100 



♦ KNOWLEDGE ♦ 



[Aug. 1, 1884. 



" Let BCnowledge grow from more to more." — Alfred Tenntsos. 



Only a small proportion of Letters received can possihly he in- 

 serted. Correspondents must not he offended, therefore, should their 

 letters not appear. 



All Editorial communications should he addressed to the Editob of 

 Knowledge; all Business communications to the Publishebs, at the 

 Office, 74, Qreat Queen-street, W.C. If this is not attended to 



DELAYS ABI8E FOB WHICH THE EDITOR IS NOT RESPONSIBLE. 



All Remittances, Cheques, and Post Office Orders should he made 

 payable to Messrs. Wyman & Sons. 



The Editor is not responsible for the opinions of correspondents. 



No COMMUNICATIONS ARE ANSWERED BY POST, ETEN THOUGH STAMPED 

 AND DIEECTED ENVELOPE BE ENCLOSED. 



SOLAE GLOW. 



[1350] — In connection with Capt. Noble's letter (13-i4, p. 77), 

 it may be also of interest to mention that the solar glow is a 

 striking object here in the north-west quarter of the heavens on 

 any tolerably clear evening at aboat an hour and a-half after 

 snnset ; and likewise in the north-cast before sunrise. The colour 

 of the meteor is not quite white, bat seems to be slightly tinged 

 •with yellow. K. Phillifs. 



Northam, Bideford, July 2C, 188-i. 



LIGHTNING— VISIBLE LUMINOUS TRACK OF A 

 METEOR. 



[1351] — The record of Thunderstorms which appeared in your 

 issue of 25th instant induces me to send you these few lines, not 

 80 much for the purpose of telling you that one passed over my 

 house on July 4 (during which one tree was struck in the front 

 lawn, another within twenty paces of the hall door, and another 

 about a quarter of a mile in the rear), as to remark that, in my 

 humble opinion, the conclusion arrived at some time ago in your 

 columns — that, because the duration of a flash of lightning is 

 but about a millionth part of a second, therefore the eye cannot 

 detect its direction — is an erroneous one. For not only have I 

 seen lightning descend from the clouds, and dart from cloud to 

 cloud, but ^also ascend out of the ground; the explanation of 

 which facts, as contrasted with the aforesaid conclusion, appears to 

 me to be very simple, viz., that the electric spark (which, owing to its 

 velocity, appears as a flash) must subtend some angle while passing 

 tlirough the air, which angle is necessarily reproduced in miniature 

 on the retina, the consequence being that the optic nerve conveys 

 to the brain, in exact sequence, the impressions made npon its 

 terminal ramifications. 



I further wish you would kindly ask your readers whether any 

 of them recollect having observed the track of a meteor remaining 

 visible long after the meteor itself had vanished, as I did on 

 one occasion for about half an hour ? Also, can you explain the 

 " quare and quomodo" of such an unusual occurrence ? W. A. 



July 26, 1884. 



[I have myself seen lightning ascend from the earth to a cloud : 

 and hear that this phenomenon was witnessed by a well-known 

 contributor during a heavy storm, on Thursday, the 24th nit. — Ed.] 



LIGHTNING. 



[1352] — I am surprised that none of your correspondents have 

 referred to the copy of the photograph of a flash of lightning, 

 which appeared in yours of the 4th inst., and which is certainly 

 quite unlike the conventional lightning-flash as it appears in paint- 

 ings, &c. I only recollect once getting a good side-view of a 

 thunderstorm. I was looking on at a cricket-match in the Phoenix 

 Park, Dublin, and the storm took place in the neighbourhood of 

 Kingstown. It is some years since, but I cannot fix the precise 

 date. I saw a large number of lightning-flashes, all of which 



strikingly resembled the photograph in question, though in some 

 cases the lines were, I think, a little more wavy In both cases the 

 breadth of the flash was very perceptible, and I should expect to 

 hear that the area of its section was considerable. Nevertheless, 

 when the earth is struck by lightning the surface of contact appears 

 to be very small. Possibly the flashes which I saw and that which 

 was photographed by Mr. Gurley presented exceptional character- 

 istics. Perhaps some of your readers could throw some light on 

 this subject. — I remain, W. H. S. MoNCK. 



13, Belvedere-place, Dublin, July 26, 1884. 



PARTRIDGES : THEIR LOVE OF YOUNG. 



[1353] — One morning here, after a heavy thunderstorm during 

 the night, accompanied by deluges of rain and a great fall in tem- 

 pera! ure, the keepers came npon a family of partridges, all dead — the 

 I lareut birds crouching close together in the grass, their nine little ones 

 between them, pressed as closely as possible to their sides, the inside 

 wing of each parent bird extended, wing over wing, making a 

 double roof for the protection of their family, and in this position 

 these devoted birds had perished, trying to the last to save their 

 young ones at the cost of their own lives. The men who found 

 them seemed quite impressed, and said " It was a pretty picture ! " 



Swigell House, Northumberland, M. J. C. 



July 23rd. 



OVERHEAD WIRES. 



[1354] — Let me say a few words about overhead wires, the 

 danger from which I think may be easily obviated. 



At present the posts to which the wires are fastened are placed 

 on the rid^je of the roof. 



Let additional posts be placed on the eave in the street-front of 

 the building on each side of the street, and directly opposite to each 

 other. 



I assume the distance betwixt these posts across the street would 

 be sixty-five feet, and the height of the building and post together 

 from the ground ninety feet. If so, and the wire broke off, even 

 riosr from the post, it would swing twenty-five feet above the traffic 

 in the street, and could not, therefore, interfere with it. 



The posts could be made longer or shorter, so as in all cases a 

 difference of twenty to twenty-five feet should be obtained. 



J. W. BCSK. 



DIVISIBILITY BY SEVEN. 



[1355]— When a vulgar fraction whose denominator is 7 is 

 reduced to a decimal fraction, its equivalent is a repetend con- 

 sisting of six figures, in which, when extended indefinitely, the 

 7th, 13th, 19th, &c., figures are repetitions of the first, and if the 

 first figure be added to the 4th, 10th, or 16th, ic, figures, the snm 

 is = unity or f . The 1st figure may, for convenience, be called 

 the complement of the 4tb, 10th, or 16th, &c. 



Now, if a number be divided into terms of three figures each, 

 and marked 0, 1, 2, 3, ic, from the right, it is evident from the 

 above that any even term divided by 7 will have the same re- 

 mainder, whether it is taken at its abstract or its local value ; also, 

 that any odd term divided by 7 will, taken at abstract value, have 

 a remainder which is the complement of the remainder when the 

 same term is taken at its local value. Take the following 

 example : — 



Terms. 2. 1. 0. 



246,474,585 -i-7 = 35,210,655? 

 or 



Local value 246,000,000-4-7 = 35,142,857^ 



„ 474.000-7-7= 67,714^ 



„ 585 -=-7= 83f 



35,210,655? 

 Abstract Value. 



246-r-7=35f true fraction. 



474-i-7 = 67f complement of true fraction. 



585-=-7 = 83f true fraction. 

 Taking local values, the remainders are }, f, i, whose sum gives 

 the true fraction. Taking abstract values, the remainders are f , 

 f, 7, and the true fraction is to be obtained from them thus, | — f •*- 

 f = 0; for the same fractional result is obtained whether we add f 

 to a quantity or subtract f from it, since the difference between 

 the two results must be (J + 4 = ) unity. If, therefore, the even 

 terms be called plus and the odd terms minus, and if their sum be 

 divisible by 7 without a remainder, then the whole number is also 



