MERCURY VENUS THE EARTH. 



71 



evidence of the convexity of the water between the eye and the object. "We also find, that 

 during a lunar eclipse the shadow projected by the earth upon the disk of the moon is 

 always of a circular shape. The common occurrence now of a voyage round the world, 

 proceeding in the same general direction, east or west, and arriving at the same point 

 again, demonstrates the figure of the earth to be either that of a sphere or a cylinder ; 

 and the latter is disproved, and the convexity of the surface shown, north and south, by 

 the gradual declination or rise of the north and south circumpolar stars, as the equator is 

 approached or receded from. Our terrestrial mansion, therefore, is a vast mass of matter 

 of a spherical form like the planets whose round disks are the objects of telescopic 

 observation. The spherical figure of the planetary bodies a result of the law of gravi- 

 tation is, on many accounts, the best shape they could have assumed. The same phe- 

 nomena could not have been offered to their surfaces, with the same machinery, supposing 

 any other form. Had the earth been a rotating cylinder, the solar beams could not 

 have reached its two extremities together, or its general superficies with either extremity. 

 But it is only an approximation to the truth of its actual shape, to speak of our world as 

 having a spherical form. It is not a globe whose circumference is everywhere at an 

 equal distance from the centre, a property essential to a sphere. A process of reasoning 

 led Newton to the conclusion, that the circle of the earth's daily rotation upon its axis 

 being the greatest at the equator, the consequent greater action there of the centrifugal 

 force would produce a bulging out of the surface in the equatorial regions of a yielding 

 mass, and a flattening at the poles ; and this deduction from the laws of forces has been 

 proved to be correct by the actual measurement of the lengths of degrees of the meridian, 

 made with care and precision by the commissioners of various nations. The certain con- 

 clusion obtained is, that our globe is an oblate spheroid, an orange-shaped ball, compressed 

 at the poles, and elevated at the equator, having the following dimensions : 

 Diameter at the equator 7925-648 miles 



Diameter at the poles 7899 '140 



The length of the axis of the poles is thus about twenty-six miles and a half less than the 

 diameter of the equator. It is highly improbable that any error of importance exists in 

 this measurement, founded upon the principle first employed by Eratosthenes, when he 

 attempted to determine the value of an arc of the meridian between Alexandria and Syene. 



Syene. 



