November 14, 1895] 



NATURE 



31 



the long, flat shore, that the right combination occurs. For, 

 when there is a ground swell on, the waves do not roll in con- 

 tinuously, but come in groups, and therefore we require the 

 coincidence of the higher waves and the right minute on the 

 shore. 



Now, Morecambe Bay has a long flat .shore, over which the 

 tide comes in so fast that it has been known to overtake a coach- 

 and-four. It is open to the south-west wind and the Irish 

 Channel tide. The time when the " Barisal guns" are heard is 

 when the great tidal wave is rushing up the estuaries in the Bay 

 of Bengal. This is a very suggestive coincidence. 



Before we go further in our investigation of air-thuds, it would 

 be of great help if we would get some exact data as to the dis- 

 tance at which the sound of great guns, of blasting, or of waves, 

 can be proved to have been heard. So far I have endeavoured 

 to explain such sounds by surface action. I believe, however, 

 so strongly in the rapidity and constant recurrence of earth- 

 movements which must produce sound, that I would add a few 

 words upon that point. 



To begin with small sounds. When the sun is setting and the 

 shadows of the mountain climb up the screes or talus, each part 

 rapidly passes from sunshine into shade, and often into very cold 

 shade. In such circumstances it is not uncommon to hear a crepita- 

 tion among the fragments. This is quite in accord with the view 

 that the downward travel of the screes is largely due to diurnal 

 changes of temperature affecting the fragments of which they are 

 composed. More rarely bangs are heard among the crags in 

 similar circumstances. The ticking of the firedogs and iron- 

 work of a grate when the fire has gone out, and the sudden 

 and terrifying crack of the oak-wainscoted room, are familiar 

 examples of the same kind of thing. In frost, of course, such 

 noisy ruptures are common. 



Now as joints in rocks are surface phenomena, due to shrink- 

 age, detonations accompanying such disruptions should be often 

 heard. Under the artificially-produced conditions of mining 

 they are frequently heard. In the limestone quarr}', from 

 which the black marble of Dent is procured, the workmen found 

 that, when they were quarrying the lower beds, and struck the 

 rock with a pick or bar, fragments flew up into the air with 

 a greater force than could be due to the blow of the pick, and 

 in an unexpected direction. Also, when the tunnel was being 

 made above Kibble Head, and the workmen wereengaged upon the 

 bed of rock which formed the floor of the tunnel, pieces used to 

 burst off with a loud noise, so that some of them thought that 

 they had discovered a detonating shale. The explanation in 

 both cases was that the hard thin bed which shelled oflT in that 

 unexpected manner, rested on compressible shale, which, 

 behaving as a fluid, transmitted the pressure due to the sides of 

 the quarr)' or the wall of the tunnel, squeezed up the floor where 

 the rock had been removed, and produced what in a coal mine 

 would be called a "creep." The thin bed of hard rock above 

 the shale rose in a slight arch over the upthrust shale, and was 

 thrown into a state of tension, so that, when it was struck, 

 chips, flakes, and sometimes larger pieces, would fly off". 

 Phillips pointed out that "the removal of one side of a vein 

 would leave the remaining side in a condition of strain resemb- 

 ling that of a strung lx)w, with a tendency to bulge outward 

 into the workings," and it is known that from such a surface, 

 especially when covered with a coating of crystalline vein-stuff", 

 fragments fly off with cracks and explosions of various degrees 

 of intensity. Earth-movements and the action of subterranean 

 waters are continually opening out channels in which all these 

 processes which produce strained surfaces and consequent 

 explosions are going on. The widespread belief that fairy or 

 goblin miners are heard working deep in the bowels of the 

 earth probably arises from these natural rock-burstings, which 

 are, from "the nature of the case, more apt to occur along the 

 line of lodes. I have myself been advised to go in for certain 

 mining speculations because the sound of workmen's tools had 

 been heard beneath the ground. 



As on a small scale along lines of tension, artificially or 

 naturally produced, so on a large scale along the lines of strain 

 due to the great earth-movements, which are continually going 

 on, analogous disruptions must repeatedly occur. When it is 

 on a great scale, and the tremors and throws accompanying it 

 have been observed, it is called an earthquake ; when the noise 

 only of the break is heard, it is unrecorded, because of the 

 difficulty of distinguishing one air-thud from another. It must 

 be a phenomenon of not uncommon occurrence along the lines 

 of more rapid earth-movement, for we must bear in mind in all 



NO. 1359, VOL. 53] 



such inquiries that time is an element in the bending of rocks. 

 In the cases I mentioned above, it is the rapidity of the action 

 due to the artificial removal of the mass that causes the rock to 

 break rather than to sag or bend, and retain its curved form. 

 This last we see commonly among the contorted strata where 

 nature has applied the pressure more gradually ; but we also see 

 evidence of more sudden movement in faults and slickensides 

 and similar phenomena, all of which are going on still, and must 

 be accompanied by sounds could we only detect them. 



T. McKenny Hughes. 



Cactaceae in the Galapagos Islands. 

 In my notice (Nature, p. 623) of Dr. Baur's botanical col- 

 lections from the Galapagos Islands, I intended to add a few 

 words respecting the Cactacere, but forgot it at the last moment. 

 This natural order of plants forms the most conspicuous feature 

 in the vegetation of some of the islands, as may be seen in the 

 excellent views illustrating the cruise of the U.S. ship Albatross ; 

 yet Darwin, so far as I can discover, is the only naturali.st who 

 ever brought away any specimens, or contributed to any more 

 exact knowledge than can be got from photographs on a very 

 small scale. The presence and abundance of Cactaceoe in these 

 islands is highly interesting, especially in relation to the age 

 and origin of the flora, and to the fact that none exist in Juan 

 Eernandez, though they abound in Chili. They are also of 

 great importance to the animal kingdom during long droughts, 

 as they are then the only source of w3ter. As we learn from^ 

 Darwin, the succulent branches are eagerly devoured by the 

 large tortoises, lizards, and various other animals. He found 

 that lizards four feet in length were easily enticed whenever he 

 threw them a piece of a branch, and small birds would come 

 within a few feet of him and peck at one end whilst a lizard was 

 eating at the other. ^ Darwin also regarded the Cactaceae as a 

 source of food ; and it is noteworthy that animals were mainly de- 

 pendent on the branches broken off by wind. Opuntia galapa- 

 gem was found by Darwin on James's Island, growing from six 

 to ten feet high, with a trunk a foot in diameter, and so densely 

 covered with strong spines as to be protected from destruction 

 by predatory animals. The younger branches bear only long 

 elastic bristles. Dr. N. J. Andersson, a Swedish botanist, who 

 visited the islands in 1851, states (Eugenics Resa, " Botanik," 

 p. 95) that this cactus growf in all the islands, and he adds that 

 he observed at least four or five other species, but had not time 

 to collect specimens or to sketch them. In another place 

 (" Linncea," xxxi., 1861-2, pp. 571-631) he particularly mentions 

 the Opuntia growing in lava where nothing else would grow, 

 forming huge candelabra-like objects with pretty red, shining, 

 fruits. It is evident, too, from the photographic views repro- 

 duced in the " General Sketch of the Expedition of the Alba- 

 tross" that one (or more) species of Cereus attains a height ot 

 ten or twelve feet, and is equally prominent in the landscape. 

 Perhaps the next botanist who visits the islands may find time 

 to study the Cactaceae. But when will any (government think it 

 of sufficient importance to attach a really qualified botanist to 

 such expeditions ? I ought to add that Dr. Baur is not a botanist, 

 but he appears to have done his best as a botanical collector. 

 W. BOTTING HeMSLEY. 



Slow Lightning, 



As far as I know, the first description of slow lightning occurs- 

 in your issue of November 7. It must be very rare, for I have 

 never met any one who would readily believe in its existence. I 

 write to testify to the accuracy of Mr. Crawford's description, 

 though I have not seen it quite as slow as the flash which he 

 timed. The best example that I ever saw was in a storm over 

 London some eighteen years ago. A thick stream poured down^ 

 in the sort of curve which liquid takes from a kettle, and was 

 then slowly joined by a similar stream from the opposite direc- 

 tion, the united stream then continuing its .slow course down- 

 wards. I was not where I could see the end of it. The 

 peculiarities — the breadth of the streams, and their deliberate 

 motion— could scarcely be an optical delusion. The streams did 

 not appear to me as "chains." 



I have observed lightning all my life, and since it has begun 

 to be photographed, I have been looking out for pictures of the 

 various types. I hope we may get some picture of this, and 



1 See Magazine of Zoology and Bctany, i. (1837), p. 467, where Prof. 

 Henslow describes and figures Opuntia galapageia. 



