August 1, 1893.] 



KNOWLEDGE 



153 



The imperfect skull of a simply enormous lemuroid 

 animal, from south-west Mada.L;ascar, has been exhibited 

 by Dr. Forsyth Major at the Zoological and Royal 

 Societies. It is part of a find sent to the British Museum, 

 and including also some very remarkable .Kiii/arnia 

 remains. The front portion of the mammalian skull is 

 injured, and only the molar and premolar teeth (of a 

 distinctly lemuroid type) are preserved. There is a high 

 and outwardly projecting zygomatic arch, and the inter- 

 orbital part of the frontals has enormous lateral develop- 

 ments above the orbits. All the bones of the skull are 

 greatly thickened. The brain case is remarkably small, 

 and the cranio-facial angle very obtuse. The remains were 

 exhumed from a marsh covered by a stratum of shell marl 

 about two feet in thickness. 



Uttttrs. 



[The Editor does not hold himself responsible for the opinions or 

 statements of correapondenta.] 



OX THE FORIIAIION OF CLOUDS IN THK ATMOSPilEEE 

 OF MAE8. 



To the FAlitor of Knowledge. 



Sir, — It is generally supposed that the atmosphere of 

 Mars is very rare, and we know from observation that it is 

 generally clear and free from clouds. This has been 

 attributed to the rarefied atmosphere being unable to 

 contain much aqueous vapour, and it has also been 

 concluded that the snow-line is very low, probably reaching 

 the ground in all extrx-tropical latitudes. It seems to me 

 that these conclusions have been arrived at by straining 

 an imperfect analogy too far, and that the observed facts 

 can be explained in another, and it seems to me, more 

 probable way. At a fixed temperature aqueous vapour can 

 attain a certain maximum tension. When it does attain 

 this tension, we speak of the air as being " saturated," 

 suggesting an anilogy between a salt dissolved in water 

 and the aqueous vapour " dissolved" in the air ; and also 

 suggesting that if the iiuantitij (apart from the volume) of 

 this air was lessened, the amount of aqueous vapour 

 necessary to saturate it would be lessened also. As a 

 matter of fact it is not so. The maximum tension is quite 

 independent of atmospheric pressure ; at the same tem- 

 perature it would be the same for a vacuum as for our air 

 at sea-level. It is therefore a mistake to suppose that, 

 because the atmosphere of Mars is rare, it cannot contain 

 mucli aqueous vapour ; volume for volume, it can contain 

 exactly as much as our atmosphere could at the same 

 temperature, whatever be its degree of rarity. The ex- 

 planation of the observed facts must, then, be sought 

 somewhere else. 



Let us consider how clouds are formed on our earth. 

 Water is evaporated at sea-level in the form of a trans- 

 parent and invisible vapour, which rises rapidly, expanding 

 and losing heat as it rises, till on attaining a certain level 

 it has lost so much heat that it can no longer remain in 

 the state of vapour ; a considerable portion condenses into 

 the liquid form, forming our lower or cumulus clouds. 

 This level may be called the level of watery condensation. 

 The portion of aqueous vapour which has escaped con- 

 densation continues to rise, and as it rises to expand and 

 lose heat, till a higher level is reached at which it can no 

 longer remain in the gaseous state. It then in its turn 

 condenses, but its temperature is so low that it condenses 

 in the solid form, forming our cirrus clouds. This 

 level may be called the level of ice precipitation. I 



say ice and not snow, because the snow-level proper 

 is a very different thing, and much lower. There is 

 no snow-level in the air ; it is confined to the solid 

 surface, generally mountains, but coming down to sea-level 

 in high latitudes. Probably the aqueous water, if rapidly 

 forced up, as by ascending currents of air m blowing 

 over a mountain slope, is, to a great extent, forced past the 

 level of watery condensation, and when condensation does 

 occur, a good deal of the aqueous vapour passes at once 

 into the solid form, and is deposited as snow. Now let us 

 consider how all this would be modified in the case of 

 Mars, and for the present we will leave out of consideration 

 the greater distance of lilars from the sun, and consider it 

 as if it were at the same distance as the earth, or, at any 

 rate, had the same temperature at the surface. Although 

 the density of the Martial atmosphere is much less than 

 ours, there would be just as much evaporation from an 

 equal area of water surface as with us. Perhaps there 

 would be even more, because evaporation would go on 

 more readily into a rare atmosphere than into a dense one, 

 although ultimately the quantities taken up would be equal. 

 Now this aqueous vapour would rise on Mars as it does 

 with us, but much more slowly, as the density of the 

 Martial atmosphere diminishes much more gradually. 

 This is not conjectural ; it can be pmird that, owing to the 

 diminished force of gravity at the surface of Mars, the 

 atmospheric density, which is halved at a height of three 

 and a half miles with us, would only be halved there at a 

 height of about nine miles. Moreover, the aqueous vapour 

 would not only rise more slowly, but would attain a much 

 greater height before precipitation would occur. If we 

 suppose that on earth it occurs when the vapour has 

 expanded to twice its original volume — that is, at a height 

 of three and a half miles — an equal expansion would only 

 occur on Mars at a height of nine miles. As a matter of 

 fact, precipitation would not occur even then, because the 

 aqueous vapour absorbs a certain amount of heat, partly 

 by stopping the direct solar radiation, partly, and to a much 

 greater extent, by intercepting the reflected radiation from 

 the body of the planet. On the earth, where the vapour 

 rises rapidly, and to a height of only three and a half 

 miles, the amount so absorbed may be inconsiderable, but 

 on Mars, where the vapour rises much more slowly, and to 

 a height of nine miles, the amount absorbed must be much 

 greater, and would raise the level of watery precipitation 

 slill further, say to a height of eleven miles. Hence the 

 great height of the Martial clouds, and the slowness 

 with which the vapour rises. We might expect that instead 

 of our dense cumulus clouds, the prevailing cloud in the 

 Martial tropics would be a high and thin stratus, covering 

 nearly the entire sky. Now, we know from experience that 

 a thin veil of stratus cloud interferes but little with the 

 definition of objects seen directly through it — in fact, not 

 unfrequently it improves definition ; so that we might 

 expect the centre of the disc would be clearly visible, and 

 as we would have no unclouded portion for comparison, we 

 might not suspect the existence of clouds at all. On the 

 other hand, near the edge of the disc, the hght from below 

 coming very obliquely, would be almost entirely inter- 

 cepted, while the clouds themselves would reflect a diffused 

 white light. Consequently we might a priori expect that 

 definition would be good in the centre of the disc, while 

 the edges were surrounded with a whitish margin. Of 

 course, the greater distance of Mars from the suu must 

 modify this to some extent, but the close agreement between 

 our theoretical deductions and the actual results of obser- 

 vation makes me think that the modification can only 

 extend to altering details, leaving the general features the 

 same. 



