THE PLANETS, AKE THEY INHABITED I 



bodies of the same class ; but let us see the purposes in the eco- 

 nomy of nature which are fulfilled by this common character 

 given to the motion of these planets and the position of the sun. 



9. We find, upon considering the qualities of organised bodies, 

 and especially of the species of the animals and vegetables upon 

 the earth, that the maintenance of their physical well-being is 

 essentially dependent on the uniformity and regularity with which 

 they are supplied with the two great physical principles of light 

 and heat. Should these, or either of them, be subject to any 

 extreme variations, such vicissitudes would be incompatible with 

 the organisation of the species. There is a cold on one hand and 

 a heat on the other, under which no organised body could continue 

 to exist, and there are still narrower limits within which it is 

 necessary to confine the temperatures they are exposed to, in order 

 to secure the perfection of their physical health. There are also 

 degrees of light, the intensity of which would be incompatible 

 with the continued perfection of the organs of vision. 



Seeing then how essential to the well-being of the creatures 

 that people this globe an uniform supply of light and warmth 

 is, we are naturally led to examine the expedient by which this 

 necessary provision has been secured to them. If we had a fire 

 in our neighbourhood which at once supplied light and heat, 

 and that circumstances obliged us continually to shift our 

 position in relation to it, how should we move so as to receive an 

 uniform degree of illumination and warmth from it ? Could we 

 move in any other path than that of a circle around the fire as 

 a centre, keeping thereby always at the same distance from it ? 

 Now this is exactly the path in which the earth moves, as repre- 

 sented in the plan ; and we find that the three other planets 

 severally also move in circles, each keeping continually at the 

 same distance from the common fountain of light and heat.* 



10. Since this motion in the case of the earth is an expedient 

 whereby an important end is attained, analogy justifies the 

 conclusion that it is to be regarded likewise as the expedient 

 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 : therefore, that although it must be admitted that 

 each planet (considered per se) is supplied uniformly with light 



* The paths of the planets in moving round the sun when submitted to 

 extremely accurate examination proved to be oval in their form, but their 

 departure from the circular form is so very minute, that if such an orbit 

 were described in its proper proportions on paper, it would be indistin- 

 guishable from a circle. For all the purposes of the argument here 

 advanced, the paths of the planet may, therefore, be taken to be concentric 

 circles with the sun in the common centre. 

 6 



