Sept. 9, 1880] 



NATURE 



A37 



remark that in a cloud-mass when just formed, if it be at 

 all uniform, the electric attractions and repulsions would 

 approximately balance one another at every point, so that 

 the mutual repulsion of any two water-drops would be 

 almost compensated, except when they came very close 

 to one another. 



But there is nothing in this explanation inconsistent 

 with the possibility that the particles of water may be 

 caused to fly about repeatedly from cloud to cloud, or from 

 cloud to an electrified mass of air ; and in many of these 

 regions the air, already in great part deprived of its 

 moisture, may have become much cooled by expansion as 

 it ascends, so that the usual explanation of the produc- 

 tion of hail is not, at least to any great extent, interfered 

 with. 



I may here refer to some phenomena which seem to 

 offer, if closely investigated, the opportunity for the large 

 scale investigations which, as I shall presently show, will 

 probably be required to settle the source or sources of 

 atmospheric electricity. 



First, the important fact, well known nearly 2,000 years 

 ago, that the column of smoke and vapour discharged by 

 an active volcano gives out flashes of veritable lightning. 

 In more modern times this has been repeatedly observed 

 in the eruptions of Vesuvius and other volcanos. 



Sabine, while at anchor near Skye, remarked that the 

 cloud-cap on one of the higher hills was permanently 

 luminous at night, and occasionally gave out flashes 

 resembling those of the aurora. I have not been able to 

 obtain farther information as to this very important fact ; 

 but I have recently received a description of a very similar 

 one from another easily accessible locality. 



My correspondent writes from Gahvay, to the following 

 cftect, on the 2nd of the present month : — 

 1 .," At the commencement of the present unprecedentedly 

 long and severe storm the wind blew from south-west 

 and was very warm. After blowing for about two days it 

 became, without change of direction, exceedingly bitter 

 and cold ; and the rain was, from time to time, mixed 

 with sleet and hail, and lightning was occasional. This 

 special weather is common for weeks together in March 

 or early April. The air is (like what an east wind brings 

 in Edinburgh) cold, raw, dry, and in every way uncom- 

 fortable, especially to people accustomed to the moist 

 Atlantic winds. During these weeks a series of small 

 clouds, whose shadows would only cover a field of a few 

 acres, seem to start at regular intervals from the peaks of 

 hills in Connemara and Mayo. They are all more or less 

 charged with electricity. From high ground, behind the 

 city, I have at one time seen such a cloud break into 

 lightning over the spire of the Jesuits' church. At 

 another, I have seen such a cloud pour doivn in a thin 

 line of fire, and fall into the bay in the shape of a small 

 incandescent ball. On one occasion I was walking with 

 a friend, when I remarked, ' Let us turn and make a run 

 for it. We have walked unwittingly right underneath a 

 little thundercloud.' I had scarcely spoken when a some- 

 thing flashed oil the stony ground at our very feet, a 

 tremendous crash pealed over our heads, and the smell of 

 sulphur was unmistakable. I fancy that I have been 

 struck with these phenomena more than others, from the 

 circumstance that they have always interfered with my 

 daily habits. My walks often extended to considerable 

 distances and to very lonely districts. Now these small 

 local spurts of thunderstorms would hardly excite attention 

 in the middle of a town, all the less as the intervening 

 weather is bright, though raw — these spurts coming on 

 every three or four quarters of an hour. Neither would 

 they excite much attention in the country, as, while such 

 a little storm was going on in one's immediate neighbour- 

 hood, you would see at no great distance every sign of fine 

 weather. In fact they always seem to me like the small 

 change of a big storm." 



My correspondent, though a good observer and eloquent 



in description, is not a scientific man. But it is quite 

 clear from what he saj's that a residence of a few weeks 

 in Gahvay, at the proper season, would enable a trained 

 physicist to obtain, with little trouble, the means of 

 solving this extremely interesting question. He would 

 require to be furnished with an electrometer, a hygro- 

 meter, and a few other simple pieces of apparatus, as well 

 as with a light suit of plate armour, not of steel but of the 

 best conducting copper, to insure his personal safety. 

 Thus armed he might fearlessly invade the very nest or 

 hatching-place of the phenomenon, on the top of one of 

 the Connemara hills. It is to be hoped that some of the 

 rising generation of physicists may speedily make the 

 attempt, -in the spirit of the ancient chivalry, but with the 

 offensive and defensive weapons of modern science. 



Another possible source of the electricity of thunder- 

 storms has been pointed out by Sir W. Thomson. It is 

 based on the experimental fact that the lower air is 

 usually charued with negative electricity. If ascending 

 currents carry up this lower air the electricity foiTnerly 

 spread in a thin stratum over a large surface may, by 

 convection, be brought into a very much less diffused 

 state, and thus be raised to a potential sufficient to enable 

 it to give a spark. 



However the electrification of the precipitated vapour 

 may ultimately be accounted for, there can be no doubt 

 of the fact that at least as soon as cloud is formed the 

 particles are electrified ; and what I have said as to the 

 immense rise of potential as the drops gradually increase 

 in size remains unaffected. I have tried various forms of 

 experiment, with the view of discovering the electric 

 state of vapour mixed with air. For instance, I have 

 tested the vapour which is suddenly condensed when a 

 receiver is partially exhausted ; the electrification of 

 cooled bodies exposed to moist air from a gasholder ; and 

 the deposition of hoar-frost from a current of moist air 

 upon two polished metal plates placed parallel to one 

 another, artificially cooled, and connected with the outer 

 and inner coatings of a charged jar. All have given 

 results, but as yet too minute and uncertain to settle such 

 a question. These experiments are still in progress. It 

 appears probable, so far, that the problem will not be 

 finally solved until experiments are made on a scale much 

 larger than is usual in laboratories. 



A great thunderstorm in summer is in the majority of 

 cases preceded by very calm sultry weather. The atmo- 

 sphere is in a state of unstable equilibrium, the lower 

 strata are at an abnormally high temperature, and highly 

 charged with aqueous vapour. It is not easy, in a 

 popular lecture like this, to give a full account of what 

 constitutes a state of stable equilibrium, or of unstable, 

 especially when the effects of precipitation of vapour are 

 to be largely taken into account. It is sufficient for my 

 present purpose to say that in all cases of thoroughly 

 stable equilibrium, a slight displacement tends to right 

 itself; while, in general, in unstable equilibrium, a slight 

 displacement tends to increase. Now, if two cubic feet 

 of air at different levels, could bo suddenly made to change 

 places, without at first any other alteration, and if, on 

 being left to themselves, each would, under the change of 

 pressure which it would suddenly experience, and the 

 consequent heating or cooling, with its associated evapo- 

 ration or precipitation of moisture, tend to regain its 

 former level, the equilibrium would be stable. This is 

 not the case when the lower strata are very hot, and fully 

 charged with vapour. Any portion accidentally raised to 

 a higher level tends to rise higher, thus allowing others 

 to descend. These, in consequence of their descent, tend 

 still farther to descend, and thus to force new portions up. 

 Thus, when the trigger is once pulled, as it were, we soon 

 have powerful ascending currents of hot moist air, pre- 

 cipitating their moisture as cloud as they ascend, cooling 

 by expansion, but warmed by the latent heat of the 

 vapour condensed. This phenomenon of ascending 



