November 17, 19^0] 



NATURE 



81 



tins negative electricity was slightly in ex- 

 >s. The charge per c.c. tended to be larger the 

 -hter the rain, but the fall in two minutes was so 

 Tiall in light rains that it seems by no means im- 

 : obable that with a more sensitive apparatus there 

 A i.uld have been a smaller total excess of positive elec- 

 ricitv recorded. Observations covering the complete 

 iinual precipitation, whether rain or snow, at a 

 umber of stations in different latitudes will be neces- 

 sary before we can safely draw conclusions respecting 

 I lie earth as a whole. 



It was discovered by Lenard many years ago that 



in the case of an ordinary waterfall, or when water 



falls on a solid obstacle, the water drops formed take 



.( positive, the surrounding air a negative charge. 



Lenard believed, however, that no such separation 



curred when drops split up without falling on an 



;i<tacle. Simpson found a similar absence of charge 



when experimenting with Simla tap-water, but on 



trvinir distilled water he found that the splitting up 



• drops bv means of a vertical air jet is accompanied 



a marked separation of electricity, the water tak- 



i,^ the positive charge. The breaking up of drops, 



,ch containing about 1/4 c.c. of water, gave the 



water a charge of about +23 x 10-^ E.U. per c.c. If 



the drops were already charged, this additional charge 



was added when they broke, so that the action is 



cumulative. Raindrops become unstable on attaining 



■ certain size, and tend to break, so that natural 



-nditions approach those of Simpson's experiments. 



rational explanation is thus given of a positive 



large on rain if it behaves as distilled water. This 



•• should expect it to do, except perhaps in smoky 



districts, but further experiments on actual rain-water 



in various localities seem desirable. The presence on 



nne rain of negative electricity is ascribed by Simp- 



on to a transfer of charge from air which has pre- 



viouslv surrounded breaking raindrops. 



The theoretical problem mainly considered by Simp- 

 - in is the relation of rain to thunderstorms. He 

 lieves that there are normally present in thunder- 

 orm areas upward currents of air with velocities of 

 : least 8 metres per second (18 m.p.h.). Such cur- 

 rents prevent raindrops from falling, and Simpson 

 supposes the drops to go through frequent repetitions 

 of the cycle ; growth, breaking up (with separation of 

 iectricitv), fresh growth, and so on, at a nearly con- 

 ant height in the atmosphere until the charge is 

 I great as to produce at a certain level a gradient 

 irger than 30,000 volts per cm., which he takes to 

 e the electric strength of air. When this limit is 

 -ached, a lightning flash neutralises the accumu- 

 ited charge over a limited area, and the process goes 

 n repeating itself. There are various difficulties in 

 he wav of accepting this explanation as complete, 

 ut some represent our present ignorance rather than 

 ositive knowledge. We should like to know, for 

 istance, whether vertical air currents such as Simp- 

 on postulates really do exist near the precise level 

 here the air breaks down, also what the true nature 

 t a lightning flash is, whether unidirectional or 

 wcillator)-, what charge passes, and what is the ex- 

 •enditure of energy. For all we know, the air may 

 e in a strongly ionised condition, possibly even there 

 nay be separation of the constituent gases, and a 

 'Otential gradient much under 30,000 volts per cm. 

 lav suffice to cause a discharge. In the meantime, 

 impson's theory of thunderstorms had better be 

 ^garded as a hypothesis, but, unlike some hypotheses, 

 promises to be useful in suggesting promising:- lines 

 or observation and experiment. The separation of 

 electricity by the breaking up of raindrops may not 

 play quite so fundamental a part as Simpson sup- 

 poses, but assuming it to take place with natural 



NO. 2142, VOL. 85] 



rain, it can hardly fail to play an important part in 

 thunderstorm phenomena. 



The memoir as a whole is most original and sug- 

 gestive, and is one on which the meteorological service 

 of India deserves to be congratulated. As many 

 readers of Nature are doubtless aware, Dr. Simpson's 

 services have been lent by the Indian Government to 

 the present British Antarctic Expedition, principally 

 with the view of his studying electrical conditions in 

 high latitudes, and we may, I think, entertain high 

 hopes that the resulting increase of knowledge will 

 be eminently satisfactory both to India and to this 

 country. C. Chree. 



A 



THE PREVENTION OF PLAGUE. 

 -MEMORANDUM on plague has recently been 

 prepared by Dr. Newsholme, medical officer of 

 the Local Goveinment Board, and has been sent to 

 the sanitary authorities of England and Wales, with 

 a request that their officers should endeavour to secure 

 the adoption of the suggestions contained therein. 

 The memorandum gives an interesting conspectus of 

 the essential features of the disease, and deals mainly 

 with its methods of spread and the measures which, 

 in the light of recent researches, must be taken for 

 its prevention. Fortunately, i)lague, although a 

 disease capable of manifesting i"tself as an epidemic 

 of a widespread and virulent character, is now so well 

 understood on its epidemiological side, that the direc- 

 tion which preventive measures should take is obvious. 

 The situation may be summarised in the dictum — " no 

 rats, no plague." Practically, however, the matter is 

 perhaps not so simple as it may seem. 



The first section of the memorandum describes 

 briefly the symptoms in plague. The injected eyes and 

 the thick, "drunken" speech are noted as character- 

 istic signs of the disease. There is no mention, how- 

 ever, of the tendency to "shouting" delirium and the 

 impulse to get out of bed and wander off, utterly heed- 

 less of their condition — well-known symptoms in the 

 natives of India. The "acute" ward of a plague hos- 

 pital is at times a very noisy place, and mild restraint 

 requires to be put upon patients to prevent their un- 

 conscious excursions. 



The "ambulant" form of plague is referred to, and 

 it is stated that persons with this type of the disease 

 may spread the infection. Spread of infection by 

 such persons would seem, however, to be very doubt- 

 ful, bv direct personal contagion at least, and it is 

 equally doubtful whether effective carriers of-, the 

 disease in the sense of typhoid carriers exist. The 

 evidence for the existence of such carriers is not satis- 

 factory, and although the possibility of the occurrence 

 of "pneumonic" carriers must be considered, the 

 rarity of this type, at least in India, and its extreme 

 fatality, considerably limit its importance from this 

 point of view. 



The statement that there is little or no liability to 

 infection from contaminated food is a comforting one, 

 and is justified by the accurate observations on the 

 pathology of human plague made some years ago in 

 Bombay by the Austrian Plague Commission, and by 

 the results of experiments on susceptible animals. 



The memorandum accepts in its entirety the results 

 of the recent investigations of the Plague Research 

 Commission, viz., that the sole infective agents in an 

 epidemic of bubonic plague to be reckoned with are 

 the infected rat and the infected rat flea — the former 

 an indirect agent and the latter the immediate infect- 

 ing agent. It follows that the measures suggested 

 for attempting to stamp out the disease are directed 

 solely towards the destruction of rats and their para- 

 sites. It has indeed been claimed that domestic 



