T2 THE SOLIDITY OF THE EARTH. 



possibly have existed. The argument is as follows : The moon at the 

 end of a century gets to be between five and six seconds of time in 

 advance of her natural place in the heavens with regard to the earth. 

 It was suggested that this difference probably resulted from the extent 

 to which the earth's movement is retarded by the friction against it of 

 tidal waters. And when this idea was worked out it was found that 

 the tides were competent to produce a retardation of the earth four 

 times as great as the difference between the motion of the earth and 

 the moon. Hence it followed, if the data for the calculation are 

 correct, that 10,000,000 years ago the earth must have been rotating 

 ith faster than it rotates now, and therefore that the centrifugal force 

 must have been to the centrifugal force now in the proportion of 64 

 to 49. So that if the earth had consolidated at that remote period, 

 the ellipticity of its upper layers would have been a curve of i in 

 230, instead of a curve of nearly i in 300, which the earth actually has 

 now. Therefore, allowing for all possible errors, Sir William Thomson 

 concludes that the consolidation of the earth from a molten state (if it 

 were ever fluid) must have taken place at some period considerably 

 more recent than 1,000,000,000 years ago ; and since that far-off period 

 when the earth began to consolidate, consolidation has progressed till 

 it is probable that at its centre no part of the earth is now fluid. The 

 evidence on which this conclusion rests is remarkably forcible, for if the 

 earth's interior were to any large extent fluid, our planet would behave 

 as a fluid body, and be drawn out of shape by the attracting forces ex- 

 ternal to it, just as the sun and moon draw the ocean towards them in 

 tidal movement, and tides would not exist. On the contrary, however, 

 the earth is extremely rigid, probably as rigid as though formed through- 

 out of solid cold steel, and would appear only to yield a few inches to 

 the attracting powers which produce tides, so that there can be no 

 ground for assuming a fluid interior, when it must be so far removed 

 from the surface, even if existing, as to be inappreciable by any effects 

 manifested at the surface. 1 



Evidence of the Globular Form of the Earth. The figure of the 

 earth has been determined more accurately by actual measurement ; 

 but its globular form has been long demonstrated by experience and 

 observation of the following facts : 



First, during an eclipse of the moon the shadow of the earth is 

 cast upon the moon so as to exhibit under all circumstances a nearly 

 circular outline ; and since this outline is sometimes cast by different 

 aspects of the earth, the demonstration by this means of the earth's 

 globular form is perfect. The Egyptians, Babylonians, and early 

 Greeks all were aware of this evidence of the earth's shape. Secondly, 

 navigators such as Ferdinando Magellan in 1519, Sir Francis Drake 

 in 1577, Sir Thomas Cavendish in 1586, and many others, sailing con- 

 stantly to the west have returned to the point in Europe from which 

 they started, and thus by sailing round the world have demonstrated 

 the earth's spherical form. Thirdly, by ascending increasing heights 

 above a plain in any part of the world, the distance to which the eye 

 1 Thomson and Tait's "Natural Philosophy.*' 



