THE DIRECTION OF MOTION. 239 



Earth is nearly the same, the lateral resistance is greatly 

 diminished. And throughout the course of each current 

 thus generated, as well as throughout the course of each 

 counter-current flowing into the vacuum that is left, the 

 direction is always the resultant of the Earth's tractive force 

 and the resistance offered by the surrounding masses of air : 

 modified only by conflict with other currents similarly de- 

 termined, and by collision with prominences on the Earth's 

 crust. The movements of water, in both its gaseous 



and liquid states, furnish further examples. In conform- 

 ity with the mechanical theory of heat, it may be shown 

 that evaporation is the escape of particles of water in the 

 direction of least resistance; and that as the resistance 

 (which is due to the pressure of the water diffused in a gase- 

 ous state) diminishes, the evaporation increases. Converse- 

 ly, that rushing together of particles called condensation, 

 which takes place when any portion of atmospheric vapour 

 has its temperature much lowered, may be interpreted as a 

 diminution of the mutual pressure among the condensing 

 particles, while the pressure of surrounding particles re- 

 mains the same ; and so is a motion taking place in the direc- 

 tion of lessened resistance. In the course followed by the 

 resulting rain-drops, we have one of the simplest instances 

 of the joint effect of the two antagonist forces. The 

 Earth's attraction, and the resistance of atmospheric cur- 

 rents ever varying in direction and intensity, give as their 

 resultants, lines which incline to the horizon in countless 

 different degrees and undergo perpetual variations. More 

 clearly still is the law exemplified by these same rain-drops 

 when they reach the ground. In the course they take while 

 trickling over its surface, in every rill, in every larger stream, 

 and in every river, we see them descending as straight as the 

 antagonism of surrounding objects permits. From moment 

 to moment, the motion of water towards the Earth's centre 

 is opposed by the solid matter around and under it; and 

 from moment to moment its route is the resultant 



