October 2, 1898.] 



KNOWLEDGE 



103 



[Prof. Hale's letter was received just as Knowledok was 

 going to press, and I have consequently not been alile to 

 reproduce for readers of Knowlkixie the interesting photo- 

 graph forwarded by him, but 1 hope to do so on a future 

 occasion. — A. C. RANVAsn.] 



THE ABSENCE OF A LUNAR ATMOSPHERE. 

 To the Editor of Knowledge. 



Deau Str, — There seems to mo so simple an answer to 

 this question — what has become of the lunar atmosphere ? 

 that it is probable the suggestion has been made many 

 times before. However, I have not seen the 

 idea suggested, and therefore submit it to your 

 readers. I would suggest that the atmosphere 

 has not been drawn away from the moon, but 

 that it is merely frozen. 



It is well known that oxygen and nitrogen con- 

 dense to a liquid at something under — 140°. At 

 a lower temperature they solidify. The moon is a 

 cold body. Let it be granted that the temperature 

 is below — 140 ', and what would become of the 

 atmosphere ? It would only exist in the liquid 

 or solid state. 



Possibly, in the absence of afiy pressure a 

 much lower temperature than —140° would be 

 required ; but that is immaterial. In the pro- 

 tracted lunar night — a fortnight long — the degri i 

 of cold might be so great, and the atmosphere 

 therefore frozen so hard, that the heat of the 

 lunar day would be utterly unable to volatilize it. 

 Yours faithfully, 



C. W. SwEETINCx. 



[Profs. Langley's and Very's observations seem 

 to point to a temperature near to the freezing 

 point of water during the lunar day in the 

 neighbourhood of the moon's equator. — 

 A. C. Eanyahd.] 



LIGHTNING PHOTOGRAPHS AND SOME 

 PHOTOGRAPHIC DEFECTS. 



By A. C. Ranyard. 



THE Graphic of 19th August contained a picture 

 made from the photograph reproduced in Fig. 1, 

 together with the following paragraph : — 

 "A PiioToOBAPn OP A Sxcnw. — Dr. W. B. Hcmpliill, of 

 Kingstown, to whom we are indobtcel for the pliotograph 

 we reprofluoo, informs us tliat the nej^ative was taken on Wednesilay, 

 Aiijiist 9th, at 11 r.ir. The liglitning on the left of the i)ietiire is 



To the Editor of Knowledge. 



Dear Sir, — A method of I'eadily comparing two photo- 

 graphs for the purpose of detecting new stars (or missing 

 stars) has occurred to me, which would obviate the 

 necessity of tedious comparisons and measurements. 



My method depends upon the fact that if a negative 

 and its positive are correctly adjusted, face to face, then 

 no image is transmitted, the field is neutral ; but if the 

 negative or positive is not identical, then the difference is 

 apparent on inspection. 



To apply this in practice, I propose to take an old star 

 negative, not too dense, and compare it with a positive 

 film, also wanting in density, which has been printed from 

 a recent negative ; when these two are exactly superposed, 

 any new stars will at once become conspicuous to the eye 

 as white spots, and any stars which have disappeared will 

 be evident as black spots ; or a print may be taken, thus 

 showing all differences between the two photographs. 



I think this method of examination would show very 

 slight changes in nebula?, &c., which might otherwise 

 escape the eye. 



If the two pictures are allowed to be slightly out of 

 adjustment, then all objects are thrown up in relief, giving 

 nebula;', &c., a very substantial appearance. 



I am sending a negative and positive, taken from one of 

 the collotypes in Knowledge, to make the matter clear, 

 and you will see that I have put a few extra stars on the 

 back of the negative. Yours faithfully, 



F. H. Glew. 



[The method proposed by Mr. Glew seems to me likely 

 to be very useful. — A. C. Ranyard.] 



Fig. 1. — Supposed photograph of ball lightning, taken by 

 Dr. W. D. Hemphill, August 9th, 1893, at 11 p.m. 



the ordinsiry stream coming from the clouds above ; but that on the 

 right is what appeared to be a ball of fire, which fell direct into the 

 sea, and like potassium when thrown into water, darted about on the 

 surface. The ball of fire carried with it a trail of light, leaving 

 smaller balls here and there, and one of these passing a gas-lamp, 

 illuminated the glass as shown in the photograph. The display was 

 one of the most brilliant and vivid ever beheld, and fortunately the 

 camera happened to be pointed in the direction, as the knotting and 

 twisting of the threads of light resembled the course of a ball of wool 

 played with by a kitten more than anything else." 



Upon seeing this paragraph I wrote a letter, addressed to 

 Dr. Hemphill, inquiring whether the camera was fixed or 

 held in the hand during the exposure, and whether he had 

 himself seen the ball of fire wandering to and fro upon the 

 sea. In reply to which he courteously sent me a silver 

 print from his negative and the following letter : — 



" 6, Windsor Terrace, Kingstown. 



"August 30th, 1893. 

 " Dear Sib, — In answer to your letter I beg to give you the follow- 

 ing particulars. I was sitting in my window opposite the sea, and, 

 anxious to take a photo of lightning, I placed my camera — a small 

 \ plate hand camera — on a table ; my daughter was sitting with me 

 watching the storm. When I opened the shutter to take the stream of 

 lightning on the left of the picture, ray daughter called out — 'A ball 

 of fire has fallen near Sandycove ! ' I at once moved the camera in 

 my hand towards Sandycove, the tower of which you see on the right 

 of the bay. The light from the heavens on sea, land, &c., was almost 

 blinding, otherwise the night was quite dark about 11 o'clock p.m. ; 

 but unfortunately, just as I had the plate exposed, a man lit the lamp 

 shown in the picture. I kept the camera pointed in the position of 

 the balls for some seconds, and my daughter tells me that she 

 distinctly saw three balls fall into the sea at short intervals. I 

 examined the negative with a high power pocket glass, and am of 

 opinion that the connecting lines between the balls were caused by 

 reflections from the gas-lamp, which was brilliantly illuminated by the 

 electric discharges, and probably caused the lines during the moving 

 of the camera (still open). I may tell you I showed the negative to 

 Sir Howard Grubb, who examined it with me, and also to Prof. 

 Tichborne. The explanation about the gas-lamp causing the lines was 



