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THE POPULAR SCIENCE MONTHLY 



in the direction S and is immediately over the point A of the earth's 

 surface. Its altitude there is 90°. As we pass along the earth's sur- 

 face away from A, the direction of the horizon continually changes and 

 the altitude of the sun continually diminishes until we reach the great 

 circle, BC, which divides the light from the dark hemisphere, and 

 there the sun is on the horizon and its altitude is zero degrees. 



-8 HP 



Fig. 1. 



As the earth is spherical, and therefore symmetrical round the line 

 OS, if we draw a circle DE on its surface, with its plane at right angles 

 to this line, the altitude of the sun, as seen from all parts of this circle, 

 will be the same ; at the point D this altitude will be represented by h, 

 the angle between the direction of the sun DS' and the horizon; for 

 the sun is so distant from the earth, that its direction is the same from 

 the center of the earth and from any point of the surface, to the degree 

 of accuracy required by explorers. Every part of the circle DE is at 

 right angles to the direction of the sun. The altitude of the sun 

 changes with, and is determined by, the distance of the circle DE 

 from A ; and, vice versa, if the altitude is known, the distance of the 

 circle from A is determined. The point A, itself, is fixed when we know 

 Greenwich time and the angular height d of the sun above the equator ; 

 this latter is called the declination; it is continually changing, but its 

 value at any time can be found in the " Nautical Almanac." An ex- 

 plorer would always take with him a copy of this work or an abbrevia- 

 tion of it; and he would also be supplied with chronometers keeping 

 Greenwich time. 



If an explorer has measured the altitude of the sun, and has at the 

 same time observed the Greenwich time by his chronometer, he has 

 merely determined that he is somewhere on a certain circle, whose 

 position he could plot on his map; but other considerations, such as 

 his last determined location, and the approximate distance he had 

 traveled from it, would make known more or less roughly in what part 



