340 



NATURE 



{August 12, 1880 



powerful artificial sources, appears to the non-scientific 

 trifling in comparison with a mile-long flash of lightning, 

 the difference (huge, if you like to call it) is as nothing to 

 others with which scientific men are constantly dealing. 

 The nearest star is as much farther from us than is the 

 sun, as the sun is farther from us than is London. The 

 sun's distance is ninety-three millions of miles. If that 

 distance be called enormous, and it certainly is so, what 

 adjective have you for the star's distance? Ordinary 

 human language, and especially the more poetic forms of 

 it, were devised to fit human feelings and emotions, and 

 not for scientific purposes. A thoroughly scientific ac- 

 count of a thunderstorm, if it were possible to give one, 

 would certainly be at once ridiculed as pedantic. 



Let us therefore, instead of attempting to discuss the 

 phenomenon as a whole, consider separately some of its 

 more prominent features. And first of all, what are 

 these features when we are /;/ the thunderstorm ? 



By far the most striking, at least if the thunderstorm 

 come on during the day, is the extraordinary darkness. 

 Sometimes at mid-day in summer the darkness becomes 

 comparable with that at midnight, very different in kind 

 as well as intensity from that produced by the densest 

 fog. Objects are distinctly visible thrCiugh it at distances 

 of many miles, whether v.'hen self-luminous or when in- 

 stantaneously lit up by lightning. The darkness, then, 

 is simply intense shadow, produced by the great thickness 

 and great lateral extension of the cloud-masses overhead. 

 Seen from a distance, the mass of cloud belonging to the 

 storm usually presents a most peculiar appearance, quite 

 unlike any other form of cloud. It seems to boil up, .as 

 it were, from below, and to extend through miles of verti- 

 cal height. The estimated height of its lower surface 

 above the ground varies within very wide limits. Saus- 

 sure has seen it as much as three miles ; and in one case 

 noticed by De I'lsle it may have been as much as five 

 miles. On the other hand, at Pondicherry and Manilla 

 it is scarcely ever more than half a mile. Haidingcr 

 gives the full details of an extraordinary case, in which 

 the thundercloud formed a stratum of only twenty-five 

 feet thick, raised thirty yards above the ground. Yet two 

 people were killed on this occasion. Other notable 

 instances of a similar extreme character are recoi'ded. 



Careful experiment shows us that the air is scarcely 

 ever free from electricity, even in the clearest weather. 

 And even on specially fine days, when large separate 

 cumuli are floating along, each as it comes near produces 

 a marked effect on the electrometer. Andrews obtained 

 by means of a kite, on a fine clear day, a steady decom- 

 position of water by the electricity collected by a fine wire 

 twisted round the string. Thanks to Sir W. Thomson, 

 we can now observe atmospheric electricity in a most 

 satisfactory manner. I will test, to show you the mode 

 of proceeding, the air inside and outside the hall. [The 

 experiment was shown, and the external air gave negative 

 indications.] 



On several occasions I have found it almost impossible, 

 even by giving extreme directive force to the instrument 

 by means of magnets, to measure the atmospheric poten- 

 tial with such an electrometer, and had recourse to the 

 old electroscope, with specially long and thick gold- 

 Iea.ves. On February 26th, 1874, when the sleet and 

 hail, dashing against the cupola of my class-room, made 

 so much noise as to completely interrupt my lecture, I 

 connected that instrument with the water-dropper, and 

 saw the gold-leaves discharge themselves against the 

 sides every few seconds, sometimes with positive, some- 

 times, often immediately afterwards, with negative elec- 

 tricity. Such effects would have required for their 

 production a battery of tens of thousands of cells. Yet 

 there was neither lightning nor thunder, and the water 

 was trickling from the can at the rate of only two and a 

 half cubic inches per minute. Probably had there not 

 been such a violent fairof sleet steadily discharging the 



clouds we should have had a severe thunderstorm. 

 Falling rain-drops are often so strongly charged with 

 electricity as to give a spark just before they touch the 

 ground. This "luminous rain," as it has been called, is 

 a phenomenon which has been o\-er and over again seen 

 by competent and trustworthy observers. In the Comptes 

 Rendu s for No\-ember last m'c read of the curious phe- 

 nomenon of electrification of the observer's umbrella by 

 a light fall of snow, to such an extent that he could draw 

 sparks from it with his finger. 



In calm clear weather the atmospheric charge is usually 

 positive. This is very commonly attributed to evaporation 

 of water, and I see no reason to doubt that the phenomena 

 are closely connected. [A few drops of water were 

 sprinkled on a heated crucible, insulated, and connected 

 with the electrometer.] 



There can be no doubt that, whatever be the hidden 

 mechanism of this experiment, the steam has carried with 

 it a strong charge of positive electricity, for it has left the 

 rest of the apparatus with a strong negative charge. We 

 will now try that form of the experiment in another way. 

 [High-pressure steam escaping from a little boiler was 

 made to play upon an insulated conductor furnished with 

 spikes, and connected with the electrometer, which then 

 showed a strong positive charge.] 



There are many substances which produce on evapora- 

 tion far greater electric developments than water does, 

 some of positive, others of negative, electricity. By far 

 the most remarkable in this respect to which attention has 

 yet been called is an aqueous solution of sulphate of 

 copper. {Proc. R.S.E., 1862.) The smallest drop of this 

 solution thrown on a hot dish gives an intense negative 

 effect — so great, in fact, that it may be occasionally 

 employed to charge a small Leyden jar. But this, like 

 the smaller effect due to water under similar circum- 

 stances, is not yet completely explained. 



The next striking features are the flashes of lightning 

 which at intervals light up the landscape with an intensity 

 which must in the majority of cases far exceed that pro- 

 duced by the full moon. To the eye, indeed, the flash 

 does not often appear to furnish more than the equivalent 

 of average moonlight, but it must be remembered that it 

 lasts for a period of lime almost inconceivably short, and 

 that the full effect of light on the eye is not produced until 

 after the lapse of a considerable fraction of a second. 

 Prof. Swan has estimated this interval at about one-tenth 

 of a second ; and he has proved that the apparent intensity 

 of illumination for shorter intervals is nearly proportional 

 to the duration. {Tj-aiis. R.S.E., 1S49.) ^ can illustrate 

 this in a very simple manner. [Two beams of light were 

 thrown upon the screen by reflection from mirrors, each 

 of which was fixed nearly at right angles to an axis. When 

 matters were so adjusted that the brightness of the two 

 illuminated spots was the same, one mirror was made to 

 rotate. The corresponding light spot described a circle 

 about the other, and its brightness became less the larger 

 the circle in which it was made to revolve.] The lightning 

 flash itself on this account, and for the farther reason that 

 its whole apparent surface is exceedingly small, must be 

 in some degree comparable with the sun in intrinsic 

 brilliancy— though, of course, it cannot appear so. The 

 fact that its duration is excessively short is easily verified 

 in many ways, but most simply by observing a body in 

 rapid motion. The spokes of the wheels of the most 

 rapidly-moving carriage appear absolutely fixed when 

 illuminated by its light alone. One can read by its light 

 a printed page stuck on a disc revolving at great speed. 

 But the most severe test is that of Sir Charles Wheat- 

 stone's revolving mirror. Seen by reflection in such a 

 mirror, howeverfast it may be rotating, a flash of light- 

 ning is not perceptibly broadened, as it certainly would 

 be if its duration were appreciable. 



The apparatus which, in our laboratories, enables us to 

 measure the ti iie which light, moving at nearly 200,000 



