640 proceedings: philosophical society 



may be accounted for, and the origin and maintenance of the Earth's 

 charge itself explained. 



The paper was discussed by Messrs. Olshausen, Bauer, Hum- 

 phreys, and Bellinger. 



Mr. J, W. Humphreys then presented a paper on Lightning discharges 

 illustrated with lantern slides. The electric separation essential to the 

 lightning discharge is produced, according to Simpson's well supported 

 theory, by the disruption of rain drops in the air and the transfer of 

 the resulting negative spray to higher altitudes by convection currents. 

 But however produced, when the potential-gradient between different 

 portions of the cloud, or between the cloud and the earth, has any- 

 where reached the disruptive value, lightning immediately occurs. 



When seen at close range lightning appears as a sinuous line or 

 streak of vivid white or, occasionally, pink — sinuous because, pre- 

 sumably, of irregular ionization and consequent irregular conductivity 

 of the atmosphere. Some of these discharges are single, as shown by 

 rotating cameras, and of short duration, while many are multiple, 

 consisting of a number of partial discharges generally of unequal 

 strength and at irregular intervals, but all following the same path, 

 the ionized and therefore conducting path of the first discharge. Occa- 

 sionally a streak of lightning persists a full second or even longer, 

 gradually fading away, and thus producing, through its end-on and 

 therefore brighter portions, the phenomenon known as beaded or pearl 

 lightning. 



Sheet lightning, so often seen in distant cumuli, presumably is only 

 the diffused and reflected light of ordinary streak lightning. The 

 objective reality both of rocket or slow moving lightning and of ball 

 lightning is often doubted, but both have the support of many excel- 

 lent observers. 



It is commonly stated that the lightning discharge consists of many 

 high frequency oscillations. A close examination of the evidence how- 

 ever indicates that it is either unidirectional or else very heavily damped. 

 But little is known of the maximum current strength in a lightning 

 discharge, though there is evidence sufficient, apparently, to show 

 that this may amount to 10,000 amperes, perhaps even 100,000 amperes 

 but that the quantity of electricity is surprisingly small, rarely enough 

 to electrolyze 0.1 cc. of water. 



The paper was discussed by Messrs. Littlehales, Olshausen, 

 Austin, Frazer, and Swann. 



F. E. Wright, Secretary pro tern. 



