1888.] Photographs of Lightning Flashes. 299 



sparks too feeble to impress themselves on the plate, but cat off parts 

 of others, thinned their images in some places, and it may be assumed 

 •with considerable certainty showed that the images that were obtained 

 were only such parts of the flashes or sparks as were visible through 

 and between the cloud masses. Before referring to .their main 

 features let me draw attention briefly to the recent report of the 

 Meteorological Society's Committee, drawn up by the Hon. Ralph 

 Abercromby. 



The Committee reports that from the consideration of sixty photo- 

 graphs sent from Europe and America in answer to their circular, " it 

 /is evident that lightning assumes various typical forms, under con- 

 ditions which are at present unknown." The following appear to be 

 some of the most typical forms : 



1. Stream Lightning^ or a plain broad, rather smooth streak of 

 light. Only two or three specimens of this form were received by 

 the Committee. 



2. " Sinuous Lightning^ when the flash keeps in some one general 

 direction, but the line is sinuous, bending from side to side in a very 

 irregular manner. This is by far the commonest type." It is very 

 noticeable that the thickness of the line varies during the course of 

 discharge. Sometimes the thinnest part is the highest, at other timefj 

 a flash in the air begins thin, broadens out in the middle and thins 

 again on approaching the earth. The Committee says that it can 

 offer no explanation of this at present, but draws attention to the fact 

 in some photographs of electric sparks taken from an induction 

 coil, those of high tension are thinner than those of low tension. 

 This, I venture to think, affords no explanation of variation in 

 intensity during the course of one flash, though it may explain the 

 variation in thickness between two flashes. 



3. Ramified Lightning^ in which part of the flashes appears to 

 branch off from or run into the main streak like the fibres from the 

 root of a tree. 



4. Meandering Lightning ; sometimes the flash appears to meander 

 about in the air without any definite course and forms small irregular 

 loops. The thickness of the same flash may vary considerably in 

 different parts of the course, as above mentioned. 



5. Beaded or Chapletted Lightning ; sometimes a series of bright 

 beads appear in the general white streak on the photograph. Occa- 

 sionally these brighter spots appear to coincide with bends in a 

 meandering type ; but often the beads appear without any evident 



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