September 1, 1892.] 



KNOWLEDGE 



1(39 



may be consistently above it, and hence the ice and snow 

 formed during the night would soon be melted during the 

 day, and we should see only the indications of water and 

 not of snow. 



The sky of Mars by day is almost cloudless. We know 

 this by observation, but we know it far more surely by 

 inference ; for observation at such a distance is apt to be 

 deceptive. Here a most fruitful source of condensation 

 is the lowering of temperature, consequent on the expansion 

 which rising currents of air experience as they attain a 

 greater height. On Mars this cause is far less eft'ective. 

 For that a body of air may expand to double its former 

 volume, it must rise nearly nine miles instead of three and 

 a half, and it will take more than six times as long to do 

 so. Nor IS this all. Its temperature would not be lowered 

 so much by such an expansion, for less work would be 

 done, and even an equal loss of temperature would make 

 a less ditt'erence to the power of the air to hold water 

 vapour. For whilst the boiling point of water would be 

 lowered some 24° Cent, by an ascent to the height of half- 

 density here, it would on Mars (assuming an atmosphere 

 proportional to the mass) be lowered only 1H°. It would 

 be tairer, however, to compare the etfects of an equal motion, 

 for the nine miles rise, which would lower the boiling point 

 on Mars by but 13', would lower it on the earth by 86°, 

 or six and a half times as much. 



It is easy to see how it comes to pass that the spectrum 

 of Mars atibrds evidence of the presence of water vapour. 

 For evaporation would be easy and rapid, the boiling point 

 of water being (on the assumption made before of an 

 atmosphere proportioned to the mass) 10° Cent, instead of 

 100° ; whilst condensation would be difficult and slow. 

 The tendency would be during the day for the atmosphere 

 to become as fully laden with water vapour as it would 

 hold. At night condensation v.ould indeed set in, 

 but the formation of a continuous cloud canopy would 

 probably interfere to check radiation, and would prevent 

 the temperature falling as low as we should suppose. The 

 traces of this night cloud canopy are probably seen in the 

 white rim always observed on the east and west limbs, and 

 always somewhat broader on the side emerging from dark- 

 ness ; for both mornings and evenings are always cloudy on 

 Mars, but more especially the mornings. The low density 

 of the atmosphere would prevent differences of pressure 

 being set up at all comparable to those we know here ; the 

 feeble gravity of the planet would make the movements in 

 response to such, differences far more languid. There are 

 no hurricanes in Mars. The clouds wiU be cirri, not 

 cumuli, hoar frost will be far more common than snow, 

 and the currents, such as they are, will not bring moisture 

 enough to the pole in winter to cover it with snow to any 

 great depth, so that the succeeding summer may well be 

 able to melt it nearly all away. 



If these considerations be correct, it is Venus and not 

 Mars which bears the greatest analogy to our own planet ; 

 it is more nearly equal to the earth in size and mass, and 

 hence I would suggest its meteorological conditions are 

 more nearly similar, ilars with the smaller amount of 

 heat that it receives, its thin atmosphere, and sluggish 

 meteorology will, in spite of our knowing the configuration 

 of its surface so well, present more dift'erences than analogies. 

 And what we do see is probably deceptive : the ruddy glow 

 and apparently open seas of the part turned towards us are 

 very likely perfectly consistent with the part which we do 

 not see being bound in ice, or perhaps we should rather 

 say in frost. 



Eeii.\rks by a. C. Eanyard. 



[I do not feel as certain as my friend Mr. Maunder 

 that the atmosphere of Mars is less dense than our own ; 



but I agree as to the strong e\'idence tending to show that 

 the Martian atmosphere is more transparent than the 

 earth's, and with Mr. Maunder's conclusion that it must be 

 loss dust-laden. This seems to follow naturally from the 

 more languid character of the storms which Mr. Maunder 

 has so ingeniously shown to be a natural consequence of 

 the ieeble gravity at the Martian surface. Most of the 

 larger dust particles in the earth's atmosphere are suspended 

 within a mile or two of the surface. During the last 

 few years, it has been shown that the dusty state of the air 

 is intimately connected with cloud formation. Thus on 

 Mars we never observe white seas of cloud such as Hoat 

 so frequently over the earth's surface, and which must 

 obscure its oceans and continents with a snow-white sheet, 

 ever drifting and changing as it forms or dissolves. 



The intense polarization of the sky, as seen from a 

 mountain-top, proves that our upper air is full of very fine 

 dust, which is possibly partly of terrestrial origin and partly 

 derived from the dehris of meteors which have been driven 

 into vapour in the upper air. But it does not even follow 

 that the number of meteors which plunge into the atmos- 

 phere of ^lars correspond with the number met by the 

 earth. The large majority of meteors which we encounter 

 evidently belong to elliptic streams, and — whether we 

 assume with Proctor and Sir K. Ball that such streams 

 had a terrestrial origin or, as I think more probable, that 

 they represent the remains of comets captured by the 

 earth — it seems likely that the earth would encounter 

 more of such closed streams of meteors than Mars ; for, on 

 the one theory. Mars, during its sun-like stage, must have 

 been a feeble little centre of disruptive action as compared 

 with the earth, and, on the other theory, Mars' power of 

 captiu-ing comets must be feeble compared with the 

 earth's. 



It is evident that the red colour of Mars is not due to 

 the absorption of its atmosphere, for, as Webb remarks, 

 its polar snows never have a ruddy tinge, even on the limb. 

 We see no evening and sunrise tints on Mars. When it 

 presents the gibbous form, the terminator where twi- 

 light effects should be visible seems somewhat more sharp 

 than the limb or outer edge of the planet, which always 

 appears rather whiter and more misty, as Mr. Maunder 

 remarks, than the rest of the planet. On the earth, 

 twilight lasts till the sun has sunk about 18^ below the 

 horizon, and if there were such a band of soft degrading 

 Ught on Mars, it ought to be easily recognizable when 

 Mars presents its most gibbous form. But this absenc3 of 

 detectable twilight is quite consistent with a very dense 

 Martian atmosphere, if the air were very clear and free 

 from dust.] 



ON SOME RECENT ADVANCES IN THE STUDY 

 OF SOLAR PROMINENCES AND TACUIJE. 



By A. C. Eanyard. 



IT may be said that till recently photography has 

 lagged behind direct observation in the study of 

 solar phenomena. The first great advance was 

 made by -lanssen, who, more than ten years ago, 

 photographed details of suuspots and of the sun's 

 ' surface which the eye could not recognize. Prof. C. A. 

 Young, some years ago, obtained a few photographs of 

 ! bright solar prominences, but they showed very Uttle 

 j structure compared with the wonderful detail which can 

 i be seen with the spectroscope around the sun's limb. 

 I There is no lack of light for the study of solar structures, 

 I but the very abundance of light and heat increases the 

 I difficulties presented by the troubled ocean of atmosphere, 



