318 FERGUSON'S LECTURES. 



LECT. then of the same length at all the rest ; for in turning 



TII & TV 



>v^;' the globe round its axis (when rectified according to the 

 sun's declination) all these places will keep equally long 

 above or below the horizon. 



12. The sun is vertical twice a year to every place 

 between the tropics ; to those under the tropics, once 

 a year, but never any where else. For, there can be no 

 place between the tropics, but that there will be two 

 points in the ecliptic, whose declination from the equa- 

 tor is equal to the latitude of that place ; and but one 

 point of the ecliptic which has a declination equal to the 

 latitude of places on the tropic which that point of the 

 ecliptic touches ; and as the sun never goes without the 

 tropics, he can never be vertical to any place that lies 

 without them. 



13. To all places in the torrid zone,* the duration of 

 the twilight is least, because the sun's daily motion is 

 the most perpendicular to the horizon. In the frigid 

 zones,* 2 greatest ; because the sun's daily motion is nearly 

 parallel to the horizon ; and therefore he is the longer 

 of getting 18 degrees below it (till which time the twi- 

 light always continues.) And in the temperate zonesw 

 it is at a medium between the two, because the obliquity 

 of the sun's daily motion is so. 



14. In all places lying exactly under the polar circles, 

 the sun, when he is in the nearest tropic, continues 24 

 hours above the horizon without setting ; because no 

 part of that tropic is below their horizon. And when 

 the sun is in the farthest tropic, he is for the same 

 length of time without rising ; because no part of that 

 tropic is above their horizon. But, at all other times of 

 the year, he rises and sets there, as in other places ; be- 

 cause all the circles that can be drawn parallel to the 

 equator, between the tropics, are more or less cut by the 



Note 91. Between the tropics. Note by the Author, 

 Note 92. Between the polar circles and poles. Ibid. 

 Note 93. Between the tropics and polar circles. IbiiL 



