COMMON THINGS THE EARTH. 



globular, and a diurnal rotation, would be that the surface would 

 consist of two vast polar continents separated by an extensive 

 equatorial ocean. 



Such not being the distribution of land and water on the earth, 

 it follows that its form cannot be that of an exact globe. 



35. It remains, then, to find means to ascertain by direct 

 measurement and observation, what is the actual form of the 

 earth. 



If a terrestrial meridian were an exact circle, as it would neces- 

 sarily be if the earth were an exact globe, every part of it would 

 have the same curvature. But if it were an ellipse, of which the 

 polar diameter is the lesser axis, it would have a varying curva- 

 ture, the convexity being greatest at the equator, and least at the 

 poles. If, then, it can be ascertained by observation, that the 

 curvature of a meridian is not uniform, but that on the contrary 

 it increases in going towards the Line, and diminishes in going 

 towards the Poles, we shall obtain a proof that its form is that of an 

 oblate spheroid. 



To comprehend the method of ascertaining this, it must be 

 considered that the curvature of circles diminishes as their 

 diameters are augmented. 



If, therefore, a degree of the meridian be observed, and 

 measured, at different latitudes, and it is found that its length is 

 not uniformly the same as it would be if the meridian were a 

 circle, but that it is less in approaching the equator, and greater 

 in approaching the pole, it will follow that the convexity or curva- 

 ture increases towards the equator, and diminishes towards the 

 poles ; and that consequently the meridian has the form, not of 

 a circle, but of an ellipse, the lesser axis of which is the polar 

 diameter. 



Such observations have accordingly been made, and the lengths 

 of a degree in various latitudes, from the Line to 66 N. and to 

 35 S., have been measured, and found to vary from 363000 feet 

 on the Line to 367000 feet atlat. 66. 



From a comparison of such measurements, it has been ascer- 

 tained that the equatorial diameter of the spheroid exceeds the 

 polar by r ^th of its length. 



Now this is precisely the form, precisely the degree of ellipticity, 

 which a globe, composed of fluid or soft materials, would assume if 

 it had a rotation on its axis once in twenty-four hours. 



Thus it appears, that the form of the earth, ascertained by 

 observation, supplies another proof of its diurnal rotation. 



36. It is not enough to know the proportions of the earth. It 

 is required to determine the actual dimensions of the spheroid. 

 The following are the lengths of the polar and equatorial diameters, 



62 



