360 ATMOSPHERIC ELECTRICITY. 



was always under an angle of 40° that the deviation of the flash of light- 

 ning was produced; butBrandes* remarks that this determination cannot 

 he regarded as exact, because, in the instrument by which it was obtained 

 the same angle would appear more or less acute, according to the 

 position of the observer's eye. But it would not be without interest 

 to science to have more certain data as to the form of linear discharges 

 of lightning, and the more so as it is thought that some connexion 

 lias been discovered between this form and the peals of thunder. 

 Possibly the property which certain substances possess of becoming 

 instantaneously phosphorescent under the influence of the light de- 

 veloped by an electrical discharge, and this at the distance of several 

 yards, as M, Becquerel has shown, f might furnish means of obtain- 

 ing phosphorescent impressions of the streaks of lightning, and thus 

 determine all the irregularities which the discharge exhibits in its 

 course through the air. 



It may be asked, What is the cause which produces the zig-zag forms 

 of lightning ? AVithout stopping to consider the hypothesis of Lo- 

 gan,! ^^^^^ regarded them as the result of irregular refractions which 

 the atmospheric vapors and the clouds cause the rays of light to 

 undergo, we shall remark that two different explanations have been 

 given of this phenomenon. According to the first, which is by 

 Parrot, § it is produced by alternate portions of the air, more or less 

 humid, and consequently of more or less conducting power, which the 

 electrical substance meets in its passage, and which oppose to it unequal 

 resistances in different directions. It is true that the atmosphere 

 contains a great variety of exhalations, and particularly of watery 

 vapor irregularly diffused ; besides this explanation agrees with 

 the phenomenon which lightning presents at the surface of the earth, 

 where we see it leave bodies which are poor conductors of electricity 

 for those which offer it a more ready passage ; but this explanation 

 is considered insufficient, because it cannot be applied to the ordi- 

 nary electric spark. In fact, in the narrow space which separates 

 a body from the conductor of the electric machine, we cannot regard 

 the air of a chamber as presenting strata alternately dry and moist, 

 and yet the spark none the less pursues a sinuous course. 



The second explanation, which is more generally admitted, is based 

 on the resistance which compressed air opposes to the motion of 

 electricity. Atmospheric air being a slight conductor of electricity, 

 it is natural to suppose that the electricity, in passing through it, 

 drives before it the molecules of which it is composed, from whence 

 results, successively, compressions along the whole line through which 

 the discharge takes place. This effect of an electric discharge is also 

 confirmed by the known experiment of Kinnersley. Compressed air 

 opposing a greater resistance, the electricity will follow the path along 

 which the air is less condensed, and will deviate from its direction in 

 a straight line to traverse that of a broken line. 



Linear discharges of lightningordinarily take placebetween the clouds;. 



« Beibriige zur Witterungskunde, p. 353. Leipzig, 1820. 

 t Coiuptes Kendus, torn. VIII, p. 216. 1839. 

 i Philosophical Transactions, vol. XXXIX, p. 210. 1735. 

 § Dictionnaire de Gehler, torn. I, p. 999. 



