THE EARTH. 



moon passes directly behind the earth, so that the shadotv winch th -earth pro* 

 jects behind it in the directkm opposite to the sun shall full upon the moon, 

 we invariably find that -shadow- to be, not as is commonly said, circular, but 

 such exactly as one globe would project upon the surface of another globe. 

 Now, as this takes place always* ift whatever position the earth may be, arnl 

 while the earth is revolving rapidly with its diurnal motion upon its axis, it. 

 follows that the earth must either be an exact globe or so little different from 

 a globe Unit its deviation from that figure is undiscoverable in its shadow. 



We may, then, consider it demonstrated that the earth may be practically 

 regarded u4 globular in its form. We shall hereafter see that it sligbtly 'de- 

 parts from the spherical figure, but our present purpose will be best hnswerei 

 by regarding it as a globe. 



- The. .objection will doubtless occur to many minds that the inequality which 

 .exists on the surface of that portion of the globe that is covered by land, espe- 

 cially the loftier ridges of mountains, such as the Andes, the Alps, the Hima- 

 laya, and others, are incompatible witli the idea of a globular figure. If the 

 term globular figure were used in the strictest geometrical sense, this objection 

 doubtless, would have great force. But Jet us see the real extent of this pre- 

 sumed deviation from the globular form. The highest mountain on the surfaceof 

 the globe does not exceed five miles above the general level of the sea. The 

 entire diameter of the globe, as we shall presently see, is eight thousand miles. 

 The proportion, then, which the highest summit of the loftiest mountains bears 

 to the entire, diameter of the gloi>o will be that of fife to eight thousand, or one 

 to sixteen hundred. If we take an ordinary terrestrial globe of sixteen inches 

 in diameter, each, inch upon the globe will correspond to five hundred miles 

 upon the earth, and the sixteen hundredth part of its diameter, or the hundredth 

 part of an inch, will correspond to five miles. If, then, we take a narrow atrip 

 of paper, so thin that it would take one hundred leaves to make an inch in 

 thickness, and paste such a strip on the surface of the globe, the thickness of 

 the strip would represent upon the sixteen-inch globe the height of the loftiest 

 mountain on the earth. We are then to consider that the highest mountain- 

 ranges on the earth deprive it of its globular figure only in the same degree 

 and to the same extent as a sixteen-inch globe would be deprived of its globu- 

 lar figure by a strip of paper pasted upon it the hundredth part of an inch 

 thick. 



It is supposed that the greatest depth of the ocean which covers any portion 

 of the globe does not exceed the greatest height of the mountains upon the 

 land. If this be true, the ocean upon the earth might be represented by a film 

 of liquid laid with a camel's-hair pencil upon the surface of a sixteen-inch 

 globe. 



It is apparent, therefore, that depths and heights which appear to the com- 

 mon observer to be stupendous, are nothing when considered with reference 

 to the magnitude of the earth; and that, so far as they are concerned, we inay 

 practically regard the earth as a true globe. 

 .(i 



THE MAGNITUDE OF THE EARTH. 



Having ascertained satisfactorily the figure of the earth, our next inqtriry 

 must be us to its magnitude ; and since it is a globe, all that vre are required to 

 know is the length of its diameter. 



If a line were described surrounding the globe, so as to form a circle upon 

 it, the centre of which should be at the centre of the globe, such a" circle is 

 called a grat circl-e of the earth. Now if we know the length of the circum- 

 ference of such a circle, we could easily calculate the length of its diameter, 



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