6o 



Marvels of the Universe 



THE SOUTHERN HEMISPHERE OF MARS. 



As observed at the Lo^vell Observatory in 1905, sbowing the extended 

 area of the South Polar cap during the Southern winter. 



cannot suppose the atmospheric pres- 

 sure on Mars to be more than a 

 seventh that upon the earth, probably 

 it is much lower. But it follows from 

 the rarit}' of the air on Mars that the 

 vicissitudes of temperature there are 

 far greater than upon the earth. 



The atmospheric circulation of 

 Mars must be very languid. This 

 follows as a consequence of the small 

 amount of heat received from the sun 

 and of the feeble force of gravity. 

 Just as a stone on Mars will only fall 

 six feet in the second, instead of six- 

 teen as with us, so a mass of heated air 

 will rise with proportionate slowness. 

 Some writers have indeed spoken of 

 Mars as a " wind-swept planet," and 

 have referred to its " hurricanes," 

 '■ tornadoes " and " dust-storms." No 

 such tumults can ever disturb its calm, 

 clear, thin atmosphere. They are the 

 creatures only of an unscientific use 

 of the imagination. 



If it be objected that the snow-caps of Mars are actually seen to melt awaj' under the influence 

 of the summer sun, a twofold answer is at hand. The rapidity with which the polar caps 



melt is the clearest possible proof that 

 the sun's rays are not working upon 

 great masses of ice. There is, indeed, 

 no sufficient evidence that any con- 

 siderable fall of snow ever takes place 

 on Mars, or could take place. In the 

 extreme temperature of the Martian 

 winter "at a temperature vastly 

 below any ever felt in Siberia, the 

 smallest particles of moisture will be 

 condensed into what we call hoar frost, 

 and will glisten with as much white- 

 ness as actual snow." And, on the 

 other hand, though the ^ean tempera- 

 ture of the planet is low, it is not 

 impossible, nor even improbable, since 

 the extreme daily and annual range 

 must be greater than anything we 

 can experience here, that in mid-day of 

 the tropics under a vertical sun — that 

 is to say, in the region of the planet 

 THE NORTHERN HEMISPHERE OF MARS. ,^3,^ favourably presented to our 



observed at the Lowell Observatory in 1905. showing the diminished i 



area of the North Polar cap during the Northern summer. SCrUtmy— thC , temperature may HSe 



