54 



THE PLURALITY OF WORLDS. 



should we move ? Should we not take care to keep always at the same dis- 

 tance from it ? And to accomplish this, should we move in any other path 

 than that of a circle, having the fire in the centre ? This, however, is precisely 

 what is accomplished by the annual motion of the earth. It traverses its course 

 round the central fire of the system, keeping always nearly at the same distance 

 from the inexhaustible fountain of light and warmth. By this simple expedient 

 of observing a circular path, with the sun in the centre, this necessary object 

 is attained. 



Now, in examining the movements of all the other planets, we find that the 

 same expedient is provided : that they severally, in their periodical courses, 

 like the earth, preserve uniform distances from the sun moving round that 

 body in circles, of which it is the common centre. 



Seeing, then, that this motion in the case of the earth is a means whereby an 

 important end is attained, analogy justifies the conclusion that it is to be re- 

 garded likewise as a means for the attainment of a similar end in each of the 

 planets. But it will probably be said that the planets are at different distances 

 from the sun ; that the most remote of them is nearly twenty times farther from 

 that luminary than the earth, while the nearest of, them is little more than one 

 third the earth's distance ; therefore, that although it must be admitted that each 

 planet (considered per se] is supplied uniformly with light and warmth by this 

 circular motion ; yet the intensity of these principles to which the several 

 planets are exposed, comparing one with another, is so extremely different as 

 to destroy all analogy between them. 



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, produced by the solar rays, depends on the density of 

 the air which surrounds the objects affected by it. Thus we find the tempera- 

 ture, 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 there- 

 fore easily imagine, provided the existence of their atmospheres be conceded, 

 that their density has been so regulated, that the nearest planets to the sun, 

 which receive the greatest intensity of its rays, may not, after all, be more 

 heated 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 mount- 

 ains 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 equalized and compensated. The means of accomplishing this 

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



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 a day in the most remote planet would be less 



than that of a day in the nearest in the same proportion as the sun's heat would 



be less. It may therefore be objected that there might be scarcely daylight 



enough in the planet Herschel to serve the purposes of social and civil life. 



Such might undoubtedly be the case if we were to deny the possibility of any 



variation, however minute, in the organs of vision ; but without denying this, 



let us consider how the matter would stand. The perception which the eye 



of any creature acquires of light, depends (cateris paribus) upon the magnitude 



of the circular aperture or foramen, in front of the eye, called the pupil, which 



has, externally, the appearance of a circular black spot ; but which is, in 



) reality, a circular hole through which the light is admitted to the interior of the 



I chamber of vision, there to affect the membranous coating which transmits its 



j influence to the brain and causes the sensation. It must be evident, even to 



W->-~. 





