CLIMATE. 



than the decrement on any succeeding day ; but still on these 

 succeeding days there is still a decrement of heat, though a less 

 one, and therefore on the whole the temperature must continue to 

 fall, and it will so continue until by the increasing length of the 

 day, and the decreasing length of the night, and the increasing 

 altitude of the sun, the increment of heat during the day becomes 

 equal to its decrement during the night; after that takes place, the 

 result of the sun's influence during each twenty-four hours will be 

 on the whole an increase of heat, and the temperature of the 

 weather will- accordingly be augmented. 



Hence it is that the winter season in the northern hemisphere is 

 the interval between the 21st of December and the spring equinox; 

 the same interval being the summer season in the southern 

 hemisphere. 



The altitude to which the sun rises at noon constantly increases 

 until the 21st of June, when it becomes as it were stationary, 

 and afterwards decreases. In the same manner the altitude at 

 noon constantly decreases until the 21st of December, after which 

 it increases, having remained in like manner stationary for a 

 certain interval. These two epochs have therefore been called 

 the solstices, one being denominated the summer, and the other 

 the winter solstice, from a Latin word which signifies the standing 

 or stationary position of the sun. 



168. When the northern hemisphere is most inclined towards 

 the sun, as shown in fig. 9, the sun is vertical at noon to all 

 places at 23| north of the equator. Before that day, and after 

 it, the sun's altitude at noon is less than 90, and consequently it 

 does not reach the zenith. It may be considered therefore 

 gradually to approach the zenith at such places until the 21st of 

 June, and then gradually to recede from it. 



From this circumstance the parallel of latitude which passes 

 through such places has been called the tropic. 



Similar phenomena are produced at the corresponding parallel 

 of south latitude, and these parallels are accordingly called 

 respectively, the northern and southern tropic, or the Tropic of 

 Cancer and the Tropic of Capricorn. 



169. It will be evident, by considering the diagrams, fig. 9 and 

 fig. 11 , that the sun can never be vertical at noon to any part of 

 the earth except to places which lie between the tropics, and at 

 all such places it is vertical at noon twice in the year. 



170. These astronomical causes of the vicissitudes of the seasons 

 may be further illustrated by the diagram, fig. 12, which pre- 

 sents a perspective view of the earth in twelve successive positions 

 which it assumes in one revolution round the sun, the observer 

 being supposed to view it from the north side of the plane of its 



171 



