1873 •] The Planet Mars in 1873. 185 



should have a vaporous envelope of greater extent, whether 

 we regard such envelope as originally a portion of the once 

 wholly vaporous mass of the planet or as partially gathered 

 in by the planet's attraction on vaporous matter in the 

 inter-planetary spaces. And if we assume that the quantity 

 of atmosphere would be proportional to the mass of the 

 planet, — that is to the centre or third power of the planet's 

 radius, multiplied by the number representing the density 

 of the planet, — then since the surface of the planet is pro- 

 portional to the square of the radius, it would follow that 

 the quantity of air vertically above each square mile of a 

 planet's surface would vary directly as the product of the 

 numbers representing the diameter and the density of the 

 planet. This will be thought as probable a conclusion as 

 Mr. Williams's, and in the present instance it leads to a 

 very similar result. We may adopt it provisionally, in 

 order to see what general results we obtain by following 

 such considerations. 



Applying this rule to Mars, whose diameter is about 

 6-nths, and density about 3-ioths of the earth's, we obtain 

 for the quantity of air above each square mile of the surface 



of Mars, the expression — x 1—, or 2 — , where the corre- 

 _ 11 10 55 _ 



sponding expression of the earth is unity, — so that, quite 

 nearly enough for our present purpose, the quantity of air 

 above each square mile of the surface of Mars would be 

 2-5ths of the quantity above each square mile of the earth's 

 surface. But the pressure and therefore the density of the 

 air at the mean level of a planet depend on the quantity 

 of air above each unit of area, and the attraction of gravity 

 at the planet's surface ; for this pressure is solely produced 

 by the weight of the air. Gravity on Mars is represented 

 by 0*387, where terrestrial gravity is unity; and multiply- 

 ing * by 0*387 we obtain 0*1548, which represents (on our 



assumptions) the pressure of the atmosphere on Mars, when 

 unity represents the atmospheric pressure at our sea-level. 

 Mr. Williams deduces from his assumption a pressure of 

 0*179. According to one view, the mercurial barometer 

 would stand at about \\ inches ; according to the other, at 

 about 5J inches on Mars. 



Now it is not difficult to perceive that with an atmo- 

 sphere such as this, and a supply of solar heat equal only 

 to 4-gths of that which we receive from the sun, Mars 

 might present most of the appearances actually observed. 

 This has been shown (very ably, in my opinion) by 

 Mr. Williams ; and although I shall proceed presently to 



vol. in. (n.s.) 2 b 



