450 THE EARTH. 



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. 



On another occasion, in our discourse 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 there- 

 fore 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. Now let the latitude of these two places be 

 exactly determined, and let us suppose that the difference between these two 

 latitudes is found to be one degree and a half; and suppose als-0 that on meas- 

 uring the distance between them, that distance is found to be one hund-red 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 conse- 

 quently the length of one degree would be two thirds of this, or sixty-nine and 

 a half miles. Having thus found the length of a degree, AVC should have to 

 multiply it by three hundred and sixty, by which we should obtain the circum- 

 ference of the earth. This would give twenty-five thousand and twenty miles, 

 and we should then find by the usual mode of calculation the diameter of the 

 earth, which would prove to be a little under eight thousand miles. 



We have made these calculations chiefly with a view of rendering the prin- 

 ciples of the investigation intelligible. The more exact dimensions of the 

 earth will be explained hereafter. 



We conclude, then, that the earth is a globe eight thousand miles in di- 

 ameter. 



ANNUAL MOTION OF THE EARTH. 



We have on other occasions shown that the distance of the earth from the 

 sun may be expressed in round numbers by one hundred millions of miles. It 

 is more exactly ninety-five millions of miles. 



We have also considered in general the path of the earth in its annual course 

 round the sun to be a circle, in the centre of which the centre of the sun is 

 placed. This is nearly but not exactly true. That the path of the earth is 

 not a circle with the sun in its centre, has been ascertained by the following 

 observations. 



In astronomical telescopes there are placed by a particular arrangemer.t, 

 within the eye-pieces, certain A'ery fine threads or wires, which are extended 

 parallel to each other across the field of view. These wires are so constructed { 

 that, by a simple mechanical contrivance, they may be moved toward each other, 

 preserving, however, their parallelism. The mechanism which so moves 

 them is made to measure exactly the distance between them. 



When such a telescope is presented to the sun or moon, the wires may al- 

 Avays be so adjusted, by turning a screw, that one of them shall touch the upper 

 and the other the lower limb of the disk, as represented in the annexed dia- 

 gram, fig. 2, where S represents the disk of the sun, and A B and C D the 

 wires. 



Now let us suppose that a telescope is pointed to the sun, and the Avires so 

 adjusted that they shall exactly touch the upper and lower limbs. Let the ob- 

 server then watch from day to day the appearance of the sun and the position ) 

 of the wires : he will find that, after a certain time, the wires will no longer ( 



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