324 PHYSIOGRAPHY. [chap. 



the same star, or its height above their horizon. To the 

 man who travels northwards, the pole star will appear to 

 mount higher and higher in the heavens ; and, if the ice in 

 the arctic regions did not bar him from getting so far, he 

 would eventually find this star over his head. In fact, it will 

 be seen from Fig. loi that the pole star is in the zenith of 

 an observer at N. Bat, to the person who travelled south- 

 wards, the pole star would appear to be steadily sinking lower 

 and lower in the sky ; and, when he got midway between 

 the north and south poles of the earth, at the line called the 

 equator, he would find that the star actually seemed to touch 

 the horizon ; while, if he continued his course to the south, 

 it would disappear altogether. But the person who stayed 

 at home, at O, would not have observed any movement 

 in the position of the star. In fact, it may be assumed 

 for our present purpose, that this star is fixed ; and its regular 

 movement apparent to the travellers, has been due to their 

 own change of position on the earth's rounded surface, as 

 shown in the figure. This, therefore, proves that the earth 

 is convex, at least in a north-and-south direction. 



If the travellers, instead of going northwards and south- 

 wards, had taken their journey due east and due west, they 

 would not have observed any alteration in the altitude of 

 the pole-star. But, the traveller to the east would have 

 found that the sun rose earlier and set earlier than it did 

 when he was at O ; while the traveller to the west would 

 have found that it rose later and set later. It can be shown 

 that this is a proof of curvature in an east-and-west direc- 

 tion ; and, therefore, by combining the two sets of observa- 

 tions the rounded shape of the earth's surface may be fully 

 established. 



Engineers and surveyors are in the habit of taking the 

 eaj-th's sphericity into account in their calculations. If, for 



I 



