/ OV GEOGRAPHY. 575 



It appears that the strata are more commonly in a direction nearly horizontal 

 than in any other; and their thickness is usually almost equable for some 

 little distance; but they are not disposed in the order of their specific gravity, 

 and the opinion of their following each other in a similar series, throughout 

 the greater part of the globe, appears to rest on very slight foundations. 



From observations on the attraction of the mountain Shehallion, Dr. 

 Maskelyne inferred the actual mean density of the earth to be to that of 

 water as 44- to 1, judging from the probable density of the internal substance 

 of the mountain, which he supposed to be a solid rock. Mr. Cavendish haS' 

 concluded more directly, from experiments on a mass of lead, that the mean 

 density of the earth is to that of water as 54- to 1. Mr. Cavendish's experi- 

 ments, which were performed with the apparatus invented and procured by 

 the late Mr. Michell, appear to hare been conducted wi^th all f)ossible ac- 

 curacy, and must undoubtedly be preferred to conclusions drawn from the 

 attraction of a mountain, of which the internal parts are perfectly unknown 

 to us, except by conjectures founded on its external appearance. Supposing 

 both series of experiments and calculations free from error, it will only follow 

 that the internal parts of Shehallion are denser, and perhaps more metallic, 

 than was before imagined. The density assigned by Mr. Cavendish is not 

 at all greater than might be conjectured from observations on the vibrations of 

 pendulums; Newton had long ago advanced it as a probable supposition that 

 the mean density of the earth might be about 5 or 6 times as great as that of 

 water', and the perfect agreement of the result of many modern experiments 

 with this conjecture affords us a new proof, in addition to many others, of 

 the accuracy and penetration of that illustrious philosopher. 



