204 SCIENCE PROGRESS 



either because it has rested for some time over ground of 

 superior radio-activity, or because it contains an excess of 

 oxides of nitrogen, or ozone. Should such air be drawn into 

 an ordinary thunderstorm, it would offer an easy path for 

 the discharge of lightning, and would have its conductivity still 

 further increased thereby. A series of discharges might take 

 place, and if an eddy were formed the heated air would not 

 be so easily dissipated into the surrounding air as would 

 happen in the case of an ordinary lightning flash. If the 

 supply of electrical energy were great enough, a tornado might 

 be formed in this way. 



Wind- und Wasserhosen in Enropa. Alfred Wegener (Vieweg 

 & Sohn, Braunschweig, 1917)- 



This book deals at considerable length with " European 

 tornadoes (" grosstromben ") ; but whirlwinds in which there 

 is not sufficient reduction of pressure for the formation of 

 either a funnel cloud or a waterspout, and which are visible 

 only as dust-whirls (" kleintromben "), are not included. 



In the early part of the book there is a catalogue of 244 

 observations of tornadoes, and a full description of eleven of 

 these is given. The author then passes on to a statistical dis- 

 cussion of the whole of the material that he has collected, 

 and gives (Chapter IV) curves showing the monthly frequency 

 of tornadoes in Europe (maximum in July) compared with those 

 for America (maximum in May). The curves in both cases 

 follow those for thunderstorm frequency very closely. The 

 hourly frequency has its maximum between 16 and 17 hours in 

 both Europe and America, the thunderstorm frequency again 

 showing a close agreement. Davis has shown that in North 

 America the tornado almost always occurs in the S.E. quadrant 

 of a depression, but of the 49 European cases which admit of 

 this kind of classification, 20 occur in the S.W. and 29 in the 

 S.E. quadrant. Turning to the question of the direction of 

 movement, in both countries this is most frequently from 

 SW. or W. ; in North America, 87 per cent, have been found 

 to move from these points, for Europe the author obtains the 

 lower figure of 52 per cent. The rate of travel is more often 

 than not under 5 metres per second, but in a few instances 

 amounts to as much as 20 metres per second. Both the length 

 of track and the duration of the phenomenon show great 



