92 Interval betzveeti the Flashy 



doubts, tlioKgh the matter is Jar from being demonstrated^ that 

 the light and the noise have been produced simultaneously. At 

 the same time, the phenomenon is so simple, that even the an- 

 cients, little advanced as they were in natural science, were ac- 

 quainted with the true cause. Take up, for example, the Sixth 

 Book of Lucretius'* Poem, and you will speedily find observa- 

 tions which are intended to establish that light moves with much 

 more rapidity than sound. Some verses later, you will also 

 find, as the inevitable consequence of these premises, that the 

 lightning"'s flash should reach the earth much sooner than the 

 thunder^ voice, although both the thunder and lightning had 

 been formed at the same instant, and'by the same shock. 



This explanation is perfectly accurate ; and the only advan- 

 tage we possess over the philosophers of antiquity, is the power 

 of assigning for every given distance, the precise time which in- 

 tervenes between the light and the sound in seconds, and the 

 fractions of seconds. 



Two astronomical phenomena (the eclipses of Jupitefs satel- 

 lites and aberration)^ have enabled us to prove, that light tra- 

 verses space uniformly with a velocity of -80 thousand leagues 

 (French) for every second of time. Hence it follows that it is 

 only one eight-thousandth part of a second in travelling ten 

 leagues. But ten leagues, beyond doubt, far surpasses the ele- 

 vation at which thunder and lightning are produced in our at- 

 mosphere ; so that disregarding a fraction of a second which is 

 wholly inappreciable, we may, in all our researches upon thun- 

 der, safely conclude, that we see the lightning at the very mo- 

 ment of the flash. 



As to the sound, it may be affirmed, according to the most 

 recent experiments, the temperature of the air being 50^^ Fahr., 

 that its velocity equals S68.5 yards (English) per second. If 

 the cloud, whence the lightning has issued, is at a distance in a 

 straight line of 368.5 yards, there will elapse an entire second 

 between the appearance of the light and the arrival of the sound. 



jneteor, prevented the sonorous vibrations from escaping from the interior 

 of the water-spout, and from being communicated to the nearly tranquil air 

 of the rest of the atmosphere. I doubt, however, whether this explanation, 

 ingenious as it is, will obtain many proselytes. Most will prefer the idea of 

 the production of light even without sound. 



