Sept. 2, 1 8 So] 



NATURE 



409 



sion. You will remember that we are at present dealing 

 with the appearances observed i?t a thunderstorm, so that 

 I do not refer to that form of sheet-lightning which com- 

 monly goes by the name of summer-lightning, and which 

 is not, audibly at least, followed by thunder. 



The next remarkable feature of the storm is the thunder, 

 corresponding, of course, on the large scale, to the snap 

 of an electric spark. Here we are on comparatively sure 

 ground, for sound is very much more thoroughly under- 

 stood than is electricity. We speak habitually and with- 

 out exaggeration of the crash of thunder, the rolling of 

 thunder, and of a fcal of thunder ; and various other 

 terms will suggest themselves to you as being aptly em- 

 ployed in different cases. All of these are easily explained 

 by known properties of sound. The origin of the sound is in 

 ail cases to be looked for in the instantaneous and violent 

 dilatation of the air along the track of the lightning flash, 

 partly, no doubt, due to the disruptive effects of electricity 

 of which I have already spoken, but mainly due to the 

 excessive rise of temperature which renders the air for a 

 moment so brilliantly incandescent. There is thus an 

 extremely sudden compression of the air all round the 

 track of the spark, and a less sudden, but still rapid, rush 

 of the air into the partial vacuum which it produces. 

 Thus the sound-wave produced must at first be of the 

 nature of a bore or breaker. But as such a state of motion 

 is unstable, after proceeding a moderate distance the 

 sound becomes analogous to other loud but less violent 

 sounds, such as those of the discharge of guns. Were 

 there few clouds, were the air of nearly uniform density, 

 and the flash a short one, this would completely describe 

 the phenomenon, and we should have a thunder crash or 

 thunder clap according to the greater or less proximity of 

 the seat of discharge. But, as has long been well known, 

 not merely clouds but surfaces of separation of masses of 

 air of different density, such as constantly occur in thun- 

 derstorms, reflect vibrations in the air ; and thus we may 

 have many successive echoes, prolonging the original 

 sound. But there is another cause, often more efficient 

 than these. When the flash is a long one, all its parts 

 being nearly equidistant from the observer, he hears the 

 sound from all these parts simultaneously ; but if its parts 

 be at very different distances from him, he hears succes- 

 sively the sounds from portions farther and farther distant 

 from him. If the flash be much zig-zagged, long portions 

 of its course may run at one and the same distance from 

 him, and the sounds from these arrive simultaneously at 

 his ear. Thus we have no difficulty in accounting for the 

 rolling and pealing of thunder. It is, in fact, a mere 

 consequence, sometimes of the reflection of sound, some- 

 times of the finite velocity with which it is propagated. 

 The usual rough estimate of five seconds to a mile is near 

 enough to the truth for all ordinary calculations of the 

 distance of a flash from the observer. 



The extreme distance at which thunder is heard is not 

 great, when we consider the frequent great intensity of 

 the sound. No trustworthy observation gives in general 

 more than about nine or ten miles, though there are cases 

 in which it is possible that it may have been heard four- 

 teen miles off. But the discharge of a single cannon is 

 often heard at fifty miles, and the noise of a siege or 

 naval engagement has certainly been heard at a distance 

 of much more than 100 miles. There are two reasons 

 for this : the first depends upon the extreme suddenness 

 of the production of thunder ; the second, and perhaps 

 the more effective, on the excessive variations of density 

 in the atmosphere, which are invariably associated with a 

 thunderstorm. In certain cases thunder has been propa- 

 gated, for moderate distances from its apparent source, 

 with a velocity far exceeding that of ordinary sounds. 

 This used to be attributed to the extreme suddenness of 

 its production; but it is not easy, if we adopt this hypo- 

 thesis, to see why it should not occur in all cases. Sir 

 W. Thomson has supplied a very different explanation, 



which requires no unusual velocity of sound, because it 

 asserts the production of the sound simultaneously at all 

 parts of the air between the ground and the cloud from 

 which the lightning is discharged. 



We now come to an exceedingly strange and somewhat 

 rare phenomenon, to which the name oi fire-ball ox globe- 

 ligJiinino has been given. As we are as yet unable to 

 produce anything of this kind by means of our electrical 

 machines, some philosophers have tried to cut the Gor- 

 dian knot of the difficulty by denying that any such thing 

 can exist. But, as Arago says, "Oa' en serious jious, si 

 nous nous mettions a nier tout cc gu'on ne salt pas 

 expliquer?" The amount of trustw-orthy and independent 

 evidence which we possess as to the occurrence of this 

 phenomenon is such as must convince every reasonable 

 man who chooses to pay due attention to the subject. 

 No doubt there is a great deal of exaggeration, as well as 

 much imperfect and even erroneous observation, in almost 

 all of these records. But the existence of the main 

 feature (the fivs-ball) seems to be proved beyond all 

 doubt. 



The most marked peculiarities of this species of 

 lightning-discharge are its comparatively long duration 

 and its comparatively slow motion. While a spark, or 

 lightning-flash, does not last longer than about a millionth 

 part of a second, if so long, globe-lightning lasts from one 

 to ten seconds, sometimes even longer, so that a suffi- 

 ciently self-possessed spectator has time carefully to 

 watch its behaviour. The general appearance is that of 

 a luminous ball, which must be approximately spherical, 

 because it always appears circular in outline, slowly and 

 steadily descending from a thundercloud to the ground. 

 It bursts with a loud explosion, sometimes before reaching 

 the ground, sometimes as it impinges, and sometimes 

 after actually rebounding. Its size varies from that of a 

 child's head to a sphere of little less than a yard in 

 diameter. On some occasions veritable flashes of 

 lightning were seen to proceed from large fire-balls as 

 they burst. It is difficult to imagine what these balls 

 can be if they be not a species of natural Leyden jar very 

 highly charged. If it be so, no ordinary lightning-rod 

 can possibly prevent danger from it ; and we may thus be 

 able to explain the verj' few cases in which damage has 

 been done by lightning to thoroughly protected buildings. 

 To guard against this form nothing short of a pretty 

 close net-work of stout copper wires would suffice. 

 Meanwhile I give a brief sketch of t-^'o out of the long 

 series of descriptions of such phenomena which Arago 

 has patiently collected. The first is given on the authority 

 of Babinet, who was deputed by the Academy of Sciences 

 to make inquiries into the case. 



Shortly, but not immediately, after a loud [peal of 

 thunder, a tailor who was sitting at his dinner saw the 

 paper ornament which covered his fire-place blown down 

 as if by a gentle breeze, and a globe of fire, about the size 

 of a child's head, came gently out and moved slowly about 

 at a slight elevation above the floor. It appeared bright 

 rather than hot, and he felt no sensation of warmth. It 

 approached him like a little kitten which desired to i-ub 

 itself in play against his legs ; but he drew his feet away, 

 and by slow and cautious movements avoided contact 

 with it. It remained several seconds near his feet, while 

 he leaned forward and carefully examined it. At last it 

 rose vertically to about the level of his head, so he threw 

 himself back in his chair and continued to watch it. It 

 then became slightly elongated, and moved obliquely 

 towards a hole pierced to the chimney about a yard above 

 the mantelpiece. This hole had been made for the 

 chimney of a stove which was used in winter. " But," 

 as the tailor said, " the globe could not see the hole, for 

 paper had been pasted over it." The globe went straight 

 for the hole, tore olT the paper, and went up the chimney. 

 After the lapse of time- which, at the rate at which he 

 had seen it moving, it would have required to get to the 



