LIGHT AND HEAT, HOW SUPPLIED. 



and warmth by this circular motion ; yet the intensity of these 

 principles to which they severally are exposed, comparing one 

 with another, is so extremely different as to destroy all analogy 

 between them. 



11. In answer to this, we are, however, to consider that the 

 influence of light and heat upon a planet does not depend solely 

 on its distance from the sun. The heat, as is well known, pro- 

 duced by the solar rays, depends on the density of the air which 

 surrounds the objects affected by it. Thus we find the tempe- 

 rature, at great elevations in our own atmosphere, considerably 

 lower than at the mean surface of our globe ; because at these 

 elevations the air becomes so thin as to be incapable of collecting 

 and retaining the sun's heat. We can, therefore, easily imagine, 

 provided the existence of planetary atmospheres be conceded, 

 that their densities have been so regulated, that the nearest 

 planets to the sun, which receive the greatest intensity of its 

 rays, may not, after all, be subject to a greater temperature than 

 the most remote ones, which are exposed to the least intensity 

 of its rays : just as we find that the temperature of the summits 

 of lofty mountains at the tropics is as low as the temperature of 

 some of the polar latitudes. It is plain, then, how the effects of 

 the various distances of the planet from the sun may be equalised 

 and compensated. The means of accomplishing this are provided 

 in the form of atmospheres, as we shall presently see. 



12. But let us turn to the consideration of the solar light. The 

 intensity of the sun's light varies with his distance exactly in the 

 same proportion as that of his heat ; and the brightness of the 

 day in each of the planets would be in the exact proportion of 

 the apparent magnitudes of the sun as seen from them severally. 

 Now, it is evident, that as we approach any object, its visual 

 magnitude increases, and, as we recede from it, its visual 

 magnitude diminishes. A balloon seen at the place from which 

 it makes its ascent appears of vast dimensions. Seen at a great 

 height in the air, it is diminished to a mere spot. Looking from 

 the summit of the cliffs of Dover, Edgar says to Kent 



Half way down 



Hangs one that gathers samphire ; dreadful trade ! 

 Methinks, he seems no bigger than his head: 

 The fishermen, that walk upon the beach, 

 Appear like mice ; and yon' tall anchoring bark, 

 Diminish'd to her cock ; her cock, a buoy 

 Almost too small for sight. 



Knowing the relative distances of Mercury, Venus, the Earth, 

 and Mars from the sun, nothing is more easy than to ascertain 

 by calculation the relative apparent magnitudes of the sun, as 



7 



