98 On the Cause of Thunder. 



Hence we are brought to this alternative, either that the thunder 

 heard by Volney proceeded from an atmosphere which was per- 

 fectly serene, or it was produced in clouds, which were situated, 

 at the most, at the insignificant height of 98 feet. Of these two 

 hypotheses, it appears the choice should be so much the less doubt- 

 ful, since the clouds which, an hour after the thunder was heard, 

 covered the atmosphere at Pontchartrain, were majestic hail 

 clouds. But whatever may be the value of this reasoning, so 

 far as the particular observation which occasioned it is con- 

 cerned, it is not the less certain, that after hearing thunder with 

 a serene sky, we ought carefully to inspect, looking all round, 

 if there be no clouds beginning to appear in any quarter of the 

 horizon.* 



In deducing these important consequences from the deter- 

 mination of the interval of time which elapses between the ap- 

 pearance of the lightning and the noise of the thunder, it has 

 not been necessary that^we should know the physical cause 

 to which thunder is to be ascribed. The researches which 

 have been undertaken to discover this cause, ought not, how- 

 ever, the less to be mentioned in this place, although they have 

 not been attended with all the success which was desirable. 



A smart clap of our two hands produces a very considerable 



* In considering the above observation of Yolney with all attention, I 

 cannot go the length which this able man did when he concluded that thun- 

 der might certainly proceed from serene skies. 



Pliny reports, that at the time of Catiline's conspiracy, a municipal De- 

 curion of Pompeii, Herennius by name, was struck by lightning which 

 proceeded from a sky without clouds : he does not however state, whether 

 thunder accompanied the lightning ; and, therefore, this quotation leaves 

 the question very much where it found it. 



Suetonius informs us, " That after the death of Caesar, a circle like the 

 rainbow was seen in the clear and serene sky, to surround the disk of the sun, 

 and lightning struck the monument of Julia, the daughter of Ca}sar. We 

 know, at the present day, that no circle similar to the rainbow, and that in 

 fact no circle, whether a halo, or a simple arch, could possibly appear round 

 the sun in a sky that Avas quite clear and serene. . The historian should 

 have been contented with remarking, that the phenomenon occurred at a 

 time when the sky generally was clear. It ought likewise to be noted, that 

 he says nothing here of thunder. The event mentioned by Crescentius 

 raises the same kind of doubts. This author very decidedly declares, that 

 on one occasion near the island of Procida about mid-day, ichen the sky teas 



