CONDITIONS OF HABITABILITY OF A PLANET. 87 



the same object on Mars would only pass through six feet in the 

 same time ; consequently, all movements on Mars that are the 

 effect of gravitation are much slower than they are here, and 

 this implies that its atmospheric circulation must be sluggish. 

 The late R. A. Proctor, unequalled in his day as a popular 

 writer on astronomy, made one of his few mistakes when he 

 described Mars as a planet swept by hurricanes. The less the 

 attractive power of the planet the more languid must the 

 movements of its atmosphere be ; we know with certainty that 

 there are no hurricanes on Mars. 



The feeble action of gravity has another effect. On the earth 

 if we ascend some three and a third miles, say about as high as 

 the top of Mont Blanc, we find that the barometer reads just 

 half of what it does at the sea level ; half the atmosphere has 

 been passed through. At double that height the pressure would 

 be halved again ; it would be only one-quarter of that at sea 

 level. On Mars the level of half pressure will be at nearly 

 nine miles from the surface, and of quarter pressure at nearly 

 eighteen miles. This relation we may briefly express by saying 

 that the barometric gradient is much steeper for the earth tlian 

 for Mars, and it follows that however thin and rare the atmos- 

 phere may be at the surface of Mars,- yet at only a few miles 

 height the pressure must be the same for the two planets, and 

 above that height the pressure for Mars would be the greater. 



It is quite clear that Mars has not much atmosphere ; its 

 surface markings are seen far too distinctly for it to be 

 possible to suppose that we view them through anything like 

 the amount of air that exists above the earth ; indeed it is very 

 doubtful whether an observer on the planet Venus could make 

 out anything of our geograph}^ through the veil that our atmos- 

 phere spreads round us. It is generally supposed that the 

 atmospheric pressure at the surface of Mars may be about 

 one-seventh of that on the earth, equivalent to the sort of 

 atmosphere that we should find about nine miles high above 

 the earth. This would be about the atmospheric density that 

 Mars might claim if atmospheres were dealt out to planets in 

 proportion to their masses. But it appears probable that with 

 planets as with people, the strongest get the lion's share ; to 

 those that have it is given, and from those which have not, 

 even that which they seem to have is taken away. The above 

 estimate, therefore, must be taken as the highest possible, 

 probably much higher than the fact ; for a little planet like 

 Mars cannot have the power of acquiring or retaining an atmos- 

 phere possessed by so much heavier a globe as the earth. 



