COMMON THINGS THE EARTH. 



afterwards sees them from the deck long before the general level of 

 the country will be observed by him. All these are natural and 

 necessary consequences of the convexity of the surface of the 

 ocean. The same effects would be seen in any part of a continent 

 which is sufficiently free from mountains and other inequalities. 



5. But we have a still more conclusive and convincing proof of 

 the general form of the earth even than those which have been 

 explained. "When the moon passes directly behind the earth, so 

 that the shadow which the earth projects behind it in the direction 

 opposite to the sun shall fall 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, in whatever position the 

 earth may be, and 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 that its devia- 

 tion from that figure is undiscoverable in its shadow. 



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

 be practically regarded as globular in its form. "We shall here- 

 after see that it slightly departs from the spherical figure, but 

 our present purpose will be best answered by regarding it as a 

 globe. 



6. 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, especially the loftier ridges of moun- 

 tains, such as the Andes, the Alps, the Himalaya, and others, are 

 incompatible with the idea of a globular figure. If the term 

 globular figure were used in the strictest geometrical sense, this 

 objection doubtlessly would have great force. But let us see the 

 real extent of this presumed deviation from the globular form. 

 The highest mountain on the surface of 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 globe will be that 

 of five 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 strip 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 



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