322 



METEOROLOGY. 



the trade-winds, of certain currents observed in tlie sea, and the 

 diurnal variations of the barometer." The subject is believed to be 

 now included among the prize questions of the French Academy, and 

 in the increasing number of researches it is hoped that its difficulties 

 may at length be effectively obviated. 



The laws of Solar Intensity here derived a priori, have a general 

 accordance with physical phenomena, and will furnish instructive 

 comparisons with analogous values obtained by meteorological obser- 

 vations. The changes of the sun's intensity upon the inaccessible 

 regions of the Pole will be included, to which the late Arctic explora- 

 tions have given unusual interest. And, among other advantages, 

 light will be thrown upon geological researches relating to changes of 

 the heat of the globe at very remote epochs. 



At the close, the course of investigation has led to the development 

 of a pecular inequality in the annual duration of sunlight. The like 

 series of values for the duration of twilight is also new, and will not 

 be devoid of interest. But the main design has been — distinguishing 

 between the sun's intensity and terrestrial temperatures — to carry out 

 one comprehensive principle, by which the laws of the sun's intensity 

 of heat and light are obtained to some degree of completeness as a 

 system. 



SECTION I. 



Irradiated surface upon the p)lanets. — It is evident that the extreme 

 rays proceeding from the sun to the planet are tangent to the two 

 spheres, as shown in the annexed diagram ; where are represented a 

 section of the 

 sun, of the 

 planet, and 

 the radius- 

 vector or dis- 

 tance of the 

 planets centre 

 from that of 

 the sun. The 

 sun being the 

 greater body 



illuminates not only the adjacent hemisphere of the planet, but also 

 the zone or belt A C lying beyond, which may be called the zone of 

 differential radiation. From the geometrical properties of the figure, 

 it is shown that the sine of the angular breadth of the zone of differ- 

 ential radiation is equal to the difference of the radii of the sun and 

 planet divided by the radius- vector of the planet's orbit. 



With this principle, we can determine by geometry the actual breadth 

 in miles, and the proportions of dark and illuminated surface. These 

 will vary with the elliptic changes of distance from the sun, as indi- 

 cated in the following table. 



