1 52 THE BORDERLAND OF SCIENCE. 



to find the pressure reduced to one-half (no matter 

 what the height we start from) ; at seven miles it is re- 

 duced to one-fourth ; and so on. But, owing to the 

 relatively small attraction of gravity in Mars, a height 

 of nine miles must be attained from his sea-level before 

 the atmospheric pressure is reduced to one-half, and a 

 height of eighteen miles before it is reduced to one- 

 fourth, and so on. And instead of forty miles (which 

 as we have seen is the lowest estimate of our air's 

 height above the level where its pressure is like that of 

 the Martian air), we find a height of fully seventy- five 

 miles as the minimum. We may fairly assume that the 

 Martian atmosphere extends to a height of at least 100 

 miles from the planet's surface. 



In such an atmosphere there is ample scope for air- 

 currents, and it is probable that owing to the tenuity 

 of the air the winds in Mars would have a high velocity. 

 They would not necessarily be violent winds, since the 

 force of wind depends on the quantity of air which is in 

 motion quite as much as on the velocity. So that we 

 need not entertain the theory which was advanced some 

 years since in the Spectator that trees in Mars must be 

 small in consequence of the great violence of Martian 

 hurricanes, by which all lofty trees would be destroyed. 

 Even at a velocity of a hundred miles per hour, Martian 

 winds would be less destructive than gales on earth 

 blowing at the moderate rate of twenty miles per hour. 

 But on a globe so small as that of Mars, compared 

 at least with the earth's, swift air currents would be 

 very effective in carrying off from the central heated 



