TO FIND THE LONGITUDE. 



noting the times at which these two points would cross the 

 meridian of the place whose longitude is to be determined. 



To comprehend fully the spirit of the celebrated problem of 

 finding the longitude, we must imagine the globe of the earth 

 turning on its axis, having around it the starry firmament. Let 

 us suppose A B c, a b c, to be the parallel of latitude of the place 

 P, whose longitude is to be determined, p being the pole, and let 

 M z N represent the firmament. Let the zenith be the point on 



Fig. 3. 



the firmament marked by z. If we suppose the globe to turn 

 upon its axis in the direction of A B c, a b c, the place p will, by 

 its rotation be carried to the right of z, and the same point z 

 will become successively the zenith of the points c B and A ; and, in 

 fact, every point in the parallel of latitude will successively come 

 under the point z, which will be, therefore, in regular succession, 

 their zenith points. In twenty-four hours, or, more accurately, 

 in twenty-three hours and fifty-six minutes, the globe will make 

 its complete revolution ; therefore three hundred and sixty 

 degrees of the parallel will successively pass under the same 

 point of the firmament. 



107 



