644 



THE NATURE BOOK 



/'.V.i/.., ;-,(/,; /•! .1/. <;. Mfs:,i, r. 



A LIGHTNING FLASH FROM THE EIFFEL TOWER 

 TO A CLOUD, PARIS, JULY 31, 1904. 



sufficiently brilliant to render the land- 

 scape very visible, so that it is impossible to 

 make a reproduction which will show both 

 flash and landscape at once. In order to 

 indicate the 

 landscape M. 

 Mesmer took a 

 photograph of it 

 the next day. 

 using the same 

 camera an d 

 pointing it in 

 the same direc- 

 t i o n. This 

 photograj^h i s 

 placed beneath 

 in the accom- 

 panying figure. 

 The flash actu- 

 aUy left the 

 E i ff e 1 Tower, 

 and, it will be 



noticed, split up into two portions and 

 disappeared in the cloud. 



There are two other points of interest 

 attached to lightning flashes which the 

 application of the photographic method 

 has cleared up completely. 



Many good observers ha\'e published 

 at different times accounts of lightning 

 flashes which they described as " beaded," 

 or " chapleted," or presented the appear- 

 ance of a string of pearls. So well did all 

 accounts agree with each other, that 

 authorities were led to beheve that a 

 peculiar form of lightning flash was in 

 question. There was, however, an ele- 

 ment of disagreement between some of the 

 descri])tions, because one obser\-er would 

 state that he saw a flash which broke up 

 into a series of points or beads, and another 

 that he saw a series of beads alone. 



Now the real origin of this " beaded " 

 lightning, as it has been called, is this. 

 \\> all now know that although a single 

 flash of lightning is instantaneous, 3-et the 

 discharge is so powerful that it heats up 

 the channel of air through which it passes 

 almost instantaneously to incandescence. 

 Further, this column of air is not a straight 

 \-ertical channel from the cloud to the 

 earth, but \'erv wavy, both in the line of 

 sight of the obser\-er and at right angles 

 to it. 



Now. when the lightning discharge is 

 over, this air column cannot cool \'ery 

 quickly, and consequently it remains 

 visible for several fractions of a second. 

 Owing to the wavy nature of the column 

 some portions of the channel will a}:)pear 



I'hotcgriif'h by M. l.tnili I omiii-t. 



TRAIL OF A LIGHTNING FLASH ON A PHOTOGRAPHIC PLATE DUE TO 



THK INCANDESCENCE OF THE AIR. 



Camera moved by hand. April 12, 1904, 9.40 p.m. Paris. 



