270 



NATURE 



\July 19, 1888 



thunderstorms as explained in Prof. Silvanus Thompson's 

 " Elementary Lessons," and similar elementary treatises : — 



"The coalescence of small drops to form large ones, though 

 it increases the electrical density on the surfaces of the drops 

 does not increase the total quantity of electric'ty, and therefore 

 cannot directly influence the observed potential." 



Surely this entirely omits the fact that the capacity of a sphere 

 is equal to its radius, and thus in the case of eight equal spheres 

 coalescing into one (which is taken by Prof. Thompson), not 

 merely would the density be doubled, but the potential of the 

 same quantity would be increased four times. 



In the well-known case given by Prof. Tait for the formation 

 of a raindrop the potential of the same quantity might be 

 increased fifty million time?. 



The source of the energy which is the cause of the increased 

 potential in this case, is prob ibly the molecular force of cohe- 

 sion released during the act of condensation and union, the 

 cohesion and the electricity being oppositely placed, so that 

 while the former is running down hill (as it were) the latter is 

 obliged to run up ; the top of the hill answering to the critical 

 moment for disruptive discharge. 



In view of these facts, it seems to me that if the above 

 sentence is not altogether erroneous, it is certainly ambiguous, 

 and liable to breed false notions in the mind of the unreflecting 

 and too credulous student. E. Douglas Archibald. 



Transparency of the Atmosphere. 



It may be, I think, desirable to correct an error which has 

 crept into all the accounts of the extraordinary transparency of 

 the atmosphere observed here last week. It occurred on Sun- 

 day, the 8th, and not on Monday, the 9th inst. I can confirm 

 the several details as to the objects visible to the unassisted eye. 

 But in one respect this effect was surpassed -on August 20, 1887, 

 when the double flash of the Dunkirk light, distant from this 

 place about forty-five miles, was visible for several hours. This 

 light could not be seen here on the 8th inst. 



Pavilion Hotel, Folkestone, July 16. J. Parxell. 



Preserving the Colour of Flowers. 



In response to the inquiry of "A. W.," perhaps you will 

 allow me to say that many years ago I met with Mdlle. d'Ange- 

 ville, the first lady to ascend Mont Blanc. She possessed the 

 largest and best preserved collection of Alpine flowers I have 

 ever seen, and she assured me she never used anything but 

 cotton-wool in her pres c , changing it, of course, frequently. 

 Her gentians, pedicularias, and other delicate plants were per- 

 fect in colour ; and having tried her plan myself, although with 

 less care, and therefore with less success, I still have Alpine 

 flowers which have retained their colour for twenty years. 



54 Doughty Street, July 17. A. W. Buckland. 



Distribution of Animals and Plants by Ocean Currents. 



In connection with Miss Buckland's letter on this subject it 

 may be interesting to note that, during a visit to Orotava, Tene- 

 riffe, in April 1887 (about the time mentioned by your corre- 

 spondent), I observed and gathered a quantity of pumice-stone 

 upon the seashore, the high tide mark being literally strewed 

 with it. It ;eemed probable that it had been deposited there 

 some weeks or possibly months previously, as, had there been 

 any quantity floating about in the sea, I should have noticed it, 

 being engaged at the time tow-netting in the neighbourhood 

 and in the adjacent Canary Islands. There was no evidence of 

 vegetable debris having accompanied the pumice, nor did I 

 notice any pieces with barnacles attached. 



Liverpool, July 13. Isaac C. Thompson. 



A Curious Resemblance 



WHILST walking by the sea on the cliffs last Sunday, I per- 

 ceived at a distance of about 15CO yards a flight of nearly forty 

 ducks, travelling at a good pace 2 or 3 feet above the level of 

 the water. To me they appeared exactly what the so-called 

 "sea-serpent" would, eight or ten of the birds flying close 

 together and forming the head, whilst the rest trailed behind 

 and formed the body and tail. At intervals they disappeared. 

 This was caused, I think, by the birds changing their course 



and flying either directly away or towards me ; the former, I 

 believe, in my case. 



Some time afterwards I saw two other flights, and these re- 

 sembled the first exactly, those with me also being surprised at 

 their "snake "-like appearance. W. J. Lockyer. 



Thanet, July 16. 



The "Sky-coloured Clouds." 



There was a very bright display of these clouds last night! 

 I could not perceive anything of them up to 10 p.m., though the 

 sky was clear, but by 10.18 they had become conspicuous, and 

 were brightest, so far as I observed, near midnight. 



I have seen very little account in any English paper of the 

 visibility of these clouds beyond England, nor do I know 

 whether they have been seen elsewhere than in Northern 

 Europe. Has there been anything published on these points in 

 English ? 



Neither have I seen any reference to the extensive observa; 

 tions of Herr O. Jesse at Steglitz, with his suggestions to 

 observers. He considers it very important that this unusually 

 favourable opportunity should be utilized for learning the 

 motions of currents at great heights in the atmosphere. He 

 suggests that photographs taken simultaneously from two places 

 at a distance of say 20 kilometres would be useful for ascertain- 

 ing the height of the clouds ; but for this purpose the necessity 

 arises of being able to calculate very accurately the azimuths 

 and altitudes of different points in the photograph. Their height 

 can likewise be determined, though less accurately, by observa- 

 tions of the limit of sunshine upon them. Herr Jesse proposes 

 another way also, viz. by throwing an intense beam of electric 

 light on the clouds ; but I should doubt the practicability of 

 this. 



The direction and rate of motion could be best made out, he 

 says, by the use of a cloud-minor. The changes that take 

 place in the forms of the clouds before they have moved far make 

 it difficult to ascertain their motion accurately. 



Herr Jesse further thinks the intensity of the light of the 

 clouds in different positions should be determined ; also that 

 the sky should be examined in the day-time with a polariscope 

 and photometer in the hope that the presence of the matter of 

 the clouds, then invisible to the eye, might be revealed. 



Sunderland, July 13. T. W. Backhouse. 



An Unusual Rainbow. 



Singularly enough I can record the appearance of a rain- 

 bow aft«-r sunset similar to that described by Mr. S. A. Hill 

 (Nature, March 15, vol. xxxvii., p. 464). I was not aware 

 there was anything unusual in it until I read Mr. Andrew's com- 

 munication, or would have written to you about it. I do not 

 remember on what day I saw the rainbow, but it was about the 

 date of that observed by Mr. Andrew. I called my wife's at- 

 tention to it, and attributed it to the bril'iant glow of the sunset 

 tints. It had a secondary bow, and Mont Kogie as a dark 

 background. E. L. Layard. 



British Consulate, Noumea, May 15. 



TIMBER, AND SOME OE ITS DISEASES} 

 IX. 



IF the leaves are stripped from a timber-tree early in the 

 summer, or during their young conditions in the 

 spring, the layer of wood produced in the current year— and 

 probably even that formed next year-- will be poor and thin. 

 This is simply a fact of observation, and does not depend 

 on what agent deprives the tree of its leaves. Those oaks 

 which suffered so greatly from the ravages of certain tiny 

 caterpillars this last summer (1887)— many of them having 

 all their leaves eaten away before July — will have recorded 

 the disaster by a thin annual ring of wood : it is true the 

 more vigorous trees produced (at the expense of what 

 stores of food materials remained over) a second crop of 

 leaves in August, and so no doubt the zone of wood will 

 prove to be a thin double one, but it is at the expense of 

 next year's buds. 



1 Continued frim p. 1^0. 



