Jtme 27, 1889] 



NATURE 



Finding, however, that other neighbours had noticed the same 

 thing, and that at Golden Grove, near Roscrea, the workmen had 

 observed that not only the water in the barrels was black, and 

 was even next morning "like ink," but also that there was a 

 decided blackness and scum in the pools on the carriage tlrive, at 

 some distance from any chimney, I was convinced that the 

 blackness had come from the thunder-cloud. 



At so great a distance (about 200 miles) from any large smoke- 

 producing town this was somewhat remarkable. 



I was able to procure from one place, from near Eyrecourt, 

 Co. Galway, through the kindness of the Rev. C. Lawrence, of 

 Lisreaghan, Eyrecourt, a sample of the water. It is stated to 

 have been of a dark blue colour, but when it reached my hands 

 it had become pale reddish brown, with a considerable amount 

 of solid matter in suspension. 



Dr. W. J. Russell, F.R S., has been good enough to examine 

 the sample, which appears lo behave in all respects much as 

 London rain water, except in being free from acid reaction 

 (this may be due to previous impurity in the collecting vessel). 

 The amount of sulphates is represented by 0*074 gramme of 

 H2SO4, and of chlorides by o '066 gramme of HCI, in a litre. The 

 solid matter is devoid of structure or of crystalline form, and 

 appears to be soot. The specimen is, however, the less satis- 

 factory from having been taken from near a house where coal has 

 been burnt. 



I understand that the rain was black near Shinrone, King's 

 County, and that a few blackish drops were noticed at Dundrum, 

 near Cashel, From Ballymore Castle, near Eyrecourt, where 

 the sample was obtained, to Golden Grove, the distance is about 

 23 statute mile?, and Shinrone is between them. From Eyre- 

 court to Dundrum is about 48 miles, Mr. Lawrence tells me 

 that the blackness of the rain was noticed by several of his 

 neighbours, and that a laundress kept the water three weeks in 

 the barrels, and had then to reject it, as it was still too much dis- 

 coloured. 



Possibly it may be worth noting that two days before (April 



10) there was an intense blackness like that of a moonless night 



in London, between 12.30 and 1.30 p.m., but I have at present 



no evidence to connect it with the phenomenon above described. 



Athenaeum Club, S.W., June 18. RossE. 



On the Theory of Hail. 



Ln last week's Nature (p. 151) Prof. Robinson gives an account 

 of a hailstone that fell at Liverpool on the 2nd instant, consist- 

 ing of an opaque nucleus, surrounded by almost clear ice, and 

 this by opaque ice. 



Hailstones formed of concentric layers, like the coats of an 

 onion, are by no means uncommon. The number of layers has 

 been known to amount to as many as thirteen {American Journal 

 of Science, 1., 403); but the statement that such a structure is 

 formed, as Prof. Robinson supposes, "during electric oscillation 

 from cloud to cloud," belongs to a theory that has often been 

 disproved. Nor would it probably have excited much attention, 

 but that it originated with no less a man than Volta, who, seeing 

 how pith balls and other light objects oscillated Ijetween two 

 metal plates in opposite electrical states, imagined that the hail- 

 stone acquired its successive coatings by oscillating between two 

 clouds in opposite states, until its weight became too great for the 

 electric force to sustain it against the gravitating force. 



The two most distinguished writers who have examined this 

 theory, and have shown its futility are Kamtz {Lehrbnch der 

 Meteorologie, ii. 525) and Becquerel ("Traite de I'Electricite," 

 iv. 151). 



The theory of hail which scientific meteorologists now accept, 

 originated with Prof. Olmsted, of Yale College (I have not the 

 reference at hand to the American Journcil of Science, but see 

 Edin. Neio Phil. Journ., ix. 244). This theory has received 

 its finishing touches in the papers of the United States Coasts 

 Survey (" Meteorological Researches for the use of the Coast 

 Pilot," Part ii., p. 85, Washington, 1880). The writer is Mr, 

 William Ferrel. 



Limiting our notice to such compound hailstones as the one 

 described by Prof. Robinson, which from their size and velocity 

 are the most destructive, and are produced during a tornado or 

 a violent thunderstorm, the following is in few words the 

 theory of their formation. When the wind gyrates rapidly 

 round an axis, more or less inclined to the earth, the space at 

 and about the axis is rarefied. When air charged with vapour 

 is drawn into this rarefied space, it may be condensed into cloud 



or rain, but at a greater elevation into snow. Now, supposing 

 the rain formed in the lower region to be drawn up by the 

 ascending current into the snowy region, and so held for a short 

 space, the drops will be frozen, and then if propelled beyond the 

 gyrations it will fall to the ground as a shower of ordinary hail. 

 But if in the de"=cent they are again drawn in by the inflowing 

 current, they will be again carried up into the cold region, and 

 so acquire another coaling of snow, or, if welted in the previous 

 descent, the water will freeze into a coat of transparent ice. In 

 this way the globule may make a number of ascents and descents, 

 and acquire a fresh coating each time. 



From the date of Franklin's experiments even to our own 

 day, the formation of hail has been attributed to electricity. 

 Even so good an observer as Peltier (" Meteorologie," p. II6),. 

 while rejecting Volta's theory, offers an electrical theory of his- 

 own, with the complacent remark that " Volta a place des 

 suppositions oil je place des fails ; " but De la Rive, ini 

 noticing Olmsted's theory, very properly remarks that although 

 electricity always accompanies the formation and fall of hail,, 

 these two phenomena are not connected as cause nnd effect. 



Highgate, N., June 20. C. ToMLlNSON, 



Curious Effects of Lightning on a Tre2. 



DlTRlNG the recent thunderstorms a large elm-tree was strucK 

 by lightning in a private park at Dulwich, but the only visible 

 effects were linear interrupted grooves about | inch deep, ex- 

 tending down one side of the tree lo the ground, where two or 

 three depressions some 3 inches deep were found. The bark is 

 scooped out as clearly as if done with a gouge, and the intervals 

 are from I to 2 feet in length, while the grooves themselves are 

 from I to 3 feet in length. The grooves are now filled with 

 mildew, which, I take it, indicates the death of the adjacent 

 bark. I have often seen trees which have been struck by 

 lightning, but none in which the effects have at all resembled 

 those I have described. Alfred S. Gubb. 



Gower Street, W.C. 



The Formation of Cumuli. 



A VERY perfect illustration of the method of formation of 

 cumuli was noted by the writer recently. A perpendicular column 

 of smoke was seen capped at a vast height liy a rounded mass of 

 cumulus cloud having a flattened under surface. The ascending 

 warm current being traceable by the smoke, and the cloud-cap 

 very distinct and persistent, the appearance was very striking, 

 the sky in the vicinity being intensely blue and otherwise 

 cloudless. M. A. Veeder. 



Lyons, N.Y. , June 10. 



Coral Reefs. 



The business of the surveyor abroad is not with theories. It 

 is to collect facts ; to apply the resources under his command to 

 the delineation of the earth's surface ; and to examine the bottom 

 of the ocean. When he begins to theorize, he may be suspected 

 with some reason of bias, and of insensibly colouring his re- 

 ports with preconceived notions of what he expected to find, 

 instead of carefully storing up evidence. He is, however, at 

 liberty to study the writings of our great naturalists, and to him 

 Darwin is, at present, the great authority : not so much the 

 young naturalist of the Beagle, as the matured thinker who, after 

 forty years of deep research into various problems of Nature, 

 published that edition of "Coral Reefs" which has been before 

 the world for the last fifteen years. 



With your permission, I desire to ask two or three questions 

 of those gentlemen who are unable to reconcile their views with 

 Mr. Darwin's theory of subsidence. 



The Fiji Islands present the most complete collection of coral 

 reefs in the world. We have there the fringe reef, the barrier, 

 and the atoll ; islands which have a barrier all around them, 

 others where it is sunken on one side ; the island, such as 

 Lakemba, where there is a fringe on one side and a barrier on 

 the other ; as Thithia, which is surrounded by fringe only. 

 There are extinct craters, such as Fulanga, and islands with ex- 

 terior rim and depression in the centre, the formation of which 

 is apparently not due to the volcanic action ; islets on the edge 

 of atolls like Ngele Levu which are wholly coral, others of coral 

 in the centre of lagoons. In the same locality may be found 

 islands which have a fringe to windward and barrier to leeward, 

 and those which have a fringe to leeward and a barrier to wind- 



