SECTIONAL TRANSACTIONS.— A. 535 



Discussion on The Mechanism of Thunderstorms. (Dr. G. C. Simpson, 

 C.B., F.R.S. ; Prof. C. T. R. Wilson, F.R.S. ; Dr. R. A. Watson- 

 Watt ; Mr. B. T. G. Schonland ; Prof. J. J. Nolan.) 



Dr. G. C. Simpson, C.B., F.R.S. — The breaking-drop theory ascribes the origin of 

 the electricity in a thunderstorm to the breaking of the raindrops, which are iield up 

 within the cloud by ascending currents having a vertical velocity of more than eighi. 

 metres per second. The water after breaking has a positive charge, the corresponding 

 negative charge going into the surrounding air. The positively charged water tends 

 to accumulate in relatively small regions called the ' regions of separation,' while the 

 negative electricity is distributed widely throughout the cloud by the air currents. 

 The direct consequences of this theory are the following, the names in brackets giving 

 the authors whose observations will be quoted to support the theory : 



Mai7i. — Heavily charged rain from the centre of "the storm is positively charged, 

 while the lighter rain from the cloud as a whole is negatively charged (Simpson). 



Form of lightning discharges. — Photographs of lightning show that the majority 

 of lightning flashes start in positively charged clouds and generally from a definite 

 small region of the cloud. Occasionally discharges take place from the ground to the 

 negatively charged cloud ; discharges of this nature are violent and branched upwards. 



Discharges in tropical storms. — The region of separation is high in a tropical storm ; 

 hence the discharges which originate in the region of separation do not pass out of 

 the cloud. The discharges which can be seen between the earth and the cloud are 

 the relatively few discharges from ground to the negatively charged cloud (Watson 

 Watt, Schonland). 



Distribution of potential gradient in the neighbourhood of a thunderstorm. — As more 

 positive than negative electricity is carried down by the rain, and as the accumulated 

 positive charge in the region of separation is constantly being dissipated through 

 lightning discharges, the cloud as a whole remains negatively charged. Thus the 

 potential gradient is predominantly negative, and the current from the ground is 

 mainly positive (Schonland, Wormell). 



Polarity of Clouds.- — A great deal has been written regarding the polarity of clouds, 

 that is, whether the positive electricity in the cloud is above the negative or vice versa. 

 Wilson and many others have concluded that a thundercloud has positive electricity 

 in its upper parts and negative electricity in its lower parts. As this is the reverse 

 of what one would expect on the breaking-drop theory it has been claimed that this 

 disproves the breaking-drop theory. The observations on which these conclusions 

 have been reached were measurements of the changes in field strength caused by 

 lightning discharges. These observations, however, tell us nothing about the polarity 

 of the cloud ; and every observed field change can be as readily explained with a 

 cloud of negative polarity as with one of positive polarity. 



Conclusions. — The breaking-drop theory gives a physical explanation of the 

 mechanism of a thunderstorm, which is quantitatively and qualitatively capable of 

 explaining all observations so far made. 



Friday, September 7. 



Discussion on The Photographic Measurement of Radiation. (Dr. R. A. 

 Sampson, F.R.S. ; Dr. H. S. Spencer-Jones ; Dr. F. C. Toy ; Dr. 

 I. 0. Griffith ; Dr. W. T. Astbtjry ; Dr. R. A. Houstoun.) 



Mr. H. F. BiOGS. — London's Theory of Valency and Stereochemistry. 



Mr. J. Thomson. — Ultra-violet Radiations emitted by Point Discharges. 



Experiments are described in which electrical methods were employed to detect 

 the radiations emitted by the gas in a spark discharge and to measure their intensities. 

 The experiments show (a) the variation of the intensity of these radiations at a distance 

 from their source when the pressure of the gas in the discharge tube is varied and the 

 current in the spark is kept constant ; (6) the variation of the intensity when the 

 discharge current is varied and the pressure is maintained at a constant value. 



