538 



(2) Argillaceous substances. 



Clay, very dry to very moist -23 to '37 



(3) Siliceous rocks. 



New red sandstone (same block dry to satu- 

 rated) -25 to -60 



Freestone -33 to '45 



Hard compact sandstones (Millstone-grit). ... '51 to '76 



(4) Hard, compact, old sedimentary rocks '50 to *61 



(5) Igneous rocks -SStol'OO 



Effect of Pressure. 



3. This effect was not appreciable for a pressure of 7500 Ibs. per 

 square inch in such substances as bees' -wax and spermaceti. Nor 

 was there any sensible effect with chalk between a pressure of 

 4300 Ibs. and 7500 Ibs. per square inch. 



Clay which when incompressed had a conducting power ='26, had 

 when compressed with 7500 Ibs. per inch, a power ='33; and the 

 conducting power of a mixture of sand and clay in equal quantities 

 rose from '36 to -378 by an increase of pressure from 4300 Ibs. to 

 7500 Ibs. per inch. 



Generally the effect of pressure is much less than might have 

 been anticipated. 



Effect of Discontinuity . 



4. When the conducting mass consists of a number of strata 

 superimposed on each other, the mathematical problem presented to 

 us requires a distinct investigation, which is here given under a very 

 general form, together with the experiments necessary to determine 

 the effect of this kind of discontinuity. The result is that if a mass 

 of sandstone consisted of a number of strata, the conducting powers 

 of which should be about '5, the mean conductivity of the whole 

 would not be diminished by more than about ^th part, supposing the 

 average thickness of strata to be 1 foot ; or by about y^th, if that 

 average thickness should be 6 inches. This effect is much less than 

 might possibly have been anticipated. 



Effect of Moisture. 



5. This effect was very considerable in those rocks which are great 

 absorbents of water. The maximum effect appears to be produced 



