148 THE BORDERLAND OF SCIENCE. 



says Tyndall, ' will not produce glaciers. You may have 

 the bitterest north-east winds here in London through- 

 out the winter without a single flake of snow. Cold 

 must have the fitting object to operate upon, and this 

 object the aqueous vapour of the air is the direct 

 product of heat.' It is manifest, then, that the sun 

 exerts enough heat on Mars to raise the vapour of 

 water into the planet's atmosphere (as indeed spectro- 

 scopic analysis has taught us), and it is also clear that 

 this vapour must be conveyed in some way to the 

 Martian arctic regions, there to be precipitated in the 

 form of snow. And then this difficulty is introduced : 

 According to our ideas the whole surface of Mars is 

 above the snow-line; any region on our earth where so 

 great a degree of cold prevailed accompanied by so 

 great an atmospheric tenuity would be far above the 

 snow-line even at the equator. How is it then that 

 the snow ever melts, as it manifestly does, since we 

 can see the ruddy surface of the planet ? 



An explanation, first suggested, we believe, in Mr. 

 Mattieu Williams's ingenious book called The Fuel of 

 the Sun, removes this difficulty. The snow actually 

 falling on Mars must be small in quantity, simply 

 because the sun's heat is not competent to raise up any 

 great quantity of water vapour. There cannot, then, 

 be anything like the accumulation of snow which 

 gathers in regions above our snow-line ; but instead of 

 this there must exist over the surface of Mars, except 

 near the poles, a thin coating of snow, or rather there 

 will be ordinarily a mere coating of hoarfrost. * Now 



