METEOROLOGY. 329 



2d. — The interior temperature of the globe. 



3d, — The elevation above the level of the ocean. 



4th. — The general inclination of the surface and its local exposure. 



5th. — The position of mountains relative to the cardinal points of 

 the compass. 



Gth. — The neighborhood of great seas and their relative situation. 



7th. — The geological nature of the soil. 



8th. — The degree of cultivation and of population to which a coun- 

 try has arrived. 



9th. — The prevalent winds. 



The same author observes, in relation to the fourth enumerated 

 cause, that northeast situations are coldest ; and southwest, warmest. 

 For the rays of the morning which directly strike the hills exposed 

 to the east', have to counteract the cold accumulated there during the 

 night. The heat augments till three in the afternoon, when the rays 

 fall direct upon southwest exposures, and no obstacle now prevents 

 their utmost action. 



With respect to the general climatic features of the globe,^ the 

 following points have been ascertained from extensive observations. 

 At an equal distance from the equator, Asia has a comparatively cold 

 winter and a hot summer ; Europe tempers both extremes ; Anierica 

 has a severe winter and a cold spring, but is allied in summer to 

 Europe, which it surpasses in the splendid climate of its autumn. 



SECTION IV. 



DETERMINATION OF THE SUN'S HOURLY AND DIURNAL INTENSITY. 



In the last section, the sun's vertical intensity upon a given point 

 of the earth's surface at any instant during the day, was shown to be 

 measured by a perpendicular drawn from the centre of the sun to the 

 plane of the horizon. If perpendiculars be thus let fall at every in- 

 stant during an hour, the sum of the perpendiculars will evidently 

 represent the sum of the vertical intensities received during the hour, 

 which sum may be termed the hourly intensity. 



The integral calculus furnishes a ready means of obtaining this 

 sum. For during any one day the sun's distance or apparent semi- 

 diameter, and the meridian declination, may be regarded as constant, 

 while the hour-angle alone varies, and the deviations from the implied 

 time of the sun's rising and setting vv^ill compensate each other.* 



* Multiplying the equation of instantaneous intensity by d H, since astronomy shows that 

 i? varies uniformly with the time, and integrating between the limits of any two liour-angles, 

 H' H", we obtain an expression for the hourly intensity. 



In like manner let H denote the semi-diurnal arc, and integrating between the limits and 

 H, we obtain the intensity for a half day, which, on cancelling the constant multiplier 2, 

 may be taken for the whole day, or diurnal intensity, as follows : — 



f a2 sin A dzzz IJ a2 Hsin L sin D -f- a2 cos L cos D sin II. 



The diurnal intensity is, therefore, proportioned to the product of the square of the sun's 

 serai-diamcter into the semi-diurnal arc, multiplied by the sine of the latitude into the sme 

 of the sun's declination, plus the like product of the square of the sun's semi-diameter into 

 the sine of the semi-diurnal arc multiplied by the cosine of the latitude into the cosine of the 

 declination. This aggregate obviously changes from day to day, according to the sun's 

 distance and declination. 



