MEASUREMENT OF A DEGREE. 23 



earth, is divided into 360 parts, and these subdivided into sixty 

 parts each, and their length ascertained, that it would take 8000 of 

 them to measure the diameter of the earth. The length of a mile 

 therefore, instead of determining the diameter of the earth, or its 

 circumference, is itself determined by that diameter or circum- 

 ference. The circle might have been divided into 1000 parts, and 

 these subdivided into 100 each, this would give 10,000 minutes 

 or miles for the circumference, but the mile in this case would be 

 shorter. Having assumed the earth's circumference 24,000 miles, 

 we next desire to know when we have passed over a mile on its 

 surface. This would seem a difficult undertaking at first thought, 

 for how can we determine when we have passed over a degree 

 upon the earth ? A diagram will explain the manner this is 



accomplished. Let A B C D represent the earth, A C being the 

 equator. A spectator at the pole B, would see the pole star directly 

 overhead, but a spectator at A, on the equator, would see the pole 

 star in the horizon. Hence, in travelling from the north pole to 

 the equator, the elevation of the pole star changes from directly 

 overhead, or in the zenith as it is called, to the horizon, or 90, 

 changing its altitude 1 for every degree traveled over the earth's 

 surface, either north or south. - The astronomer is furnished with 

 the means of measuring the altitude of the pole star, or its 

 distance above the horizon by means of the quadrant, or the 

 astronomical circle which we shall describe, together with some 

 other astronomical instruments in the next chapter. We have 



