WIND DISPERSAL 63 
the rivers flow into the sea,” that cemetery of seeds, 
and their courses are at best mere spider-lines on a 
map. But the wind, blowing where it listeth, is every- 
where, always ready to snatch up in its arms any seed 
of sufficient lightness, and to bear it away from the 
parent; in fancy we can see tiny seeds borne by gales 
across mountains and oceans. But we have to leave 
imagination out of account, and examine prosaically 
the mechanical laws according to which such trans- 
port is of necessity conducted. Any body liberated 
in still air will fall vertically with a velocity which in- 
creases according to well-known laws until the increas- 
ing resistance of the air to its passage equals the 
effect due to gravity; it thenceforward continues to 
fall at a uniform velocity, that velocity depending upon 
the nature of the falling body. In all seeds which are 
sufficiently light to be at all suitable for wind dis- 
persal, the resistance of the air almost at once counter- 
acts acceleration due to gravity, so that the rate of 
fall may be taken as uniform from the beginning. 
If the seed on liberation is carried along by the wind, 
it will acquire almost immediately the horizontal 
velocity of the air-current, but it will at the same time 
move downward through the air with the same 
velocity as if the air was still—just as a body dropped 
in a railway carriage will fall at the same rate whether 
the train is moving or standing still. If we measure 
the speed of fall of a seed in still air, then we can 
easily deduce the distance to which it will be carried 
by a horizontal air-current of given velocity if liber- 
ated at any given height above the ground. Thus, if 
a seed liberated 100 feet from the ground falls that 
distance in half a minute, and the wind is blowing at 
