THE USE OF THE TERRESTRIAL GLOBE. 307 



LECT. 

 PROBLEM XVIII. 



The latitude of any place, not exceeding 6'b'i** degrees, and 

 the day of the month, being given ; to jind the time of 

 sun -rising and setting, and consequently the length of the 

 day and night. 



Having rectified the globe for the latitude, and for 

 the sun's place on the given day (as directed in the pre- 

 ceding problem) bring the sun's place in the ecliptic to 

 the eastern side of the horizon,, and the hour-index will 

 shew the time of sun-rising ; then turn the globe on its 

 axis, until the sun's place comes to the western side of 

 the horizon, and the index will shew the time of sun- 

 setting. 



The hour of sun-setting doubled, gives the length of 

 the day ; and the hour of sun-rising doubled gives the 

 length of the night. 



PROBLEM XIX. 



The latitude of any place, and the day of the month, being 

 given ; to Jind when the morning twilight begins, and the 

 evening twilight ends, at that place. 



This problem is often limited ; for, when the sun does 

 not go 18 degrees below the horizon, the twilight con- 

 tinues the whole night ; and for several nights together 

 in summer, between 49 and 66i degrees of latitude : and 

 the nearer to 663, the greater is the number of these 

 nights. But when it does begin and end, the following 

 method will shew the time for any given day. 



Rectify the globe, and bring the sun's place in the 

 ecliptic to the eastern side of the horizon ; then mark 



Note 84. All places whose latitude is more than 66j degrees, are in the 

 frigid zones : and to those places the sun does not set in summer, for a 

 certain number of diurnal revolutions, which occasions this limitation of 

 latitude, Note fry tht Author. 



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