324 



METEOROLOGY. 



tion is thirty-five miles wide ; while those of Mercury, the earth, and 

 Saturn, are nearly eighteen miles in breadth. In the last column oi 

 the table, it will be observed that the asteroid Vesta, though situated 

 beyond Mars, yet has, in consequence of its smaller size, a greater 

 proportion of illuminated surface than the earth. 



By computation it is found that the zone of differential illumination 

 upon the earth extends over 455,400 square miles; or, including the 

 additional area due to 34' horizontal refraction, it comprehends an 

 aggregate of 1^430,800 square miles of surface. The position of this 

 great zone is continually changing, and in turn it overspreads every 

 island, sea, and continent. At the vernal equinox, when the sun is 

 vertical to the equator, it will readily be perceived that the larger 

 base of this zone is a great circle passing through the poles and 

 having the earth's axis for its diameter. From this position it grad- 

 ually diverges, till at the summer solstice one extremity of its diameter 

 Avill be in the Arctic, and the other in the Antarctic circle. Thence 

 it gradually returns to its former position at the poles at the autumnal 

 equinox, all the while revolving in every twenty-four hoiirs, like a 

 Iringed circle around the globe, and accompanied with the lustrous 

 tints and shadows which variegate the dawn and close of day. 



SECTION II. 



LAW OF THE SUN'S INTENSITY UPON THE PLANETS IN RELATION TO THEIR 



ORBITS. 



The preceeding section represents the sun's action upon a distant 

 planet at a given distance, or at rest. It is here proposed to examine 

 the effect when the distance is variable ; that is, supposing the planet 

 to commence its motion from a state of rest, in an elliptical orbit, to 

 determine the intensity received during its passage through any part, 

 or the whole of its orbit. 



In the annexed figure, let S denote the sun situated in one focus ; 



F the planet's position at a 

 given time ; A, the perihelion 

 or point in the orbit nearest 

 the sun, B, the aphelion or 

 point farthest from the sun, 

 SF, the radius-vector, and 

 the angle ASP the true 

 anomaly. From the pro- 

 perty of the ellipse, combined 

 with the principle that heat 

 and light vary inversely as 

 the square of the distance, it 

 is i^roved that in its orbital 

 motion the earth does not 

 receive equal increments of heat and light in equal times ; but the 

 amount received in any given interval is exactly projjortional to the true 

 anomaly or true longitude described in that interval. This important 

 law, or one less correct, for the mean longitude^ appears to have been 

 first published in the Pyrometry of Lambert. 



