BAIN-MAKING. 485 



that if a little condensation could be started in one place it would 

 at once spread out in all directions, like the benign influence of 

 the little homoeopathic pill. How a rainfall started in this way 

 is ever to stop as long as any aqueous vapor remains in the air, 

 they have not condescended to tell us. This question has not, so 

 far as I know, ever been raised by the results of their incanta- 

 tions. 



As a matter of fact, every drop of water taken from the air de- 

 creases the number of vapor molecules remaining, and, conse- 

 quently, lowers the temperature of the dew point. Likewise, every 

 free molecule which is brought to rest by striking against a solid 

 body, gives up its energy of motion to that body and increases 

 the total energy of its molecular vibration, so that a body upon 

 which water molecules are condensing is having its temperature 

 continually raised, and it must be continually giving off heat to 

 surrounding bodies, or it will soon be warmed above the tempera- 

 ture of condensation. In the case of the dust particles of the at- 

 mosphere, they must give off this acquired heat to the molecules 

 of the air which come in contact with them ; hence the condensa- 

 tion of moisture from the air raises the temperature of the air. 

 There are, accordingly, two reasons why heat must be continually 

 taken from the air in order to keep up condensation. The tem- 

 perature of the dew point is being continually lowered by the loss 

 of vapor molecules, and the temperature of the air is being con- 

 tinually raised by the amount of heat which these molecules lose 

 when their motion is stopped. 



In the formation of rain by natural causes this continuous de- 

 crease of temperature is provided by ascending currents of air 

 which carry the water molecules upward into continually cooler 

 and cooler regions. These ascending currents of air may be caused 

 by mountain ranges which deflect upward the winds that blow 

 against them, by the expansion of the air over a heated area of 

 the earth's surface, and possibly by other agencies not yet under- 

 stood. In the case of our California storms, these ascending cur- 

 rents are usually persistent for several days, frequently moving 

 across the whole continent. They are marked upon our weather 

 maps as areas of low barometric pressure. Whenever there is an 

 area over which the barometric pressure is less than the normal, 

 it is an indication of an ascending current of air, and wherever 

 there is an ascending current of air there is a probability of rain- 

 fall, though if the air be very dry it may not be carried to a suffi- 

 cient height to be cooled below its dew point. 



On the other hand, wherever there is an area of increased baro- 

 metric pressure, or of high barometer, it is an indication that there 

 is a descending current of air over that area ; and since air which 

 is settling toward the earth is continually having its temperature 



