June 1, 1889.] 



♦ KNO^A^LKDGE * 



165 



Photograph 1889. 2nd pair. 

 „ ,, Line at . 



„ „ Srd pair . 



Photograph 1888 : 

 Lines across star spectra, 1st group , 



.about. . A 328.5 



i „ . . 32750 



„ . . 30600 



f „ . . 30530 



I „ . . 3047-0 



„ . . 41160 



( .. . . 4J23 



41300 



41420 



4154-0 



41670 



(approximate 30980 



o 1 ■> 39880 



2°flgro"P \ ;; 3075-0 



[ „ 39590 



r „ 38960 



I „ 3887-0 



„ . 38780 



„ 38700 



„ , I „ 3859-0 



Srd group .J '^ ?,i,rA{y 



" 3848-0 



3842-0 

 38320 

 3825-0 



As to the chemical significance of the lines lie has 

 observed, Dr. Huggins says : 



" Until I can obtain more pbotograph.s taken on different 

 parts of the nebula, I wi.sh to be understood to speak on 

 this point with much hesitation, and provisionally only. 

 We know certainly that two of the lines are produced by 

 hydrogen. The fineness of these lines points to a high 

 temperature and condition of great tenuity of the hydrogen 

 from which the light was emitted. This condition of the 

 hydrogen may give us a clue as to the probable interpreta- 

 tion of the other lines. These may come from substances 

 of very low vapour-density, and under molecular conditions 

 which are consistent with a high temperature. It is in 

 accordance with this view that the recent measures of Dr. 

 Copeland, since confirmed by Mr. Taylor, show with great 

 probability that the line known as Dj, which has been sup- 

 posed to indicate some subsfcxnce of very low vapour- density, 

 which shows itself only at the hottest region of the sun, is 

 present in the nebula spectrum. The great simplicity of 

 the three pairs of lines seen in the photograph of 1889 sug- 

 gests a substance of a similar chemical nature. 



" If hydrogen can exist at half its usual vapour-density, 

 with a molncnle of one atom only, we might possibly expect 

 to find it in some of these bodies, but at present -vve do not 

 know what its spectrum would be in such a condition. It 

 may be possibly that it is in molecular states of our elements 

 other than those we are acquainted with that we may have 

 to look for an interpretation of some of the lines of these 

 bodies." 



PHOTOGRAPHS OF LIGHTNING. 



By W. Marriott and A. C. Eanyahh. 



^51! VERY one who reads this has probably ex- 

 ^In perienced, at some time or other, strange 

 feelings of awe during the passiige of a 

 great thunderstorm. Our want of know- 

 ledge with respect to the behaviour of 

 lightning adds to the terror. As the 

 thunder crashes and rolls we feel tliat we 

 are in the presence of unknown forces. Some people even 

 close all shutters and doors, and shut themselves up in fear 

 and dread of observing the storm. Those, however, who 

 have had the courage to watch a thunderstorm have found 

 that they are not blinded by the lightning, though they 

 have frequently learnt very little as to tlie form of the 

 flashes or the direction in which they travel. 



The duration of a lightning flash is inconceivably small, 

 being in some instances about a mUlionth part of a second. 

 This is proved by the observation of radial lines on a rapidly 

 rotating disc, which ajijiear absolutely stationary when 

 illuminated by a lightning flash. The eye is dazed by the 

 sudden brightness, and is unable to follow the fl:ish, or to 

 determine whether it proceeds from the clouds to the earth 

 or from the earth to the clouds. The generally received 

 idea has been that the lightning passes from the clouds to 

 the earth ; it is, however, probable that it most frequently 

 proceeds from the earth to the clouds. 



It is popularly supposed that lightning takes the zigz-ig 

 course so frequently depicted by artists, but, as will be seen 

 from the plates accompanying this article, lightning gene- 

 rally pursues a curious sinuous course. Mr. James Nasmyth, 

 who was perhaps the first to give a correct picture of natural 



.!.- -BRANi-llliD 1.U;1IT.\1NG, ACCOKDIXG IM XasMVTH. 



lightning, believed the error of the aitists originated in the 

 form given to the thunderbolts in the hand of Jupiter as 

 sculjitured by the (ireeks. In a paper communicated to the 

 Uritish As.sociation in 1850, Jlr. Nasmyth stated thai he 

 had never observed the zigzag form of lightning usually 

 represented in works of art (as indicated in fig. 1). but that 



