THE EARTH. 467 





would have a tendency to move them toward the line that is, ascending the 

 acclivity while their gravity, on the other hand, would have a tendency to 

 make them descend, or to move them from the acclivity. When the protu- 

 berance would attain the limit at which these two tendencies would become 

 equal, so that the descending force of gravity should be equal to the ascending 

 force proceeding from the rotation, the particles of the fluid would be at rest, 

 and would neither approach the line nor recede from it. It is within the prov- 

 ince of mathematical physics to calculate what the limit of this protuberance 

 would be which would produce this state of equilibrium, and the result of such 

 calculations has given us a form which corresponds nearly to that which the 

 earth is actually found to have. 



But it may be objected that such reasoning would apply only to fluid matter 

 upon the earth, whereas the oblate form is known to belong to its solid as well 

 as its fluid surface. 



This circumstance has been explained in two ways. 1. It is said that the 

 earth in its original formation was altogether fluid ; that in that fluid state it 

 received its diurnal rotation, and consequently took the form corresponding 

 with that rotation which we have just explained ; that, by cooling down, the 

 fluid matter partially hardened into a solid matter, leaving the liquid ocean cov- 

 ering about two thirds of the globe. 



But if this original fluid state of the globe be denied or doubted, and if it be I 

 maintained that the globe received its revolution upon its axis when it was com- 

 posed as it is, partly of land and partly of water, it is nevertheless contended 

 that its present figure is explicable. If a true globe, diversified by land and by 

 water, received a diurnal rotation like that of ours, the water would in the first 

 instance flow toward the equator, and the geographical condition of the globe 

 would be, two polar continents, separated by an extensive equatorial ocean. 

 But after the lapse of ages, the ocean, washing continually upon the shores of 

 the continents, would cause the constant abrasion of their solid matter, which, 

 in the form of mud and sand, would mix with the liquid of the ocean, and would 

 obey all its tendencies. In fact, in process of time the land by decadence and 

 abrasion would obey the same principles which would affect a fluid ; and the 

 earth would at length, though after a long lapse of time, assume the form of 

 fluid equilibrium. The present distribution of land and water which characterizes 

 it has arisen from causes belonging more properly to geology than astronomy. 



Such is the theoretical reasoning applicable to the form of the earth. We are 

 still, however, required by the rigorous principles of inductive philosophy to 

 ascertain, as a matter of fact, independent of all theory, the actual figure of the 

 globe. This has accordingly been done. 



The section of an oblate spheroid made by a plane passing through the poles, 

 is au oval, the longer axis of which is in the equator. It will be evident upon 

 mere inspection that the curvature of the earth having such a form, would in- 

 crease as we approach the equator, and diminish as we approach the poles ; 

 that i% to say, a piece of a meridian taken near the equator would be part of a 

 less circle than a similar piece taken near the poles. This is equivalent to 

 stating that a degree of latitude near the equator would be shorter than a degree 

 of latitude near the poles. 



Thus, then, the question of the figure of the earth is in fact resolved into the 

 measurement of a degree of latitude at different parts of the globe. 



Such measurement has accordingly been executed with great precision, and 

 it has been found, as was anticipated, that the degrees of latitude become 

 shorter as we approach the equator, and longer as we approach the poles. A 

 comparison of their lengths has given the degree that characterizes the oblate- 

 ness of the earth. 



