William’s estimate of this temperature, which he takes to be, at 
a maximum, 7,000° F—a most abundant estimate is 
reduces the expression to a relation between three unknown 
quantitigs, x, ¢, and de’ If we desire to ascertain the rate of 
per foot, ry. Eo 
about 100 million, and the latter about 130 million, years. 
The accompanying graphical representationt exhibits the law 
of increase of geothermal temperatures in accordance with Sir W. 
omson’s discussion of Fourier’s theorem. . 
Of the general correctness of this theorem there can be no 
doubt. We may, however, for the moment qualify this asser- 
tion by an inquiry as to the nature of one of the quantities 
entering the expression of it. The coefficient. of conductivity 
x is regarded as a true constant. Whether it is so in reality is 
questionable. Experiments upon the conductivity of some 
Materials show that there is probably a variation,} which is 
* The exponential factor having become so ae e°=1 that it may be omitted. 
Pgh lmagrebegih earn J. D. Forbes to be the case with iron, the 
ivity diminishing with the temperature. : 
Cae oes 
