ATMOSPHERIC ELECTRICITY. 361 



"but it is not rare to see them launch out from one group of clouds to 

 another; sometimes, even, the electric fluid, after several zig-zag move- 

 ments, returns toward the region from whence it came. This last 

 phenomenon is frequently observed in volcanic clouds.* Exact obser- 

 vations place it bej'ond doubt that streaks of lightning sometimes 

 become forked, and are divided into three branches, which often sepa- 

 rate to points very remote from each other. f 



Mr. FaradayJ has given a new and ingenious explanation of the 

 phenomena. As he regards them, the different forms which the flashes 

 of lightning take do not belong to the discharges, but to the edges 

 of the clouds behind which they are produced. Let any one imagine 

 a dark cloud, presenting an irregular and well defined border, placed 

 between the eye of the observer and the place of electrical discharge; 

 at the moment when this is effected the cloud will present an illu- 

 minated edge, similar to the shining edge which it would exhibit 

 were it placed between the sun and the observer. Such may be, in 

 many cases, the cause of the apparent form of the flashes ; what the 

 observer really sees being only the irregular outline of a cloud. 

 Mr. Faraday states that he has frequently been able to verify this 

 opinion, and according to him, we are thus to explain the branching of 

 the discharges of lightning, their return to the region from whence 

 they came, and also the remarkable phenomenon of the flashes having 

 frequently exactly the same form and succeeding each other rapidly 

 in the same point of the sky, without, however, being produced by the 

 same electrical discharge. We readily believe that illusions of this 

 kind sometimes take place ; but we have observed the branching of the 

 lightning exhibited under such circumstances that it becomes difficult, 

 not to say impossible, to apply the explanation of the learned English 

 philosopher. For example, the Abbe Kichard§ states that he saw a 

 luminous discharge, single at its departure from the cloud, divide itself 

 in two at some distance from the earth, and each half strick a separate 

 object. Here is certainly a case of forking in which the bright line 

 delineated in the space belonging clearly to the discharge of lightning. 



The streaks of lightning are not always of the same color. Those 

 which present themselves under the form of a narrow winding dis- 

 charge are, in general, white ; they have sometimes been observed, 

 but rarely, of a purplish violet or bluish tinge. The light of the dif- 

 fused flashes of lightning neither presents this whiteness nor an 

 equal brilliancy ; its tint is often of a very intense red, sometimes 

 mixed with blue or violet. Sometimes it appears only to illumine the 

 edges of the clouds; at others it includes their whole superficial ex- 

 extent. The differences which are observed in the intensity and color 

 of the light of the flashes may depend upon several causes, among 

 which we must reckon more or less the density and moisture of the 

 stratum of the air in which the clouds are found, as well as its greater 

 or less degree of conductibility. In fact, experiment proves that 



* Philosophical Transactions for 1795, Ist part, p. 73. 



t Aunuaire for 183S, p. 252; Dictionnaire de Gehler, torn. T, p. 1000; and Lehrbuch der 

 Meteorologie, torn. II, p. 42':^. 

 t Philosophical Magazine, vol. XIX, p. 101. 1841. 

 $ Annuaiie for 163:): p. 253. 



