152 PHILOSOPHY OF STORMS. 



derneath the base sinks proportionally, and the upward 

 motion of the air in the middle of the cloud is sometimes 

 so great as to carry up the drops of rain above the region 

 of perpetual congelation, and throw them out on both 

 sides of the cloud frozen into hail. 1 Does hail of great 

 size ever come down through the base of the cloud ? And 

 when a summer cloud begins to rain down through its 

 base, does it always stop the upward current in the middle 

 of the cloud, and finally invert it, causing the air to move 

 downward in the middle of the rain, and outwards at the 

 surface of the earth, contrary to the motion it had inwards 

 while the cloud was forming? Or does the air sometimes, 

 even after the commencement of the rain, continue to run 

 in under the base of the cloud round the borders of the 

 rain, glancing up over air pressed downwards in the centre, 

 and outwards at the surface of the earth, by the weight of 

 the rain and its cooling effect on the air through which it 

 passes ? If the latter is sometimes the case, in what direc- 

 tion does the rain then move along the surface of the earth? 

 If the former is sometimes or always the case, then, as 

 the individual cloud will of course cease to rain in a short 

 time, how is the rain continued? If it is by new clouds 

 springing up in its borders, do these new clouds generally 



1 The velocity of this upward motion may be calculated by an observer, 

 over whom the middle of the cloud passes, by the following formula : Note 

 the height of the barometer at the moment of the calm which precedes the 

 storm, and also at the moment of the calm in the middle of the storm; take 

 the difference in inches ; eight times the square root of 900 times this dif- 

 ference will be the velocity in feet per second of the upward motion of the 

 air in the centre of the storm, nearly. For example : if the barometer should 

 sink one inch in the centre of the storm, the velocity upwards in that centre 

 would be 8 X V 900 X 1 = 240 feet per second. This formula is founded on 

 the fact that nine hundred feet of air in height weighs about as much as an 

 inch of mercury, and then the formula for spouting fluids applies. No al- 

 lowance is to be made for resistance, as the up-moving column passes through 

 the surrounding air ; for it is known by experiment, that water, under a given 

 head of pressure, flows through an aperture with the same velocity as into 

 air, its velocity depending entirely on the head of pressure. 



