358 ATMOSPHEEIC ELECTRICITY. 



■which the indications were complicated by elements foreign to those 

 sought; in the mean time we await new observations which may 

 serve to correct them. It is only by means of many series of regular 

 observations, made with proper instruments, that wo can hope to 

 illustrate, and resolve the various questions which are raised by the 

 results under discussion. 



Confining ourselves to the province of the historian, we proceed to 

 give in the last part of our task an epitome of the information hitherto 

 gained relative to the phenomena of lightning. 



PART III. 



SUMMABY OF OUR KNOWLEDGE OF LIGHTNING. 



CHAPTEE I. 



OF THE FLASHES OF LIGHTNING. 



When the two electricities accumulated in the contiguous portions of 

 two clouds, or of a cloud and a body placed within its sphere of activity 

 at the surface of the earth, have a,cquired a tension capable of over- 

 coming the resistance the air opposes to their union, an electrical 

 explosion takes place, and we see the lightning dart through the air 

 under different forms, such as, according to M. Arago,* diffused flashes, 

 linear and zigzag streams, and globular masses ; the latter having a 

 progressive motion. 



Some attempts have been made by Helvigf to determine the velocity 

 of the linear flashes of lightning. His observations by means of the 

 camera lucida have led him to attribute to them a velocity of 40,000 

 or 50,000 feet in a second ; but, as Messrs. Pfaff and Kaemtz| observe, 

 so many illusions may have existed in the mode of observation pur- 

 sued by this philosopher that we can only consider this data as a 

 rough approximation. But we know, according to the beautiful expe- 

 riments of Mr. Wheatstone^§ that the velocity with which the electric 

 fluid passes through a conducting wire is about 115,000 leagues per 

 second, and we may conceive that the velocity of linear discharges of 

 lightning must be the same. [This does not follow.] 



The length of these flashes does not appear to be well known ; we have 

 only a single direct determination of this element. In a storm which 

 happened on the 2d of May, 1839, M, Weigsenborn,|l of Weimar, 

 counted 19 seconds as the time comprised between the appearance of 



'-' Anmiaire du Bureau des Longitudes pour I'an 1838, pp. 249, 255, 257. 



t Annales de Gilbert, torn. LI, p. 136. 



f Dictionnaire de Gehler, torn. I, p. 1001, et Lehrbuch der Meteorologie, torn. II, p. 430. 



§ Philosophical Iransactions for 1834, 2d part, p. 589. 



II Coniptes Eeudus, torn. IX, p. 218. 1839. 



