72 THE WORLD. 



C A of the great circle C H A, according to the latitude of the 

 spectator E. Now as the earth turns once on its axis in 24 hours, 

 it is evident that the several meridians P, P I, P II, PHI, &c., 

 will come successively under the sun at exact intervals of 1 hour, 

 if they are all 15 apart, for 24 multiplied into 15 gives 360, the 

 whole number of degrees to the circle. Suppose, for a moment, 

 that instead of the earth turning up on its axis, once in 24 hours, 

 that the sun moves around the earth in this time, the effect will 

 be the same If the sphere of the earth was transparent, but its 

 axis P D B opaque, then P D would, as the sun passed around, 

 cast a shadow in the directions D A, DI, DII, Dili, &c., when 

 the sun was in the apposite direction, and the progress of this 

 shadow would mark the hour, according to the meridian in which 

 it should fall. It will be observed, that the intervals A-I, I-II, II- 

 III, are not equal intervals, but vary, because the circle C H A, 

 cuts the meridians obliquely. Now the sun is so far distant, that 

 if the observer at E should locate a horizontal plane, which, of 

 course, would be parallel to the large plane C H A, and describe 

 on it a small circle, and then divide this circle in proportion as 

 the meridians divide the large circle C H A, and should, likewise, 

 erect from its centre a gnomon, or shadow stick, inclined so as to 

 point to the north star, or in other words, to be parallel to P D, the 

 progress of this shadow would mark the hour. We have here, 



then, the principle of the horizontal Sun-dial, and all that is 

 necessary to construct one, is, to graduate it proportionally accord- 

 ing to the latitude. This can easily be done by calculation, which, 

 however, would involve more of mathematical skill than we shall 

 suppose the reader to possess ; we will, therefore, show how it 

 may be done experimentally, and thus any one, with the least 

 ingenuity, can construct a horizontal dial. Referring back 



