132 NOMOS. 



size can be no objection these rays must be brought 

 to a focus at a considerable distance within the sur- 

 face which is most removed from the sun, for the re- 

 fractive indices of various bodies increase with their 

 density, and the density of the earth is much greater 

 than the density of chromate of lead. Compared 

 with water as 1, the density of the earth indeed is 5*67. 

 But, it may perhaps be asked, is there not reason 

 to believe that the heat of the sun does not pene- 

 trate into the earth beyond that stratum of 



The objec. invariable temperature where the fluctua- 

 tions to this * 



view may be tions of atmospheric heat cease to be per- 

 ceptible ? Now it is certain that the fluc- 

 tuations of solar heat which are perceived on the 

 surface of the earth become imperceptible at a very 

 small depth below the surface ; but this does not prove 

 that the heat of the sun does not penetrate any 

 further. What are the facts? The facts are, that 

 the diurnal fluctuations do not extend to a greater 

 depth than 3 feet, and that the wider fluctuations 

 of winter and summer cannot be traced below 100 

 feet. It is established also that the extreme range 

 of fluctuation becomes less and less the deeper we 

 descend, until at last that is, at the stratum of inva- 

 riable temperature the mean temperature of the 

 surface is maintained without any apparent change. 

 At the depth of 25 feet the range of fluctuation is 

 not more than 2 of Fahrenheit; at 50 feet, 0-2; 

 at 60 feet, 0-02. It is established, also, that the 

 length of time which elapses before a part responds 

 to a particular temperature at the surface is directly 

 proportionate to the depth of this part below the sur- 



