July 1, 1889.] 



♦ KNOWLEDGE ♦ 



177 



%r^ AN ILLUSTRATED "^^ 



MAGAZINE OF SCIENCE 



SIMPLY WORDED — EXACTLY DESCRIBED 



L(^NDON: JULY 1, 1889. 



(• 



CONTENTS. 



Eow Long Does a Flash, of Ligbtning Last ? By A. C. Ranyard 



Tiger-Beetle?.— n. By E. A. Butler 



Phospbatcsas Fertilife'rs'j- By D. A. Loan, F.LC, F.C.S 



Oar Micro£Copic Foea. ^Jf A. Winkelried Williams 



On Earth-WormB. B^/ff. "Mansel Sympsou, 5I.A., M.B., Cantab 



Some Propertis of Xumb.TS. By Robt. W. D. Christie 



Photograplis of Nebu'^'t By A. C. Hanyard 



Star-torn Meteors. By the late Richard A. Proctor 



Letters :— Prof. W. H. Piclcering, Prof. E. Holden, G. H. Clarke, T. H. Bn 



The Vanilla 



Bcfraction of ilagnetic Hadiatioa 



TheFaceot the Sky for July. By Herbert Sadler, F.R.A.S 



Ohess Column 



Whist Column. By W. Montagu Gattie 



HOW LONG DOES A FLASH OF 

 LIGHTNING LAST? 



By a. C. Ranyard. 



HE recent thunderstorms came most oppor- 

 tuneh', while the information given in the 

 last number was fresh in the minds of the 

 readers of Knowledge. A great storm 

 swept acro.=s England on June 2, travelling 

 northward from the coast of M'iltshire, 

 ])ast Liverpool and Edinburgh, and up 

 into Scotland. According to ob.servations collected by Mr. 

 Marriott, the storm centre travelled at the rate of 50 miles 

 an hour, and was attended by violent electrical disturbances 

 as well as the fall in some places of extraordinaiily largo 

 hailstones, some of which, according to tru.stworthy reports 

 sent to the Meteorological >Society, measured 7 inches in 

 circumference and weighed 7 ounces. On the evening of 

 Thursday, June 6, London was visited by a great storm, 

 which also drifted northward. Mr. H. S. Wallis, a 

 statistically-minded observer, counted 1,214 flashes in two 

 hours as seen by him from Higbgate, which gives an 

 average of more than ten discharges per minute. 



Most observers of this storm agree that the flashes were 

 generally multiple flashes; they either de.scribe a pulsation 

 about the light of the lightning, or state that they saw dis. 

 tinct flashes, sometimes four on five in number, travelling; 

 along the same course with a perceptible interval of time 

 between them. To my eye it srcined that the fir.st flash 

 was generally the brightest, and that sometimes a flash was 

 succeeded by a glow which lasted an appreciable part of a 

 second. I .saw no ribbon flashes and no dark flashes. 

 There wei-e a very few observers who thought that they .saw 

 both ; we will deal with their observations later on. 



Amongst the photographs that were taken on the tith were 

 several of the so-called " dark flashes," and several photo- 



graphs of ribbon flashes. There are also some photographs 

 which show three or four flashes which evidently travelled 

 along the same course. Photographs Nos. 1 and 2 are of 

 this description. It might at first sight be thought that 

 these were photographs of parallel flashes; but an examina- 

 tion of the original negative shows in each case a triple 

 image of a railw.ay .signal-post, and that the three images of 

 the signal-post are separated by about the same distance as 

 the three flashes. The three images of the post correspond 

 in intensity with the three lightning flashes, and the 

 displacement was in the same direction on the plate : so that, 

 if the three images of the post were made to coincide, the 

 three images of the flash would also coincide turn for 

 turn and knot for knot. We can, therefore, hardly 

 doubt that the plate shifted between the three flashes, and 

 that the lightning really passed, in each case, along the 

 same path of least resistance. This is more probable, when 

 we come to consider it, than that there should have been 

 three parallel flashes in the air, which followed one another 

 at an even distance through all the irregularities of the 

 sinuous course which lightning pursues. 



Indeed, from the facts already at our command, we 

 might expect that the first flash would heat the air and 

 slightly rarefy it, leaving a course of least resistance, along 

 which subsequent discharges would flow as certainly :is 

 water follows the twLsts and turns of a pipe. When the 

 original negatives are examined, the evidence as to the shift 

 of the plates in the interval between the three flashes is 

 overpowering. The evidence which we are able to present 

 to our readers is not so strong as that derived from the 

 original negatives, owing to the faintness of the photo- 

 graphic images of objects which are only illuminated Ijy the 

 light of a lightning flash. At the bottom right-hand corner 

 of photograph No. 5 the summit of the signal-box referred 

 to above is shown. The camera hardly moved during this 

 flash, and it will be noticed that each post is surmounted by 

 a pointed ornament. Photographs 1, 2, and 5 were all taken 

 from the same spot. The signal-box is also shown at the bottom 

 right-hand corner of {)hotograph No. 2, but the overlapping 

 images are ghostlike and hazy. On the original negative 

 the three images of the pointed summits of the posts and 

 the distance between them can be measured, and is found 

 to correspond with the shift of the image of the lightning 

 flash. In photograph No. 1 the image of the signal-box is 

 lost, but three images of it are distinctly seen on the original 

 negative, and the direction of the shift of the plate can be 

 determined with certainty. There is a faint gauze, ribbon- 

 like structure, which joins the middle fl;isli with the left- 

 hand flash. It is rather more dense in places where the 

 flash has been moved obliquely across the photographic 

 plate, but the nebulous veil is so f^iint that I fear no traces 

 of it will be shown in the photographic copies published 

 with this. However, a somewhat similar phenomenon will 

 probably be recognisable on jihotograph No. 2, when 

 examined with a lens. The right-hand flash is nebulous on 

 its inner or left-hand side, and the left-hand flash is also 

 nebulous on its inner side, proving that the one flash began 

 and the other ended w-ith a glow. There are two other 

 small flashes which can be traced running parallel with the 

 three larger flashes. The whole system of flashes appear to 

 have passed behind a cloud in their upper portion. Where 

 they appear above the cloud they arc fainter, but the nebulous 

 character of the outer flashes is more easily recognised than 

 below. 



In addition to these photographs in which we have pho- 

 tographic evidence that the plate was shifted during the 

 exposure, a most interesting ])hotogra]ih was taken by Dr. 

 H. H. Hotfert, who intentionally moved his camera to and 

 fro while ho pointed it in the direction in which flashes 



