539 



by a quantity of moisture which falls considerably short of producing 

 complete saturation. The conducting power of a piece of dried 

 chalk was ='19, but became ='30 when the substance was very 

 moist. That of a well-dried piece of new red sandstone was ='25, 

 but became as much as -60 when saturated. Both these substances 

 absorbed a large quantity of water. Ancaster oolites absorbed con- 

 siderably less, and their conductivity was affected in a smaller degree. 

 For a block of dry clay the conductive power was '23, and became 

 37 when well moistened. Close indurated sandstone, palaeozoic 

 rocks of close texture, and igneous rocks are bad absorbents, and are 

 very little affected in their conductive powers by moisture. 



Comparison of Deductions from Theories of Terrestrial Temperature 

 with the Results of Observation. 



6. It has long been established by mathematical investigation, that 

 if a large globe like the Earth be heated in any manner and in any 

 degree, its temperature at points not too remote from its surface, and 

 after a sufficient lapse of time, will necessarily become such that the 

 increase of temperature in descending along a vertical line will be 

 proportional to the increase of depth. In this enunciation, however, 

 it is assumed that the conductive power throughout the mass, or at 

 least throughout its more external portion, is uniform. The differ- 

 ence of conductive power between the unstratified and sedimentary 

 portion of the earth's crust, or that between one sedimentary portion 

 and another, has not hitherto been taken into account*. The author 

 has investigated the problem assuming the crust of the globe to consist 

 of any number of strata of different conductive powers and bounded 

 by parallel surfaces, the problem being much simplified by consider- 

 ing their surfaces as plane instead of spherical. Then, assuming the 

 temperature of the crust of the globe to be due entirely to the trans- 

 ference of heat from its central portions to its surface, it is shown 

 that the increase of temperature in descending vertically through 

 any two strata, ought to be in the inverse ratio of the conductive 

 powers of those strata, whether the two strata belong to the same 

 group of stratified beds, or to two different groups in different local- 

 ities. Such at least must be the result unless we introduce very 

 * Except in the case in which Poisson investigates the state of temperature of 

 a sphere surrounded by a single concentric spherical shell of different conductivity. 



VOL. VIII. 2 R 



