180 FEOM NEBULA TO NEBULA 



explicit statement from no less an authority than Sir 

 John Herschel (Outlines of Astronomy, Art. 224) : 



Now, it is highly satisfactory to find that the general elliptical 

 figure thus practically proved to exist, is precisely what ought 

 theoretically to result from the rotation of the earth on its axis. 

 For let us suppose the earth a sphere, at rest, of uniform mate- 

 rials .throughout, and externally covered with an ocean of equal 

 depth in every part. Under such circumstances it would ob- 

 viously be in a state of equilibrium ; and the water on its surface 

 would have no tendency to run one way or the other. Suppose, 

 now, a quantity of its materials were taken from the polar regions, 

 and piled up all around the equator, so as to produce that differ- 

 ence of the polar and equatorial diameter of 26 miles which we 

 know to exist. It is not less evident that a mountain ridge or 

 equatorial continent, only, would be thus formed, down which the 

 water would run into the excavated part at the poles. However 

 solid matter might rest where it was placed, the liquid part, at 

 least, would not remain there, any more than if it were thrown on 

 the side of a hill. The consequence, therefore, would be the 

 formation of two great polar seas, hemmed in all around by 

 equatorial land. Now, this is by no means the case in nature. 

 The ocean occupies indifferently all latitudes, with no more 

 partiality to the polar than to the equatorial. Since, then, as we 

 see, the water occupies an elevation above the center no less than 

 13 miles at the equator than at the poles and yet manifests no 

 tendency to leave the former and run toward the latter, it is evi- 

 dent that it must be retained in that situation by some adequate 

 power. No such power, however, would exist in the case we 

 have supposed, which is therefore not conformable to nature. In 

 other words, the spherical form is not the figure of equilibrium, 

 and therefore the earth is either not at rest, or is so internally 

 constituted as to attract the water to its equatorial regions, and 

 retain it there. For the latter supposition there is no prima facie 

 probability, nor any analogy to lead us to such an idea. The 

 former is in accordance with all the phenomena of the apparent 

 diurnal motion of the heavens ; and, therefore, if it will furnish us 

 with the power in question, we can have no hesitation in adopting 

 it as the true one. 



Now, everybody knows that when a weight is whirled round, 

 it acquires thereby a tendency to recede from the centre of its 

 motion; which is called the centrifugal force. A stone whirled 

 round in a sling is a common illustration ; but a better for our 

 present purpose will be a pail of water, suspended by a cord and 

 made to spin round, while the cord hangs perpendicularly. The 

 surface of the water, instead of remaining horizontal, will become 

 concave. The centrifugal force generates a tendency in all the 



