168 



KNOWLEDGE. 



[September 1, 1892. 



and of the light which fell on the actual surface only a 

 small proportion would be reflected back, and this would 

 again be reduced on its outward journey. It is scarcely 

 likely that the difference in the reflective mdices of land 

 and water, especially when we remember how much of the 

 former is covered by dark vegetation, would be sufficiently 

 great to make itself noticeable. Of course, whenever our 

 sky was covered by cloud the details of oui- geography 

 would be effectively hidden. We may take it as pretty 

 certain that our earth, so far from being as easy to observe 

 as we find Mars, would be probably more like Venus ; so 

 that, instead of giving a greater amount of atmosphere 

 above each square inch to Mars than we possess, there is 

 little doubt but tbat there must be less. Lideed, if the 

 total amount of the Martian atmosphere bears the same 

 proportion to the mass of the planet as is the case here, then 

 its density at the surface will be one-seventh of ours, corre- 

 sponding to the state of things we should find if we could 

 ascend in a balloon to a height of very nearly ten miles. 



All these considerations point to the existence of intense 

 cold upon Mars, cold distinctly below the freezing-pomt 

 even for the equator ; and yet observation does not seem 

 to confirm this view. 



Instead of the white icy glitter we should expect if Mars 

 were one vast glacier, he sends to us a most conspicuously 

 ruddy light. The intensity of its redness is, indeed, the 

 most obvious thing about the planet. Then, again, it 

 seems that its atmosphere contains a very appreciable 

 amount of water vapour, a thing which we cannot recon- 

 cile with an extremely low temperature, for Dr. Huggins, 

 obsening the spectrum of the planet on February 14th, 

 1867, detected traces of some of the telluric bands due to 

 water vapour, and though the observation is a very 

 delicate one, and one may readily be deceived in the 

 matter, I have repeated it myself on two occasions, and 

 have little doubt as to its accuracy. If we accept these 

 observations, then the wliite polar caps may be reasonably 

 ascribed to ice or snow ; mdeed, as Proctor pointed out, 

 even if we had never seen them, we could, after Dr. 

 Huggins' observation, have confidently predicted their 

 j)resenee, and their gradual increase during the planet's 

 winter, and diminution through its summer. Take the 

 two facts together, and we come as nearly to a demon- 

 stration of the existence on Mars of land, water, ice, 

 snow, and cloud as we could hope with our present 

 resources, and sceptics must be able to produce a very 

 strong case to overthrow it. 



But how is it then that we do not find Mars completely 

 ice-bound ? For it clearly is not. In the summer of the 

 southern hemisphere the white pole cap has been watched 

 to shrink, until in 1877 the pole itself was actually clear, 

 and the cap itself had a radius of only 180 miles ; much 

 as if the legend of "an open polar sea,'' so false for the 

 earth, were true for Mars, and only the highland glaciers 

 of some Martian Greenland were left unthawed under the 

 summer sun. 



So that the ]\Iartian summer would seem to be actually 

 hotter than our own. And the winter, on this way of 

 looking at it, is no colder. For the winter pole cap 

 only reaches Lat. 40° or 4.5°, iif mdeed so far as that ; 

 that is to say, no lower than it does for terrestrial con- 

 tinents. Thus, if we refer to a beautiful series of drawings 

 by Mr. Knobel, the President of the Royal Astronomical 

 Society, published in Vol. 48 of the Memoirs of that 

 body, we find that the Newton Sea was clear on February 

 11th, 1884, up to S. Lat. SO'', the Zollner Sea on February 

 26th to S. Lat. 50°, and the Maraldi Sea on March 8th up 

 to S. Lat. 40^ These dates were all after February 3rd ; 

 the mean date between the autumnal equinox and winter 



solstice. Another series by Dawes, in Vol. 34 of the same 

 publication, shows us Nasmyth Inlet in N. Lat. 4.3° on 

 November 20th, 1864, the Delambre Sea in N. Lat. 55° 

 on November 26th, and the Campani Sea in N. Lat. 6.5° 

 on December 1st ; the winter solstice having fallen on 

 July 28th, and the spring equinox not coming on until 

 .January 4th, 1865 ; whilst the superb series by Mr. 

 Green in Vol. 44 show the Nasmyth Inlet in N. Lat. 43° 

 on September 10th, 1877, only a fortnight before the 

 winter solstice, September 26th. 



Now, although broad oceanic expanses are not covered 

 here on our world to any low latitudes, none of these 

 Martian markings can fully claim to be of that character. 

 The Nasmyth Inlet is most strictly a " mediterranean " 

 sea, and may fairly correspond to the Black Sea, Sea of 

 Azov, and Caspian Sea. But the northern portion of the 

 latter is ice-covered every winter ; the Straits of Yenikale 

 are often bridged with ice, and na\"igation is stopped on 

 the Sea of Azov. It would seem therefore that the winter 

 of Mars is little if any severer than here. And we must 

 remember, in addition, that we cannot discriminate 

 between snow and cloud, and that Mars is free fi'om hotit 

 to as high a latitude as the earth is free from snow. 



How are we to reconcile results so contradictory '? The 

 first circumstance to bear in mind is that the heat and 

 hght incident upon a planet are no measure of that 

 effective upon it. Of that which falls upon the earth, 

 probably nearly one half is reflected off again from the 

 atmosphere alone, and a yet further amount is reflected 

 from clouds. All this is ineft'ective in warming either air 

 or soil, and if we could assume that all the heat falling on 

 JIars was used directly by it, we should find it would be 

 practically as well off as ourselves. And no doubt some 

 approach is made to this condition, for although the deter, 

 miuation of the alhcJoea of the respective planets has 

 been much neglected, and such results as have been 

 obtained are very rough, yet it is most plain that the 

 albedo of Mars is very low, not differing much from that 

 of white sandstone, whilst the albedoes of Jupiter and 

 Venus are very high, and are best explained by supposing 

 that the reflection takes place from a cloud shell. Zollner 

 gives the moon as below Mars, the former being one-sixth, 

 and the latter one-quarter. Chacoruac rates Venus as 

 five times the moon ; whilst Seidel only credits Mars with 

 an albedo of an eleventh. It is plain that we cannot put 

 much reliance on the actual figures, but at the same time 

 we may safely conclude that the reflection fi'om Mars is 

 almost as completely from the surface as it is with the 

 moon, whilst from the earth, and yet more from Venus, 

 it is chiefly fi'om the atmosphere, either from cloud or 

 from suspended dust, the latter agent being probably the 

 more effective in the case of the earth, and the former 

 with Venus. 



It may be concluded then that the effective amount of 

 heat received by the three planets is far more nearly equal 

 than would at first appear. Mars is, therefore, much 

 warmer than its distance from the sun would imply ; but 

 on the other hand its mean temperature is by no means 

 so high as we might infer from the size of its polar caps. 

 For we must not forget that we see the planet at the best 

 advantage ; the side it turns to us is the side it turns 

 towards the sun ; the parts of Mars with which we are best 

 acquainted are its tropics at midsummer and at noon. 

 The experience of our elevated plateaux and mountain tops 

 shows how great may be the dift'erence between the 

 highest and mean temperatures where the air is rare, as it 

 certainly is on Mars. There is nothing improbable in 

 supposing that even if the mean temperature of its tropics 

 is considerably below zero Centigrade, the day temperature 



