62 OUTLINES OF NATURAL PHILOSOPHY. 



which the planes of these meridians make with 

 one another. 



The hour at any place, as we have already seen, is 

 determined by the passage of a certain star, or a 

 certain point in the heavens, over the meridian of 

 that place. But the star comes to any meridian 

 sooner than to another farther to the west, by a 

 space of time which is to 24? hours as the angle 

 made by the two meridians to 360 degrees. For 

 every 15, therefore, contained in the angle which 

 the meridians make, or in the difference of longi- 

 tude, one hour is to be reckoned, by which the ac- 

 count of time at the more westerly place is later, 

 or the clock slower than at the other. For all 

 other angles, the proportion is the same ; for one 

 degree, four minutes of time ; for one minute of a 

 degree, four seconds of time, &c. 



66. If, therefore, we could find under one me- 

 ridian the time which they reckon at the same 

 instant under another, we should have the dif- 

 ference of longitude, by converting the differ- 

 ence of time into degrees at the rate just men- 

 tioned. 



. An obvious way, therefore, of discovering the dif- 

 ference of longitude of any two places, is to have a 

 watch, or portable chronometer, well regulated, ac- 

 cording to the time at one of the places, and then 

 to carry it to the other ; where, on being compared 



with 



