i873-] The Planet Mars in 1873. 183 



a reliable indication of the general climate of the planet. 

 Indeed, it must be noted that when he pointed out the fact 

 to which I have referred, Prof. Tyndall offered no expla- 

 nation of it. He simply noted the error of those who would 

 seek to explain the former presence of enormous glaciers 

 solely by the action of cold. " Vast masses of mountain ice 

 indicate infallibly," he said, " the existence of commensurate 

 masses of atmospheric vapour, and a proportionately vast 

 action on the part of the sun. In a distilling apparatus, if 

 you require to augment the quantity distilled, you would 

 not surely attempt to obtain the low temperature necessary 

 to condensation by taking the fire from under your boiler; 

 but this, if I understand them aright, is what has been done 

 by those philosophers who have sought to produce the 

 ancient glaciers by diminishing the sun's heat. It is quite 

 manifest that the thing most needed to improve the glaciers 

 is an improved condenser ; we cannot afford to lose an iota of 

 solar action ; we need, if anything, more vapour, but we 

 need a condenser so powerful that this vapour, instead of fall- 

 ing in liquid showers to the earth, shall be so far reduced in 

 temperature as to descend in snow. The problem, I think, 

 is thus narrowed to the precise issue on which its solution 

 depends." Now, it is important to notice that what is here 

 affirmed of glaciers does not apply with equal force to snow 

 regions at the poles of a planet like, the earth or Mars. All 

 the snow which covers these regions must have been formed 

 originally by the action of heat. But a degree of heat, very 

 moderate in amount, would cause the evaporation of sufficient 

 quantities of vapour to produce the snow which covers a 

 widely-extended region. In fact, we know that even in the 

 arctic regions mists and clouds are formed, whence even- 

 tually snow is produced, and that these mists and clouds 

 are not due in all cases to aqueous vapour which has been 

 formed in warmer latitudes, but are actually produced over 

 ice-covered regions in calm weather; when, therefore, no 

 air is arriving from warmer places. Now, manifestly the snow 

 which covers the polar regions of Mars must either have 

 been formed from vapour raised and condensed in those 

 very regions, or else from vapour raised in lower latitudes 

 and condensed near the poles. In the former case, there 

 must be heat enough in the arctic regions to produce eva- 

 poration, and therefore, a fortiori, the heat in lower latitudes 

 must in that respect resemble the heat we experience in our 

 temperate zones. In the other case, there must be great 

 processes of evaporation, corresponding to those which take 

 place on our earth ; there must be winds carrying the moist 



