148 TH w<mtr>. 



scribed. At these seasons of the year the changes from winter 1 

 to spring 1 , and summer to autumn, commence, and the sun is said 

 to be crossing the line. 



The distribution of heat in the neighborhood of the equator is 

 tolerably equal, for twice during the year, viz : March 21st and 

 September 21st, the sun's rays fall vertically, and they do not 

 fall very obliquely at any time between these two periods. From 

 the 21st of March, the sun begins to move northword of the equa- 

 tor apparently, until at the 2lst of June, its angular distance from 

 the equator amounts to 23 28'. This is the angle which the line 

 S S' makes witli the line E E', eeo the figure on page 57, the 

 former representing the plane of the ecliptic, the latter the plane 

 of the equator. At this time, as the earth turns on its axis, the 

 sun is vertical at noon at nil those places which lie in a cirele 

 drawn upon its surface parallel to the equator, and at an angular 

 distance of 23 28' north of it. This circle is called the tropic of 

 Cancer, for a reason we have already explained. From the 21st 

 of June to the 21 st of September, the sun approaches the equa- 

 tor, which it crosses on the latter named day, it then moves far- 

 ther south, until, on the 21st of December, its angular distance 

 from the equator becomes 23 28', and, if we suppose a circle 

 drawn upon the earth at a distance of 23 28' from the equator, 

 but south of it, the sun will now be vertical at all places situated 

 on or near to this circle, which, for reasons already given, is called 

 the tropic of Capricorn. All places therefore lying upon these 

 tropics, receive once in the year, the sun's rays perpendicularly at 

 mid-day, this being on the 2"lst of June for the tropic of Cancer 

 and the 21st of September for the tropic of Capricorn. At all 

 places within these two tropics the sun is vertical at noon twice 

 in the year j and at all places without or beyond them, it is never 

 vertical. The nearer \vc approach the tropic?, leaving the equa- 

 tor, the more marked are the difTerentsessons of the yf or, and for 

 the following reason; once during ths year, as we have just remark- 

 ed, the sun's rays fall vertically nt the tropics, and once they 

 make an angle of 47 or tw'ce ;23 n 2; : ;', witti the d?:-prtion of tho 

 plumb-line, and which is the angle S S' C', see fiVuro on page 

 57, falling conrequcntlv, with considerable obliquity. The hot- 



