THUNDER is an explosion accompanied by a loud noise, which is heard after a discharge of lightning from the clouds. The character of the noise is variable : it sometimes resembles that which is produced when a single piece of ordnance is fired ; at other times it is a rolling sound like the successive discharges of several great guns ; and occa sionally it may be compared to a series of sharp reports from a fire of musketry.
The identity of lightning with the electric fluid is now well known [Liouramea], but the physical cause of the detonation which accom panies the flash is still the subject of conjecture ; in general it is con sidered that lightning, by its heat, creates a partial vacuum in the atmosphere, and that the sudden rushing of air into the void space produces the sound ; but various reasons have been assigned for its prolongation. It was formerly supposed that the rolling noise is merely the result of several echoes caused by the sound being reflected from mountains, woods, buildings, or clouds, or from the latter alone when a thunder-storm takes place over the ocean : this opinion seems to have been founded upon the fact that the report of a fire-arm dis charged in a mountainous tract is prolonged by the echoes during at least half a minute, which is about the time that the rolling of thunder continues. But though the reflections of sound are, very probably, in part, or at times, tho causes of the prolongation of the report arising from the explosion, yet it must be admitted that these will not always afford a satisfactory explanation of the phenomena. It may happen, for example, that, when the sky is uniformly covered with clouds, a flash of lightning will dart from the zenith, and, after a few seconds, the crash of thunder will take place accompanied by a rolling sound : soon, a second flash may pierce the clouds in the zenith and thunder may follow, but now the crash, though loud, may not be prolonged. It is justly observed by M. Arago that this is very different from the phenomena of echoes; and the explanation which was first proposed by Dr. Hooke (' Posthumous Works,' 1705) is perhaps that which possesses the highest degree of probability. The flashes of lightning, Dr. Hooke observes, are either simple or multiple : the first occupies but one small portion of space, and gives rise to an instantaneous re port ; the multiple flash takes place at different parts of one long line : if these parts should be situated in a circular are, and the observer should be in its centre, all the reports would arrive at his car at the same time, and still one loud crash only would be heard ; but if the parts were nearly in a straight line, and the observer were at one of its extremities, the reports, whether they take place at the same instant or in succession, would arrive at his ear at different times, depending wholly or partly on the distances. It may be considered therefore
that the rolling arises from the circumstance that the points of ex plosion are at different distances from the observer ; and it will follow that tho duration of the noise is equal to the time in which sound travels through an interval equal to the difference between the lengths of two lines drawn from the observer to the two extremities of the flash. The flesh of lightning and the report of the thunder take place in reality at the same moment ; but since sound travels at the rate of 1100 feet per second, while the passage of light from the cloud to the observer may be considered as instantaneous, it follows that, on count ing the number of seconds which elapse between the time of seeing the flash and hearing the report, the distance of the thunder-cloud from the observer may be ascertained if 1100 feet be multiplied by that number of seconds.
An opinion prevails that thunder has been heard when the sky was without a cloud, but the fact can scarcely be said to be satisfactorily established ; for the mounds which, in countries subject to earthquakes, have been supposed to he thunder, proceed from under the ground, and may result from a different cause. Volney however relates that, being one day at Pontchartrain near Versailles, when no cloud was visible, he heard distinctly four or five claps of thunder ; he adds, that about an hour afterwards the sky became overcast, and a violent hail storm followed. On this relation M. Arno observes, that the sounds could not have been heard if they had come from clouds at a greater distance than six leagues; and if the clouds had been at, or a little within, that distance, they must have been visible, unless it be sup posed that they were not more than a few yards above the ground ; but the hail which followed the thunder must have proceeded from clouds having great elevation, though at the time the claps were heard they were too remote to allow any sound from them to reach the ear ; and therefore he concludes that the sounds must have been produced in the air itself.
From the meteorological observations made by Dr. Scoresby, and Captains Phipps, Parry, and Ross, it appears that neither thunder nor lightning is known to take place beyond the 75th degree of north lati tude ; even so low as the 70th degree those phenomena are very rare : and in the tables of Captain Parry the occurrence of thunder and lightning is mentioned but once between June, 1821, and September, 1823. Captain Franklin also, in 675° N. lat., heard thunder on one day only between September, 1825, and August, 1826.