PLANETARY GRAVITY. 



inquiry with respect to most of the planets. The very presence 

 of those atmospheres and the clouds with which they are loaded, 

 offers a serious obstruction to any observations having for 

 their object to ascertain the geographical character of their 

 surfaces. The great distance of some of them is a formidable 

 obstacle to such an inquiry ; still, where some peculiar circum- 

 stances favour the observation, something has been done in this 

 investigation. 



7. Venus and Mars, the two planets in the system which come 

 nearest to the path of the Earth, are evidently the most eligible 

 objects for such an inquiry, and sufficient has been ascertained, 

 especially with regard to the latter planet, to draw very closely 

 indeed the ties of analogy by which the planets are associated 

 with the earth. 



8. The existence of continents and oceans, and even the configu- 

 ration of their outlines has been clearly traced on Mars. The 

 ?now which covers his polar regions during the winter, has been 

 distinctly seen, and has even been observed partially to dissolve 

 and disappear under the influence of the summer heat. The 

 clouds with which Venus and Mercury are so constantly enve- 

 loped, combined with other obstructions peculiar to the positions 

 of these planets, have rendered like observations respecting them 

 impracticable. It has, nevertheless, been ascertained that their 

 surface, like that of the Earth, is marked by mountain-chains of 

 great elevation. 



9. In tracing the analogies which prove the suitableness of tho- 

 planets for habitable globes, and which connect them by ties ot 

 kindred with the earth, one of the most important and interest- 

 ing is dependent upon the quantity of matter composing these- 

 planets, compared with their volumes or bulks. Let us see 

 how this affects the condition of the organised creatures that 

 dwell upon them. 



10. All organised beings, whether animal or vegetable, are 

 endowed with a certain limited amount of bodily strength. In 

 the case of animals, which have powers of locomotion, this 

 strength is regulated with reference to their weight, and the 

 extent and quantity of motion necessary for their well-being on 

 the surface of the globe. The structure of every animal is such, 

 in the first place, as to give it strength to support and move its 

 own body ; but this is not enough ; it must have a further amount 

 of disposable force to enable it to supply its own wants by the 

 pursuit of its prey; by the collection of its food ; by the erection 

 of its dwelling ; and, in general, by its labour in the supply of 

 its physical wan'?. In the case of vegetables, the strength must 

 be sufficient to support its weight, and resist those external 



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