WEATHER SIGNS. 69 



colder current of air. While the sun continues to shine 

 upon the earth, a sufficient quantity of moisture is sup- 

 plied by evaporation from the earth's surface to keep up 

 the cloud-forming process. As soon, however, as the 

 sun begins to decline, this supply of moisture is cut off. 

 The surrounding air, on cooling, deposits all supera- 

 bundant moisture in the shape of dew, and no more 

 vapor ascends to complete the organization of the 

 clouds in the upper air. Hence they are gradually re- 

 absorbed into the atmosphere by a process which, in 

 the daytime, was not sufficiently rapid to keep pace 

 with the reinforcements from beneath. 



After the evaporation from the earth has been dimin- 

 ished by the withdrawal of the sun's heat and rays, if 

 the clouds still continue to thicken, there is evidence 

 that they are receiving supplies from a source indepen- 

 dent of immediate evaporation from the earth's surface. 

 The upper currents of the atmosphere are probably 

 saturated with moisture which renders it incapable of 

 absorbing the clouds in contact with it. If, at this con- 

 juncture, a damp wind from the ocean were to set in 

 underneath, the clouds between these two damp strata 

 of air would be constantly gaining density, and would 

 soon become so heavy as to descend in rain. If the 

 clouds, therefore, which are formed during the day, 

 evaporate soon after sunset, we know that they are de- 

 pendent on immediate supplies of vapor from the earth's 

 surface ; but if they continue to increase after sunset, 

 there is proof that the atmosphere above and below 

 them is saturated with moisture, and rain will be likely 

 to follow. 



When all these conditions are present, the lower ani- 

 mals are instigated to perform certain unusual actions 

 and to make a temporary suspension of their usual 



