872 On the figure of the Earth. 



the interior, and that the density of the strata increases from the 

 surface to the centre. 



But though the earth be, in a mathematical sense, heteroge- 

 neous, it may notwithstanding be chemically homogeneous, if the 

 increase of density of its strata is caused onlv by the increased pres- 

 sure they suffer as they approach the centre. It may easily be 

 conceived that the immense weight of the superior strata may 

 considerably increase their density, though they may not be fluid; 

 for it is known that solid bodies are compressed by their own 

 weight. The law of the densities which result from these com- 

 pressions being unknown, we cannot tell how far the density of 

 the terrestrial strata may be thus increased. The pressure and 

 the heat which we can produce are verv small, compared to those 

 which exist at the surface, and in the interior of the sun and 

 stars. We cannot even form an idea of the effect of these forces, 

 united in those immense bodies. Every thing tends to make us 

 believe that they existed at one time in a high degree on the 

 earth, and that the phsenomena which they have occasioned, mo- 

 dified by their successive diminution, form the present state of 

 the surface of our globe ; a state vvhicli is nothing more than the 

 element of a curve, of which time is the abscissa, and of which 

 the ordinates will represent the changes that this surface has suf- 

 fered without ceasing. We are far from knowing the nature of 

 this curve, and we cannot therefore ascend with certainty to the 

 origin of what we observe on the earth ; and if, to satisfy the 

 imagination, always troubled by ignorance of the cause of the 

 phsenomena which interest us, a few conjectures are hazarded, 

 thev should be offered with the utmost caution. 



The density of a gas, the temperature remaining the same, is 

 proportional to its compression. But this law, though true within 

 those limits of density in which we have been able to prove it, 

 cannot be applied to liquids and solids, of which the density is 

 very great, compared to that of gas, when the pressure is little 

 or nothing. It may naturally be supposed that these bodies re- 

 sist compression the more thev are compressed ; so that the ra- 

 tio of the differential of the pressure to that of the density, in- 

 stead of being constant, as with gases, increases with the density. 

 The most simple function which can represent the ratio, is the 

 first power of the densitv, multiplied by a constant (|uantity : and 

 this I have adopted, because it unites to the advantage of repre- 

 senting in the simplest manner what we know of the compression 

 of liquids and solids, a facility of calculation in researches on the 

 figure of the earth. Hitherto, mathematicians have not included 

 in this research the effect resulting from the compression of the 

 strata. Dr. Young has called their attention to this object, by 

 the ingenious remark, which may be thus stated, the increase of 



density 



