[ 334 ] 

 XLVII. Intelligence and Miscellaneous Articles. 



ON THE PROBABLE CAUSE OF ELECTRICAL STORMS. 

 BY DR. J. P. JOULE, F.R.S. 



I^HE very close correspondence between the theoreticalrateof cool- 

 ing in ascending, and the actual, indicates a rapid transmission 

 of the atmosphere from above to below, and vice versd, continually 

 going on. We may believe that during thunder-storms this inter- 

 change goes on with much greater than ordinary rapidity. At a 

 considerable distance from the thunder-cloud, where the atmosphere 

 is free from cloud, the air descends, acquiring temperature according 

 to the law of convective equilibrium in dry air. The air then tra- 

 verses the ground towards the region where the storm is raging, 

 acquiring moisture as it proceeds, but probably without much dimi- 

 nution of temperature, on account of the heated ground making up 

 for the cold of evaporation. Arrived under the thunder- cloud, the 

 air rises, losing temperature, but at a diminished rate, owing to the 

 condensation of its vapour to form part of the immense cumulus 

 cloud which overcasts the sky on these occasions. The upward 

 current of air carries the cloud and incipient rain-drops upwards, but 

 presently, in consequence of the increased capacity of the mass from 

 the presence of a large quantity of water, the refrigeration of the air 

 in consequence of its dilatation will be so far diminished as to pre- 

 vent the condensation of fresh vapour, and ultimately to redissolve 

 the upper portion of the cloud. This phenomenon, w r hich has been 

 noticed by Rankine in the cylinder of the steam-engine, will account 

 for the denned outline of the upper edges of cumulus clouds. The 

 upward current no doubt extends occasionally to regions below the 

 freezing temperature. If cloud be carried with it, snow or hail will 

 be formed, which, if sufficiently abundant, will pass through the 

 cloud and fall to the ground before it is melted. Now the dry cold 

 air in which the snow and hail are formed is a perfect insulator. Ice 

 has also been proved, by Achard of Berlin, to be a non-conductor and 

 an electric. Even water, in friction against an insulator, is known, 

 from the experiments of Armstrong explained by himself and Faraday, 

 to be able to produce powerful electric effects ; and this fact has been 

 suggested by Faraday to explain powerful electric effects in the atmo- 

 sphere. Sturgeon has noted the remarkable developmentof electricity 

 by hail-showers. Few heavy thunder-storms occur without the fall of 

 hail. Hail, whether in summer or winter, is almost, if not invariably, 

 accompained with lightning. In the presence of these facts it seems 

 not unreasonable to consider the formation of hail as essential to 

 great electrical storms, although, as has been pointed out by Prof. 

 Thomson, very considerable electrical effects might be expected from 

 the negatively charged air on the surface of the earth being drawn 

 up into columns, and although, as the same philosopher has observed, 

 every shower of rain gives the phenomena of a thunder-storm in 

 miniature. The physical action of insulators and electrics in mutual 

 friction must certainly produce very marked effects on the grand 

 scale of nature. If we suppose that the falling hail is electrified by 

 the air it meets, the electrification of the cloud into which the hail 



