METEORIC RIVERS OR WATERFALLS. 425 



Biscay," on reaching the earth, was the same in kind as 

 that which formed the rivers of water from the clouds. In- 

 deed, if any violent hail storm were to stop its motion along 

 the surface of the earth, for even fifteen minutes, as this 

 storm probably did on reaching the hill on whose side it 

 fell, it would throw down just such a mass of hail as fell 

 at Offley. 



It would appear, indeed, impossible that many thousand 

 times as much water should be collected into one column in 

 a few minutes, in the air, as a column of the atmosphere of 

 the same diameter contains in vapor, even if it all could be 

 condensed. 



But this difficulty will vanish when we consider that if 

 the barometer should only fall one inch and a half in the 

 middle of one of these land spouts, the air would spout up 

 in it with the velocity of six thousand yards per minute; 

 and it is known by experiment, if the dew point in this case 

 should be about 71, which is common in the summer, that 

 a sufficient cold would be produced by the expansion of the 

 air in rising to this height, even after making allowance for 

 the latent caloric given out by the condensing vapor, to 

 condense one half the vapor contained in the air. There- 

 fore, with the assumed velocity of the upmoving column of 

 air, 1\ times as much vapor would be condensed in fifteen 

 minutes as one of these columns contains; or about enough 

 to cover a space equal to the base of the upmoving column, 

 forty-one inches deep. Now if this ascending spout of air 

 should not be perpendicular, it would throw the drops of 

 rain or hail which might be formed by the condensing vapor 

 in its upward motion, all out on one side, and these drops, 

 in descending, would evidently tend inwards towards the 

 centre of the falling mass, because there the drops would 

 meet with less resistance to their descent by following 

 others which had preceded them. 



Thus a column of descending water or hail might be 



54 



