NATURAL RAIN-MAKERS. 647 



namely, that the water in it may be cooled below the freezing 

 point and yet not frozen. A snowfiake or ice crystal falling into 

 it may suffice to start a sudden congelation, just as we may see ice 

 needles dart in all directions when the chilled surface of a still 

 pond is disturbed. We liken this monstrous cloud to a huge gun 

 loaded and quiet, but with a trigger so delicately set that a fall- 

 ing snowfiake would discharge it. The sudden puffs, gusts, and 

 elongations of the thunder cloud may have their origin in this 

 way. Again, there is every reason for believing that electricity 

 plays an important part in the enlargement and subsequent his- 



CUMULO-MMBUS. 



tory of this cloud. We have ourselves measured with sensitive 

 quadrant electrometers the pull in volts experienced by the air be- 

 tween one of these clouds and the ground. The approach of the 

 cloud can be foretold without seeing it and the sky mapped out 

 roughly by the changes in the electrical potential caused by the 

 passage of the cloud. 



From what precedes it will be readily understood that cloud 

 motion is not always a true exponent of air motion. Meteorolo- 

 gists know that it is not safe to obtain the motion of the air cur- 

 rents from the motion of the clouds, for the latter may move 

 faster or more slowly, or even apparently stand still in the wind, 

 as in the " table-cloth " cloud on Table Mountain at the Cape of 



