176 J. L. LOIJLEY — THE STUDY OF GEOLOGY. 



opinion that the earth was at one time a fluid or plastic mass, orat least 

 that it either has, or has had, a yielding exterior shell, or crust, and 

 that the present shape of the globe is entirely due to the combined 

 action of gravitation and centrifugal force. The amount of flat- 

 tening at the poles is, however, very slight, as will be perceived 

 from a consideration of the size of the earth, given in the following 

 dimensions. The longer, or equatorial diameter, has been ascer- 

 tained to be 7925"648 miles, and the shorter, or polar diameter, 

 7899-170 miles, so that the equatorial exceeds the polar diameter 

 by 26*478, or rather less than 26i miles, which is not more than 

 the l-300th part of the diameter ; the circumference round the 

 equator, or in other words the length of the equator, is 24,912 

 miles ; and the entire surface of the globe is computed to be about 

 197,552,160, or nearly 200 millions of square miles, and the cubic 

 contents, 263,858,149,120 cubical miles. 



These figures, however, great though they be, convey a very 

 inadequate idea of the vastness of the globe on which we dwell. 

 It has been estimated that were we able to see an entirely fresh 

 portion of the earth's surface every day, and were we able to see 

 on each day an extent of surface 80 miles in diameter, or an area 

 of 5,000 square miles, it would require no less than 110 years 

 to enable us to survey the whole earth even in this most rapid 

 manner ; or, again, were a man to wish to travel over every square 

 mile of the earth's surface, at the rate of 30 miles every day, it 

 would require 18,264 years for the completion of such a tour. 



The calculations of astronomers have given us the weight of 

 this vast globe with great accuracy, and we find that the specific 

 gravity of the whole earth is about bb, or 5^ times the weight of 

 water, at 60° Fahr. The result of a calculation made by the 

 Astronomer Royal is as high as 6-56, but the officers of the 

 Ordnance Survey give only 5-32, and a very reliable one by Mr. 

 Baily is 5-67. The majority of results approximate to the lower 

 rather than to the higher figures above given, and we may there- 

 fore consider 5-5 to be very near the truth. Now, the specific 

 gravity, or density, as it is termed, of ordinary rock-substances or 

 of those substances which we find composing the earth wherever we 

 can observe its structure, is 2-5. It is assumed from the weight 

 of our globe that the interior is difierently composed from that 

 portion which has come under human observation. It is, however, 

 because of the lightness of the globe, and not because of its heavi- 

 ness, that this is inferred. For rocks, although having a density 

 of only 2*5 at the surface, would, were they nearer the centre, 

 have a greater specific gravity, which would continuously increase 

 with approximation to the centre. The whole globe would there- 

 fore, it is said, were it composed entirely of the rocks we know, 

 be very much heavier than we find it to be. 



From this fact, from the flattening at the poles, from volcanic 

 phenomena, and from the increase of warmth as we descend below 

 the 'surface in mines, it is supposed that our world consists of a 

 shell or crust, comparatively thin, and having the interior fiUed 



