PHYSIOGRAPHICAL GEOLOGY. r;$ 



eastern Alps and Carpathians yielded results which showed as 

 a rule relative " defects of mass " in the mountains, and 

 "surplus of mass" in the plains, and such results suggested 

 in geological circles, a correlation between crust-movements 

 and conditions of density in the crust. But, since the publica- 

 tion of these measurements, more recent observations taken in 

 the leading European and foreign observatories, have led to the 

 conclusion that there is no immediate connection between the 

 density of the earth's crust and the tectonic structure of the 

 crust. 



Pendulum observations are even more important for the 

 determination of the specific gravity of the earth than for 

 questions regarding its form. According to the law of gravita- 

 tion, the action of two masses is proportional to their size, and 

 inversely proportional to the square root of the distance of 

 their central points of attraction. Hence if a body be simul- 

 taneously subjected to the attractive forces of the earth, and of 

 another mass of some considerable gravity, the density of the 

 earth may be calculated from the result. 



The two Scotsmen, Maskelyne and Hutton, made in the 

 years 1774 to 1776 a series of admirable experiments at the 

 mountain of Schiehallion, in Perthshire. Their aim was to 

 arrive at the density of the earth by means of the pendulum 

 deviations in the presence of the mass of Schiehallion. The 

 size, form, and weight of the solitary mountain were calculated 

 by trigonometry, and the local deviations of the pendulum 

 were observed as the pendulum was brought into the neigh- 

 bourhood of the disturbing mass of Schiehallion , the result 

 was a gravity of 4.713 for the earth. Observations have since 

 been taken at many different parts of the world, and various 

 figures have been in later years given for the earth's gravity 

 (4.39, 6.62, and 5. 77). 



All determinations of the earth's gravity agree in showing 

 that the gravity of the earth as a whole is very much greater 

 than the gravity of the rocky crust, which has an average 

 gravity not exceeding 2.5. Thus we know the important geo- 

 logical fact that the interior of the earth is neither empty nor 

 can it be filled with water, but it must consist of substances of 

 very great weight. 



The Earttts Internal Heat and the Constitution of its 

 Interior. It has long been known that the heat of the sun 



