MAGNITUDE. 



mountain-ranges on the earth, deprive it of its glohular figure only 

 in the same degree and to the same extent as a sixteen-inch globe 

 would be deprived of its globular 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 common observer to be stupendous, are nothing when con- 

 sidered with reference to the magnitude of the earth ; and that, so 

 far as they are concerned, we may practically regard the earth as 

 a true globe. 



7. Having ascertained satisfactorily the form of the earth, 

 our next enquiry must be as to its magnitude ; and since it 

 is a globe, all that we 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 great circle of the earth. Xow if 

 we know the length of the circumference of such a circle, we could 

 easily ffclculate the length of its diameter, for the proportion of 

 the circumference to the diameter is exactly known. But we 

 could calculate the circumference if we knew the length of one 

 degree upon it, since we know that the circumference consists of 

 three hundred and sixty degrees ; we should therefore only have 

 to multiply the length of one degree by three hundred and sixty 

 to obtain the circumference, and should thence calculate the 

 diameter. 



8. In our tract upon latitudes and longitudes, it was shown how 

 the latitude of a place can be ascertained. Now, let us suppose 

 two places selected which are upon the same meridian of the earth, 

 and therefore have the same longitude, and which are not very 

 far removed from each other. Let them, moreover, be selected 

 so that the distance between them can be easily and accurately 

 measured. Xow let the latitude of these two places be exactly 

 determined, and let us suppose for example that the difference 

 between these two latitudes is found to be one degree and a half ; 

 and supposing also that on measuring the distance between them, 

 that distance is found to be one hundred and four miles and thirty- 

 five hundredths. We should thence infer that such must be the 

 length of one degree and a half of the earth's surface, and that 

 consequently the length of one degree would be two thirds of this, 

 or sixty-nine and a half miles. Having thus found the length of 



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