40(5 DE. G. C. SIMPSON ON THE ELECTEICITY OF 



About this table Prof. LENARD says : " One sees from it that the velocity quickly 

 reaches a limiting value as the size of the drops increases (very nearly equal to 

 8 metres a second), above which it does not increase ; it even decreases a little as the 

 drops grow still greater." 



He then showed that this apparent anomaly is due to the drops becoming deformed, 

 so that instead of retaining the shape of spheres they become flattened out, thus 

 presenting an increased resistance to the air through which they fall. In consequence of 

 this deformation large drops rapidly break up in the air into smaller drops, and 

 LENARD found that drops of 4 mm. diameter were stable under all conditions, but that 

 drops 5'5 mm. and above in diameter could not exist for more than a few seconds after 

 attaining their final velocity relative to the air. 



This fact plays an important part when drops of water are falling tl iron gli ascending 

 currents. According to the table given above, all drops of water of a smaller diameter 

 tlian 4'f> nun. will be carried upwards by a current of 8 metres a second, while 

 all drops of a larger diameter than this will be held in suspension, neither rising nor 

 falling. Hut the latter are unstable, and after floating for a few seconds in the 

 current break ii]> into small drops which are carried upwards. Thus no water could 

 possibly fall through an ascending current of air having a velocity of 8 metres a 

 second or more. 



That thunderstorms are accompanied by strong ascending currents is admitted 

 by all meteorologists, but I know of no actual measurements of the ascending 

 currents within a thundercloud ; still the question can be discussed from indirect 

 evidence. 



Then; is no essential difference in kind between a tornado, a hailstorm, and an 

 ordinary thunderstorm, all of which are accompanied by electrical discharges. 



Now in the first two of these we know definitely that ascending currents of 

 excessive velocity do occur. The many authenticated cases in which heavy structures 

 and implements have been raised to considerable heights during tornadoes give 

 absolute proof of ascending currents comparable with the greatest horizontal winds 

 known. Now a horizontal velocity of 8 metres a second (29 kilometres, or 18 miles 

 an hour) is defined as a moderate breeze, and wind velocities of 40 metres a second 

 (approximately 100 miles an hour) have been measured during tornadoes; thus we 

 see that ascending currents having velocities many times greater than 8 metres 

 a second must occur during tornadoes. 



In the formation of hailstones we have equally certain evidence of strong ascending 

 currents. A hailstone cannot grow appreciably above the size which would be 

 sufficient to cause it to fall to the ground through the ascending currents below it, so 

 that the size of a hailstone gives a rough measure of the upward velocity of the air 

 current in which it was formed. Now hailstones have been met with having all sizes 

 between those of peas and those of melons. A hailstone as big as a pea would 

 require a vertical velocity of at least 10 metres a second to hold it in suspension ; 





