When I say that the thunder was simultaneous with the lightning, I speak as an inexact person. My meaning is, that the time which elapsed between seeing the flash and hearing the report was inappreciable to me. I wish to speak with all possible precision, and there are two points in regard to this storm upon which I can speak with some accuracy. The first is in regard to the distance of the lightning from our party. We might have been eleven hundred feet from it if a second of time had elapsed between seeing the flashes and hearing the reports; and a second of time is not appreciated by inexact persons. It was certain that we were sometimes less than that distance from the lightning, because I saw it pass in front of well-known points on the ridge, both above and below us, which were less (sometimes considerably less) than a thousand feet distant.
THE CRAGS OF THE MATTERHORN, DURING THE STORM, MIDNIGHT, AUGUST 10, 1863
Secondly, in regard to the difficulty of distinguishing sounds which are merely echoes from true thunder or the noise which occurs simultaneously with lightning. Arago entered into this subject at some length in his Meteorological Essays, and seemed to doubt if it would ever be possible to determine whether echoes are always the cause of the rolling sounds commonly called thunder. I shall not attempt to show whether the rolling sounds should ever or never be regarded as true thunder, but only that during this storm upon the Matterhorn it was possible to distinguish the sound of the thunder itself from the sounds (rolling and otherwise) which were merely the echoes of the first, original sound.
At the place where we were camped a remarkable echo could be heard (one so remarkable that if it could be heard in this country it would draw crowds for its own sake): I believe it came from the cliffs of the Dent d’Hérens. It was a favorite amusement with us to rouse this echo, which repeated any sharp cry in a very distinct manner several times, after the lapse of something like a dozen seconds. The thunderstorm lasted nearly two hours, and raged at times with great fury; and the prolonged rollings from the surrounding mountains after one flash had not usually ceased before another set of echoes took up the discourse, and maintained the reverberations without a break. Occasionally there was a pause, interrupted presently by a single clap, the accompaniment of a single discharge, and after such times I could recognize the echoes from the Dent d’Hérens by their peculiar repetitions, and by the length of time which had passed since the reports had occurred of which they were the echoes.
If I had been unaware of the existence of this echo, I should have supposed that the resounds were original reports of explosions which had been unnoticed, since in intensity they were scarcely distinguishable from the true thunder, which during this storm seemed to me, upon every occasion, to consist of a single harsh, instantaneous sound.[[13]]
[13] The same has seemed to me to be the case at all times when I have been close to the points of explosion. There has been always a distinct interval between the first explosion and the rolling sounds and secondary explosions which I have believed to be merely echoes; but it has never been possible (except in the above-mentioned case) to identify them as such.
Others have observed the same. “The geologist, Professor Theobald, of Chur, who was in the Solferino storm, between the Tschiertscher and Urden Alp, in the electric clouds, says that the peals were short, like cannon shots, but of a clearer, more cracking tone, and that the rolling of the thunder was only heard further on.” Berlepsch’s Alps, English ed., p. 133.
Or if, instead of being placed at a distance of less than a thousand feet from the points of explosion (and consequently hearing the report almost in the same moment as we saw the flash, am the rollings after a considerable interval of time), we had been placed so that the original report had fallen on our ears nearly at the same moment as the echoes, we should probably have considered that the successive reports and rollings of the echoes were reports of successive explosions occurring nearly at the same moment, and that they were not echoes at all.
This is the only time (out of many storms witnessed in the Alps) I have obtained evidence that the rollings of thunder are actually echoes, and that they are not, necessarily, the reports of a number of discharges over a long line, occurring at varying distances from the spectator, and consequently unable to arrive at his ear at the same moment, although they follow each other so swiftly as to produce a sound more or less continuous.[[14]]