82 THE YEAR. 



brilliant in those states of the atmosphere when it is 

 difficult to decompose the light by a prism. The progress 

 of growth too is greater when there are gentle showers 

 with the sky clouded, than when the sun is in all its 

 strength ; and the days of greatest drought are those 

 on which the upper atmosphere is overcast. 



Thus when we speak of the phenomena of the year 

 as depending upon the action of the sun, we speak 

 only of part of the explanation ; and daily observation is ' 

 required to find out the circumstances by which that 

 general action is modified. With these precautions, 

 however, the changes which the influence of the sun 

 produces upon the mechanical constitution of the at- 

 mosphere, are the best general index that we have to 

 the natural history of the seasons. From the perfect 

 ease with which its parts move, the air has a constant 

 tendency to preserve its equilibrium, or to restore it 

 when deranged ; and from the facility with which its 

 volume is changed, by heat and by other causes, it is 

 kept perpetually in motion. At the point of greatest 

 solar action, which is about three hours on the average 

 behind the point where the sun is vertical, its motion 

 is upward, because the heat makes it lighter ; and thus 

 there is upon the surface a general motion toward that 

 point, and in the upper regions a general motion from 

 it, or rather from the opposite point of the same parallel 

 of latitude. The apparent motion of this point west- 

 ward prevents the motion of the air from being ap- 

 parent on the parallel where the sun is vertical, or for a 

 degree or two on each side of it : on land it is inter- 

 rupted by the action of different surfaces, and without 

 the tropics, the action is too weak to overcome the 



