ZONES. 



ing its rays perpendicularly downwards, produces the greatest 

 possible calorific effect. 



At these times, also, the sun, rising precisely in the east, 

 ascends the heavens at right angles to the horizon, until at noon it 

 reaches the zenith ; after which it descends perpendicularly in like 

 manner, and sets precisely in the west. 



174. Sun vertical twice a year in the Torrid Zone. 

 Every point of the torrid zone is presented, at one period or 

 another of the year, directly to the sun, so that there are two 

 days in the year upon which the sun at noon is vertical at such 

 places, and its extreme departure from the zenith at noon is 

 always very limited. Thus to places on the Line, the sun at noon 

 never departs from the zenith more than 23|, and even at the 

 limits of the torrid zone, that is, at the latitude 23^ north or 

 south, the sun which on the 21st of June is vertical at noon in 

 the northern, and on the 21st December in the southern hemi- 

 sphere, is not more than 47 from the zenith at noon on the 2 1st of 

 December in the northern, and on the 21st of June in the southern 

 hemisphere ; for all places within the tropics the extreme distance 

 of the sun from the zenith at noon must always be less than 47. 



Indeed it may be stated generally, that for places between the 

 tropics, the greatest distance of the sun at noon from the zenith 

 will be found by adding 23| to the latitude of the place, and 

 that twice in the year it passes through the zenith at noon. 



When it is considered that the temperature of the weather 

 mainly depends on the distance of the sun from the zenith at noon, 

 increasing as that distance decreases, it will be easily understood 

 why the climate of the torrid zone is characterised by that ex- 

 tremely elevated temperature from which it takes its name ; for 

 although the sun at noon, during a certain part of the year, is at 

 a distance more or less considerable from the zenith, the interval 

 during which it has this distance is comparatively short, and that 

 during which it is in or near the zenith much more considerable. 



The hottest seasons occur, not as might be first supposed, upon 

 the Line, but at and within a limited number of degrees of the 

 tropics; because at such latitudes the sun at midsummer con- 

 tinues to cross the meridian very near to the zenith for a much 

 more considerable time than on the Line at the epochs of the 

 equinoxes, where its change of declination is much more rapid. 



175. Temperate Zone.The parts of the globe included 

 between the limits of the torrid and frigid zones, that is between 

 the latitudes 23| and 66|, is exempt from the extremes of 

 temperature which characterise the one and the other of these 

 regions. Within its limits the sun never ascends to the zenith, 

 nor on the other hand are the phenomena of continuous day or 



175 



