420 PHILOSOPHICAL TRANSACTIONS. [aNNO 1778. 



mean density may be greater or less, and that in a degree wliich is not certainly 

 known. A considerable degree of" accuracy in this point could perhaps only be 

 obtained by a close examination of the internal structure of the hill. And the 

 easiest method of doing this would be to procure holes to be bored in several 

 parts of it, from the surface to a sufficient depth, after the manner that is prac- 

 tised in boring holes to the coal mines from the surface of the ground ; for by 

 such operation it is known what kind of strata the borer is passed through, 

 together with their dimensions and densities. The proper mean among all these 

 would be the mean density of the hill, as compared to water or to any other 

 simple matter ; and thence we should obtain the comparative density of the 

 whole earth with respect to water : but in the present instance, we must be 

 satisfied with the estimate arising from the report of the external view of the 

 hill ; which is, that to all appearance it consists of an entire mass of solid rock. 

 It is probable therefore that we shall not greatly err, if we assume the density of 

 the hill equal to that of common stone ; which is not much different from the 

 mean density of the whole matter near the surface of the earth, to such depths 

 as have actually been explored either by digging or boring. Now the density of 

 common stone is to that of rain water as 2-|- to I ; which being compounded with 

 the proportion of 9 to 5 above found, there results the ratio of 4-l to 1 for the 

 ratio of the densities of the earth and rain water ; that is to say, the mean den- 

 sity of the whole earth is about 4-i- times the density of water. But since this 

 calculation has been completed, Mr. Playfair, the learned professor of natural 

 philosophy in the university of Edinburgh, has made a mineralogical survey 

 of the hill Schihallien, by which he has discovered that the varieties of rock of 

 which it consists may be reduced to 3 kinds ; a granular quartz, which occu- 

 pies all the middle part of the mountain ; a micaeous schistus which encompasses 

 the former nearly all round like a zone, to within 600 feet of the bottom ; and 

 lastly a calcareous zone, which may be said to surround the mountain at its base. 

 Though there is some irregularity in the disposition of these zones, this is at 

 least a general idea of the structure of the mountain that does not differ greatly 

 from the truth. By what Mr, Playfair could conjecture, the mean specific 

 gravity of the whole would be about 2.7, that of one stratum being about 2.64, 

 another about 2.75, and some rocks as high as 3, and even 3.2. On the 

 whole then, it appears not unreasonable to suppose the mean specific gravity of 

 the mountain to be from 2.7 to 2.75 or 2^. Now |^ X 24- gives |-§- or almost 

 5 i that is, under these circumstances, the medium density or specific gravity of 

 the whole mass of the earth, in proportion to that of water, is nearly as 5 to 1, 

 or that it is 5 times the weight of water. 



To what useful purposes the knowledge of the mean density of the earth, as 

 above found, may be applied, it is not the business here to show. Dr. H. there- 

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