538 



: LATITUDES AND LONGITUDES. 



) to the west of P ; and, for the same reason, it is ten o'clock at a place thir'y 



? degrees to the west of P, and so on. 



Again : if O be a place fifteen degrees to the east of P, O must have been 

 under Z an hour before P reached it. It will be noon, therefore, at O, an hour 

 before it is noon at P ; therefore, when it is noon at P it is one o'clock at 0. 

 In the same manner, and for like reasons, if N be a place thirty degrees east of 

 P, N will pass under Z two hours before P ; and therefore when P passes under 

 Z it will be two o'clock at N. 



It will be apparent from these explanations, that, in general, the hour of the 

 day at different places upon the earth, at the same time, will depend upon their 

 relative position east or west of each other. If one place be east of another, 

 the hour at that, place will be later with respect to noon than the hour at the 

 other ; and the extent to which it is later will depend on the distance which 

 one place is east of the other. In calculating this difference of time from the 

 difference of position east or west, we may take fifteen degrees to correspond 

 with an hour, as already explained. 



But this distance of one place east or west of another, expressed in degrees, 

 is, in fact, the difference of their longitudes ; and if one of the tv/o places in 

 question be that from which the longitudes are measured, the determination of 

 the longitude of a place would resolve itself into the discovery of the hour 

 of the day in the place whose longitude we, want to find, and also at the place 

 from which the longitudes are measured. 



Thus, for example, let us suppose that we ascertain the hour of the day in 

 New York, and find that it is 2 o'clock in the afternoon, and that we have a 



