24 LECTURES TO SCIENCE TEACHERS. 



consequently, if we measure the time which the maximum 

 takes to travel down to a given depth in the earth's surface, 

 we shall be able to measure the conductivity of the rocks. 



Again, as we descend to greater depths the variation in 

 temperature at any particular depth becomes less and less. 

 Thus, at a depth of two yards, the daily variations of 

 temperature are quite imperceptible, to any instrument 

 that we may employ. But the annual variations in tem- 

 perature can be recorded to very much greater depths, in 

 fact to about nineteen times that amount, or to about 38 

 yards. Now, this is a very important point to observe, 

 that the depth to which the annual variation of tempera- 

 ture goes is nineteen times as great as the depth to which 

 the daily variation goes. This is completely in accordance 

 with the theory of Fourier. He shows that the depth to 

 which a periodical fluctuation of temperature will descend 

 is proportional to the square root of the period of that 

 variation. Now, the periods of variation in this case are 

 one day and one year, and, therefore, the square roots of 

 the period are in the proportion of the square root of 1 

 t> the square root of 365, therefore, we ought to find that 

 1 : 19 :: v/IT v/36lT If you multiply 19 by itself you get 

 361, which is almost exactly the ratio of a year to a day. 

 Some experiments were instituted at Edinburgh, by the 

 late Principal Forbes, with different thermometers, which 

 were sunk to various depths in the soil in different 

 districts about Edinburgh. One series were sunk in the 

 rocks forming the Calton Hill, on which the Observatory 

 stands. Another set were sunk in the Experimental 

 Gardens to conceal the sand, and a third set were esta- 

 blished in a sandstone bed. These thermometers were 

 placed at different depths, going down to 24 feet, and it 

 was found that the retardation of the maximum and 

 minimum followed exactly the laws of Fourier, and also 

 that the variations in amplitudes of difference in tempera- 

 ture followed Fourier's law as we descended to greater 

 depths. Other observations of the same kind have since 

 been taken up in different regions, and a very valuable 

 set has been put at Greenwich with exactly analogous in- 

 struments. These results were reduced by Principal 

 Forbes, and afterwards a much larger series of them were 

 reduced with every degree of accuracy possible by Sir 



