INTENSITY OF SUN'S HEAT AND LIGHT. 15 



The distinction here explained has not only engaged the attention of the most 

 eminent meteorologists of modern times, but was equally adopted in ancient philo- 

 sophy, as appears in the following passage from Plato's Phsedon, LVIII : " For 

 around the earth are low shores, and diversified landscapes and mountains, to which 

 are attracted water, the cloud, and air. But the earth, outwardly pure, floats in the 

 pure heaven like the stars, in the medium which those who ar.e accustomed to dis- 

 course on such things call ether. Of this ether, the things around are the sediment 

 which always settles and collects upon the low places of the earth. We, therefore, 

 who live in these terraqueous abodes, are concealed, as it were, and yet think we 

 dwell above upon the earth. As one residing at the bottom of the sea might think 

 he lived upon the surface, and, beholding the sun and stars through the water, 

 might suppose the sea to be heaven. The case is similar, that through imperfection 

 we cannot ascend to the highest part of the atmosphere, since, if one were to arrive 

 upon its upper surface, or becoming winged, could reach there, he would on emerg- 

 ing look abroad, and, if nature enabled him to endure the sight, he would then 

 perceive the true heaven and the true light." 



In modern times, the researches of Poisson led him to the philosophic conclusion 

 now generally received, that the highest strata of the air are deprived of elasticity 

 by the intense cold ; the density of the frozen air being extremely small, Theorie 

 de la Chaleur, p. 460. An atmospheric column resting upon the sea may thus be 

 regarded as an elastic fluid terminated by two liquids, one having an ordinary 

 density and temperature, and the other a temperature and density excessively 

 diminished. 



Although the sun's intensity, which is here the subject of investigation, is the 

 principal source of heat, yet its effects are modified by proximate causes of climate; 

 of which, the following nine are enumerated by Malte Brun : — 



1st. — Action of the sun upon the atmosphere. 



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. 



6th. — 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 country has arrived. 



9th. — The prevalent winds. 



The same author observes, in relation to the fourth enumerated cause, that north- 

 east 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 accu- 

 mulated 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. 



