150 



KNOWLEDGE 



[August 1, 1892. 



posed m the circuit, and the discharge is fouud to be 

 sufficiently prolonged to fire the gunpowder before it is 

 blown away. 



From what we know of the travelling of waves along a 

 narrow channel, it seems probable that the maximum 

 electric disturbance would lead the way, and that it would 

 be followed by a decreasing wave, rather than that a small 

 disturbance should arrive first and be followed by an in- 

 creasing disturbance, but it is desirable that this question 

 should be set at rest ; possibly Mr. Glew, or somebody in 

 the future, may succeed in taking lightning photographs 

 with a lens moving uniformly in a circle of sufficient size, 

 with respect to the photographic plate, to enable us to 

 determine the part of the circular path which was being 

 described at the time that the photograph was taken. 



The photograph at the top right-hand corner of our 

 plate is beheved by Mr. Dunn, who took it, and by his 

 father, who was standing beside him at the time, to be a 

 photograph of ball lightning. At about a quarter past ten 

 on the evening of the 17th of July, 1891, Mr. W. F. 

 Dunn and his father were standing at the open window of 

 an upper room of their house in Westmoreland Eoad, 

 Newcastle-on-Tyue. A severe thunderstorm was passing 

 over the town, and the sou had his camera pointed out of 

 the window ready to take a photograph. The window 

 overlooks the valley of the Tyne from a point above the 

 Elswick Works. Suddenly the father saw a ball of fire, 

 which appeared to him to be over the river, and to be 

 moving in an easterly direction down towards the sea — ' 

 not very rapidly, but faster than a man could run. The 

 ball appeared to Mr. Dunn to be about two feet in ' 

 diameter, and when nearly opposite to the house it i 

 stopped and then disappeared. Mr. Dunn thinks that j 

 about the time it stopped he called to his son to make 

 an exposure. The son says that he took off the cap of the ' 

 camera for an instant, and replaced it. The plate was 

 developed in the ordinary way. It was by no means ' 

 the first plate which Mr. Dimn, juu., had developed. 

 He had been practising photography for some months, 

 and had obtained several successful photographs, and ' 

 never before or since has developed a plate which showed 

 a patch or streaks similar to those found upon this plate. 

 Both father and son have made statutory declarations 

 before a magistrate as to what they .saw and beheve with 

 respect to the photograph, and they have kindly submitted 

 the statutory declarations, as well as the original negative, 

 to me for examination. I have also had anopportumty of 

 asking Mr. Dunn, sen., some questions as to what" he 

 remembers. He says that the ball of fire appeared to be 

 about twice the diameter of the moon. If it was two feet in 

 diameter, this would correspond to a distance of about 

 115 feet; but the place over the Tyne where the ball of 

 fire appeared to be is certainly at a much greater distance 

 than this from his house. The camera with which the 

 photograph was taken has a focal length of about 

 5i inches ; consequently, a photograph of the full moon I 

 taken with it would only have a diameter of about | 

 oVth of an inch, but the bright patch which he beheves 1 

 to correspond with the ball lightning has a diameter 

 of li inches on the original negative. Therefore, if it 

 represents the ball of fire, the ball must have had a 

 diameter of about 1,5^, and it must have either been much 

 nearer to the window than Mr. Dunn supposed, or its 

 diameter must have been much greater than two feet. I 

 am far from feeling convinced that Mr. Dunn has succeeded 

 in photographing ball lightning, but it seemed desirable 

 to reproduce his photograph, with such particulars as I 

 could gather ; for a photograph showing somewhat similar 

 streaks and patches was shown to me in 1889. It had 



been taken during the thunderstorm which broke over the 

 metropolis with great %-iolence on the Gth of -June, 1889, 

 and was believed by the young man who took it — and who 

 kindly called upon me and allowed me to question him — 

 to be a photograph of lightning, though I was inclined to 

 attribute the marks upon his plate to want of care in 

 developing. 



The two lower photographs shown on the plate 

 published with this number of Knowledge are copied 

 from photographs sent me by Mr. S. W. Burnham, who, 

 our readers will regret to hear, is about to leave the Lick 

 Observatory and return to his old profession in Chicago. 



Utttcvs. 



[The Editor does not hold himself responsible for the opinions or 

 statements of correspondents.] 



To the Editor of Knowledge. 



Dear Sir, — I beg to enclose a photograph of a discharge 

 of lightning. Tlie photograph was taken about 9.50 p.m. 

 on Tuesday, June 28th, 1892. The camera was pointed 

 S.E. from the door, at 156, Clapham Road. The lens was 

 secured to the hammer of an electric bell, giving nine com- 

 plete vibrations per second, the amplitude being about \ of 

 an inch. It is evident that the main flash and its side forks 

 all took place in less than the half of one vibration, and the 

 motion of the lens separated the three component flashes, 

 which to the eye would have appeared to be superposed. 

 I compute from measurements that the whole discharge 

 occupied about the o\jth part of a second, and the interval 

 between the components about ^^th of a second. It will be 

 seen that the right-hand side of each spark is fainter than 

 the left, showing that the duration of each component was 

 considerable, and not of the same intensity throughout its 

 existence. The original negative shows this a little clearer 

 than the print. Thomas's " Sandell " plates were used, 

 which being thickly coated, in separate layers, has prevented 

 halation defects. Some of the shadows of chimney-pots are 

 due to sheet lightning previous to the grand flash. The 

 vibrations of the lens were in the same plane as the plate, 

 and in the direction of its length. 



Yours faithfully, 

 156, Clapham Road, London, S.W., F. H. Glew. 



July 11th, 1892. 



P.S. — It will be seen that one portion of the flash is 

 converted into chain lightning, this being due to the 

 motion of the lens being almost in the direction of the 

 length of this part of the flash, and in consequence of this 

 the diflferent curves have been more or less superposed m 

 direction of length, in the form of a chain. I think this 

 throws some light on the so-called chain lightning as seen 

 by the eye, for if the duration of a compound spark is so 

 much as j-^gth of a second, it is possible for the eye 

 (corresponding to the lens of the camera) to move through 

 a certain distance, and if this movement of the eye happens 

 to be in the direction of the length of the compound spark, 

 then its components will be drawn out into a chain-like 

 structure or complex ripple. Perhaps the duration may 

 be even greater than -r}^i\i of a second in some cases, and 

 the eye may be set in motion by the first component of the 

 flash. As the tendency of the eye would be to move in order 

 to bring the object into the best position for distinct vision, 

 so I think the ett'ect of chain lightning might be formed in 

 this way, or by accidental motion of the eye of the observer 

 during the display. — F. H. G. 



