308 FERGUSON'S LECTURES. 



LECT. that point of the ecliptic with a chalk which is in the 

 ii^J^' western side of the horizon, it being the point opposite 

 to the sun's place : this done, lay the quadrant of alti- 

 tude over the said point, and turn the globe eastward, 

 keeping the quadrant at the chalk-mark, until it is just 

 IS degrees high on the quadrant ; and the index will 

 point out the time when the morning twilight begins : 

 for the sun's place will then be 18 degrees below the 

 eastern side of the horizon. To find the time when the 

 evening twilight ends, bring the sun's place to the wes- 

 tern side of the horizon, and the point opposite to it, 

 which was marked with the chalk, will be rising in the 

 east: then, bring the quadrant over that point, and 

 keeping it thereon, turn the globe westward, until the 

 said point be 18 degrees above the horizon on the 

 quadrant, and the index will shew the time when the 

 evening twilight ends ; the sun's place being then 18 de- 

 grees below the western side of the horizon. 



PROBLEM XX. 



To Jind on what day of the year the sun begins to shine con- 

 stantly without setting, on any given place in the north 

 frigid zone ; and how long he continues to do so. 



Rectify the globe to the latitude of the place, ami 

 turn it about until some point of the ecliptic^ between 

 Aries and Cancer coincides with the north point of l!.; 

 horizon where the brazen meridian cuts it : then find, 

 on the wooden horizon, what day of the year the sun is 

 in that point of the ecliptic ; for that is the day on which 

 the sun begins to shine constantly on the given place, 

 without setting. This clone, turn the globe until sonic 

 point of the ecliptic, between Cancer and Libra, coin- 

 cides with the north point of the horizon, where the 

 brazen meridian cuts it; and find, on the wooden hori- 

 zon, on what day the sun is in that point of the ecliptic ; 

 which is the day that the sun leaves off constantly shir>- 



