the Theory of Hail. 179 



gravitation. In a storm described by Luke Howard 28 , that 

 occurred at Tottenham on the 19th April, 1809, at 5 p.m., the 

 icy bullets, some of them a full inch in diameter, were dis- 

 charged almost horizontally, and with such velocity that in 

 many instances a clean round hole was left in the glass they 

 pierced, and one large pane had two such perforations, dis- 

 tinctly formed, the glass being otherwise whole. The com- 

 paratively small width of the hail's track is also in favour of 

 the cyclonic theory, although the length may be considerable. 

 The great storm that began in the south of France early on 

 the 13th of July, 1788, extended in a few hours over the 

 whole kingdom, even as far as Holland. It proceeded in two 

 parallel zones from S.W. to N.E. ; one zone was 175 leagues 

 in length, and the other 200 ; the breadth of the western zone 

 was four leagues, and of the other only two. The zone between 

 the two was five leagues wide, but no hail fell there, only 

 heavy rain. There was also rain on the outer boundary of 

 the two zones. A thick darkness accompanied the hail, and 

 spread on both sides. The storm travelled at the rate of 

 16^ leagues an hour in both zones. Upwards of a thousand 

 parishes were ravaged by this storm. 



Turning now to Mr. William Ferrel's theory 29 of the 

 formation of hail in connexion with a tornado or cyclone, he 

 calculates in an assumed example that the plane or stratum 

 of zero temperature is 6428 metres above the base of the 

 cloud, and in the absence of friction may be supposed to be 

 brought down to the earth in the centre, where the gyrations 

 are very rapid. Below this base aqueous vapour is condensed 

 into cloud and rain, but above it into snow. The rain-drops 

 below may also be carried up into the snow-region in the ascend- 

 ing currents, and if kept suspended there for a short time they 

 may become frozen into small hail. They may then be kept 

 suspended near the base of the snow-cloud, and increase in 

 size by the rain, which is carried up into this region, coming 

 into contact with them before it has had time to freeze. In 

 this way compact homogeneous hailstones of ordinary size 

 are formed. At the height of nearly 7000 metres the density 

 of the air in comparison with that at the earth's surface is 

 0*42, and it is calculated in the assumed example that a 

 velocity of 20 metres per second in the ascending current will 

 sustain a hailstone one centimetre in diameter at that altitude. 

 This is no unusual velocity for ascending currents in tornados. 

 It is not necessary that the hailstones should remain long in 



28 Climate of London. 



29 United States Coasts Survey. Meteorological Remarks for the 

 Use of the Coast Pilot, part ii. p. 85. Washington, 1880. 



