34 



KNOWLEDGE 



[Februaby, 1902. 



east, wliilst dose to the great dark ray at the pole a Ijroad 

 but short bright ray is clearly seen, which may \l^ry well 

 have its rise in a latitude some 15^ or 20^ from the pole, 

 either on thti side of the sun nearer to us, or ou that more 

 remote. 



The indications of relief in the corona, which the present 

 plate aifords, slight though they may be, are of importance, 

 as they teud to remind us that the corona, in spite of that 

 essentially flat appearance which it generally presents, 

 must in reality have a great extension in the lino of sight. 

 It is a fact of which we need to be reminded, since it is so 

 easy to forget it, and it has often been ignored in discus- 

 sions upon its structure and nature. And indeed it seems 

 very difficult to reconcile it with some features of its 

 actual appearance. The dark rays, for instance, whether 

 we regard them as mere interspaces between bright forma- 

 tions, or as having a distinct and separate objective exis- 

 tence, are exceeding hard to account for in a structure in 

 relief. Then, again, iu the corome of 1900 and of 1901, 

 and we might add also iu the more complicated one of 

 1898, the great mass of the corona was comprised in a 

 very few striking formations of the character known as 

 synclinal groups. In 1900, three of these were recognised, 

 in the other two years, four. On all three occasions these 

 structures had the most enormous extension, reaching on 

 the average a distance of about three solar diameters from 

 the centre of the sun. It is clearly not conceivable either 

 that the structures which were visible were exactly where 

 they appeared to be, in a plane at right angles to the line 

 of sight, or that the moon concealed from us at the most 

 more than two or three structures of similar character 

 and extent. The picture, therefore, that is conjured up to 

 our mind of what would be visible to us, could we look 

 down uj)on the sun and see the corona ou all sides at once 

 and in full relief, is that of about half a dozen or so 

 of these monstrous excrescences distributed irregulai'ly 

 round the sun ; whilst about one-tenth of the solar sur- 

 face, that immediately surrounding the two poles, would 

 be given up to a few "plumes," few, that is to say, 

 relatively to the area engrossed by them. 



Uttttrs. 



[The Editors do not hold themselves responsible for the opinions 

 or statements of correspondents.] 



THE SPECTRUM OF LIGHTNING. 



TO THE EDITORS OF KNOWLEDGE. 



SiES, — The reproduction of a photograph of the 

 spectrum of lightning ou page (i of the January number 

 of this magazine is intensely interesting, and, so far as my 

 experience extends, uuique. I should indeed be glad to 

 know how the hydrogen lines in that spectrum are to be 

 accounted for. I presume that that element must for the 

 time being exist in a free state in that part of the atmos- 

 phere in which a " thunder-storm " may be raging; but 

 the question arises, How comes it there in suihcieut 

 quantity to thus indicate its presence? Are we allowed 

 to assume that the passage of au electric discharge through 

 a portion of the atmosphere decomposes the water-vapour 

 therein into its constituents, oxygen and hydrogen ; and 

 that, further, the freed hydrogen is thus raised to incan- 

 descence and so made to manifest its presence r I should 

 like to hear through some aeronaut if samples of our 

 atmosphere have ever been taken at an altitude of from, 

 say, one to two miles, which have been found to contain 

 even ammonia in any appreciable quantity. Prof. 

 Pickering remarks, in connection with the photograph 

 under consideration, " A number of photographs were 



taken in this way, and showed the curious fact that the 

 spectrum of lightning is not always the same." I do not 

 just now see how they are likely to hi' precisely similar. 

 38, Burrard Koad," " William Godden. 



West Hanipstead, N.W., 

 12lh Jduuarv, 1902, 



KAINBOW BEFORE SUNRISE. 



TO THE EDITORS OF KNOWLEDGE. 



Sirs, — Perhaps it may interest Mr. Johnson to know 

 that I saw the same rainbow at Blackheath which he 

 witnessed at South Croydon (see Knowledge, January. 

 1902, p. 11). I have a particular reason to remember the 

 date, as I was going a few miles later in the day to see a 

 relative in the country, and was rather anxious to learn 

 whether it was also" visible there, but could get no 

 information on that point a cause de la bonne heure. The 

 time during which I noticed the bow agrees with that by 

 Mr. Johnson, from 7.15 to 7.30, and I remarked that I 

 did not remember ever before seeing a morning rainbow 

 in which the whole arch was so complete and of uniform 

 brilliancy. W. T. Lynn. 



Blackheath, Jauuarv 1st, 1902. 



HOAR FROST. 



TO THE EDITORS OF KNOWLEDGE. 



Sirs, — In Mr. Bells interesting article on this 

 phenomenon in Knowledge for November last two 

 rather curious facts are not referred to. Ou calm frosty 

 nights there is generally a slight air-current moving, 

 and the direction in which it moves is always indicated 

 by the position of the lines of ice-spicules, especially 

 en the edges and corners of railings, walls, leaves, etc. 

 One's first impression is that these lines of icy points 

 are blowai away from the wind and point down the 

 cun-ent like flags. But the contrary is the fact. They 

 are built up in the teeth of the cm-rent and point " up 

 wind " to the direction from which it comes, like a 

 vane. Such lines of hoar frost are often formed along 

 the telegraph wires, and it is not uncommon to see 

 the wires on such occasions curiously vibrating. The 

 cause of tliis is a little puzzling, but I think it may 

 be accounted for by a slight change in the duectiou 

 of the current after the formation of the line of frost 

 spicules. These spicules stand out like the teeth of a 

 comb on on© side of the wire only. If the current 

 changes and now sti-ikes them at an angle the pressure 

 will cause a slight toi-siou of the wire tmtil the resistance 

 overcomes the pressure, when the wire will spring back 

 and the same process will be repeated. F. T. Mott 



Birstal Hill, Leicester. 



December 17, 1901. 



A POISONOUS SHRUB. 



TO THE EDITORS OF KNOWLEDGE. 



Sirs, — I hear, from the best authority, that there 

 is an oat ti-ee supposed to possess ciu'ious properties 

 in the domain of the Earl of Anuesley (Co. Down). For 

 several yeara certain members of the family had suffered 

 during autumn from a sort of skin eruption, and this 

 season the symptoms were more pronounced, head and 

 hands became swelled and inflamed, and feverishness 

 supervened, the whole malady somewhat resembling 

 erysipelas. It has been discovered that only those who 

 touched the leaves of this oak were thus affected, and 

 the secret cause of the complaint was definitely traced 

 to in-itatiou produced by handling them. We all know 



