THE EARTH. 



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tent and in the degree which it ought to do in virtue of the earth's diurnal 

 motion. 



SPHEROIDAL FORM OF THE EARTH. 



Although the earth be said to be a globe in the ordinary sense of the term, 

 and when extreme accuracy is not sought, yet, strictly speaking, it deviates 

 from the globular form. It has been ascertained that its figure is that which 

 in geometry is called an oblate spheroid. To acquire a notion of this form, we 

 have only to imagine an oval, such as A B C D, fig. 11, to revolve upon its 

 short axis B D. The figure it would produce by such a revolution would be 

 an oblate spheroid. It will differ from that of a sphere, inasmuch as the polar 

 diameter B D will be shorter than the equatorial diameter A C. 



Fig. 11. 



A familiar example of this figure is presented by a turnip, or in a less ex- 

 aggerated form by an orange. 



The degree in which the earth has this peculiar form is, however, so very 

 slight, that if we made a model of it in a lathe, the eye could not discover that 

 it was not a true globe. Its oblateness could only be detected by accurate 

 measurement, or by causing it to revolve in different positions in the lathe, and 

 applying to it a tool fixed on a rest. In fact, the equatorial diameter of the 

 earth is to the polar diameter in the proportion of three hundred and one to 

 three hundred ; or, in other words, the diameter of the equator exceeds the 

 length of the polar axis by one part in three hundred. If, then, we take in 

 round numbers the polar diameter to be eight thousand miles, we shall find the 

 equatorial diameter to be eight thousand and twenty-six miles ; thus the parts 

 of tha earth's surface at the equator are twenty-six miles further from the centre 

 of the earth than the parts near the poles. 



Such being understood to be the real figure of our globe, it will be asked 

 how it has been ascertained to be so. This question may be examined in 

 either of two ways either as one of theory or one of fact. We may show, 

 that, from the known laws of mechanics, a globe like the earth revolving on an 

 axis in twenty-four hours, must become an oblate spheroid of the above dimen- 

 sions ; or we may show by measurements made on different parts of the earth's 

 surface, that it is, in fact, such a spheroid, whatever cause may have imparted 

 that figure to it. 



It is well known that when any particle of matter revolves in a circle, it has 

 a tendency to recede from the centre of the circle, in virtue of what is called 

 centrifugal force. Now all points on the surface of the earth revolve very 

 rapidly in circles by reason of the diurnal motion of the globe. Any point, for 

 example, on the equator, revolves in a circumference of twenty-five thousand 

 miles in twenty-four hours. A point at a higher latitude revolves in the same 

 time in a less circle ; arid the circles of diurnal revolution become gradually 



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