October 1, 1892.] 



KNOWLEDGE 



193 



atmosphere, which consists mainly of permanent gases 

 ■whose quantity is not altered by evaporation or liquefaction. 

 With this is joined a much smaller atmosphere of aqueous 

 vapour, which varies greatlv with the conditions of time 

 and place. But these variations would be much greater if 

 the permanent gases were removed ; for I think there is no 

 doubt that their presence retards both evaporation and 

 precipitation of vapour. 



^Yhen a star is occulted by the moon, the disappearance 

 is usually instantaneous ; but to this rule there are many 

 exceptions. It seems to me that both rule and exceptions 

 can be explained by ascribing to the moon an atmosphere 

 of vapour which never attains large dimensions, and is at 

 many times and places wholly, or almost wholly, condensed. 

 It need not, of course, be aqueous vapour. Oxygen and 

 nitrogen would probably act in the same way if the 

 temperature were sufficiently low, but it is not likely to be 

 low enough for this purpose on tlie moon. The absence of 

 cloud may be considered an objection to the theory of an 

 atmosphere of vapour. But, as you pointed out in the last 

 number, dust has a good deal to do with the formation of 

 cloud, and so I believe has atmospheric air. In the 

 absence of air and dust, precipitation would perhaps take 

 place with little or no formation of clouds or fog. 



The existence of water on the moon is doubtful. On 

 Mars, however, it appears to be certain, and it is not un- 

 likely that vapour plays a much more important part in 

 the Martian atmosphere than in ours. On one point, how- 

 ever, we are at present wholly in the dark, viz.. What is 

 the effect of a diminution in the force of gra\-ity upon 

 water ? We have only the power of studying this to a 

 very limited extent on the earth, and I am not aware that 

 it has been studied at all. From the marked effects of 

 pressure on both the freezing and the boiling points of 

 water it seems probable that variations in the force of 

 gravity would affect these points also ; but how, we can 

 only conjecture. Water, however, might exist in the 

 Uquid state at a temperature far below freezing point if it 

 were sufficiently salt. Again, supposing that Mars has no 

 atmosphere of oxygen and nitrogen, the result would be to 

 modify the properties of water considerably. Water when 

 deprived of air becomes viscous, its boiling point is raised 

 and it boils with explosive \-iolence ; even its visible 

 appearance is altered. 



Aqueous vapour, according to the experiments of 

 Tyndall, has a very high absorptive power. This might 

 account for the small reflection of light from the surface 

 of Mars. If, moreover, it absorbs blue or green rays by 

 preference, the red tinge of those reflected from the solid 

 body of the planet would be accounted for. The red tinge 

 of the eclipsed moon has sometimes been ascribed to a 

 similar cause, the absorbing agent being the vapour in our 

 own atmosphere. The atmosphere would probably be more 

 dense in the equatorial than in the polar regions, but this 

 would be modified by the distribution of water on the 

 planet and the existence of mountains or high table- 

 lands. 



With regard to the relatiiv albedo of difl'ei'ent planets, it 

 ought, I tbink, to be easy to detect. Supposing that the 

 entire surface of the planet is illuminated (otherwise 

 allowance can be made), the illuminated surfaces are pro- 

 portional to the squares of the planets' diameters. If the 

 whole of the incident light was reflected the intensity, for 

 different planets, would vary directly as this square, and 

 inversely as ths square of the planet's distance from the sun 

 multiplied by the square of its distance from us. If the 

 relative Ughts thus computed for -Jupiter and Mars diS'er 

 from those directly observed, the difference is due to the 

 relative albedo of the two planets. One planet should be 



taken as the standard and the albedo of each of the others 

 expressed in tjrms of its albedo. 



Trulv yours, 

 IG, Earlsfort Terrace, Dublin. W.' H. S. Moxck. 



[The presence of an- or other gases makes no dif- 

 ference in the amount of water that will be evaporated 

 at a given temperature, but it makes a considerable dift'er- 

 ence in the rate of evaporation. Thus, if a pint and a half 

 of water were brought into a room measuring ten feet each 

 way, and filled with perfectly dry air, or any other gas or 

 vapour, at a temperature of 80° Fahrenheit nearly the 

 whole of the water would be evaporated, and the air would 

 still remain transpai-ent, and the walls of the room dry. 

 When this amount of water had been evaporated the pro- 

 cess would stop, and no more vapour would be given off" 

 unless the temperature of the room was raised. Experi- 

 ments show that the same amount of water would be 

 evaporated, at the same temperature, whatever amount of 

 dry air there was in the room to start with ; even though 

 the room was devoid of air, the samg amount of water 

 would disappear, and the pressure of the resulting water 

 vapour would raise a column of mercury in a barometer 

 by about an inch. On Mars the same tension of water 

 vapour would raise the column of mercury a little more 

 than two and a half inches. I do not feel as sure as Mr. 

 Monck or my friend Mr. Maunder as to the evidence with 

 regard to the absorption of the sun's Ught by aqueous 

 vapour in the atmosphere of Mars, and I am quite 

 prepared to believe that the white polar caps of Mars are 

 due to the white snow-like crystals of carbonic acid gas, or 

 even to condensed atmospheric air. — A. C. Raxvard.] 



CAX A PLANET ABSORB ITS OCEAXS WITHOUT HAVIXi; 

 ITS SURFACE TEMPERATURE LOWERED ? 



To the Editor of Knowledge. 



Sm, — In the June number of Knowledge, p. 114, the 

 editor criticizes the suggestion that if Mars formerly had 

 more extensive oceans than n3w, owing to internal cool- 

 ing they might have been absorbed. He makes the point 

 that in the case of the earth such an absorption could 

 only occur in case the sufface temperature, even in the 

 equatorial regions, were reduced, owing to internal cooling, 

 below the freezing point. 



Now it seems to me that the internal temperature of the 

 earth has no influence whatever upon our climate, and 

 that if its internal temperature should be reduced to that 

 of interplanetary space our surface temperature would 

 still be practically the same as at present. Let us imagine 

 an iron bar one thousand feet long and several feet in 

 diameter, surrounded by some perfectly non-conducting 

 substance. Let one 

 end, CR, be main- 

 tained at a constant 

 temperature of 1000^ t- 

 C, and let the other 

 end, SF, be exposed 

 to the outside air, 



which we will say has "' ^^'^ 



a mean temperature of 0" C. In the course of several days 

 variations of temperature inside the bar will cease, and 

 the temperatures at varying distances from CR can be 

 represented by the ordiuates of the line TF. Thus, at one- 

 tenth of thedistance from F to R, that is at one hundred 

 feet distance, the temperature will be 100° very nearly. 



If, instead of iron, the bar were made of clay we should 

 have practically the same condition of aft'airs, only instead 



