KNOWLEDGE 



[January 2, 1893. 



rapidly in fine and hot weather, espeeiallv in the long days of June 

 and jiilv. It certainly needs some such contribution to account for 

 so va?t a store of water from mere drainage at that height. The 

 surface snow of the nerc is porous, and though a cake of ice often 

 forms at night, it is sure to disappear witli a tew hours of sunshine, 

 and to leave free access to soft snow beneath, and the surface 

 meltings, which are never very great on the neve, filter into this, and 

 do not run off as they do off hard ice — such as you meet with in your 

 way up the Her de Glace, for instance. 



The circumstance of this vast accumulation of water in such a 

 place is so very exceptional that a notion has been suggested (I forget 

 where I have seen it — my son says it is in Freshfield's paper) that ( 

 there are underground springs tliere. It would be very curious if 

 this really were so. The distribution of underground water has 

 always seemed to me a very interesting subject, and one which 

 remains to be efficiently treated. That water travels very far under 

 ground there can be no doubt I have seen in the Bay of Bengal, 

 about forty miles north of Sumatra, a little island called Pulo Rondo. 

 It is not two miles round, and rises about 2.')0 feet out of the sea. A 

 broatl stream of water flows from its highest ])art down into the sea. 

 In shape the island is like a barber's basin turned upside down, and 

 tliere is no gathering ground, or none but the very smallest, above the 

 top of the waterfall. That water must travel at least forty miles 

 underground if the fall is brought about by liydi'Ost;itic pressure. 

 But there are other collections of water whicdi it is very dillicult to 

 account for by any such simple means. I know a small and very deep 

 lake, about 1000 feet higher than the Lac Cornu (which is behind the 

 Brevent from C'hamonix), and nearly 90OO feet high, above which there 

 is MO gathering ground the least adequate to explain its existence, and I 

 have always thought the Lac Cornu itself much larger than the area 

 of which it receives the drainage can account for. In 1872 I noticed, 

 during the X.W. monsoon and after a long period of dry weather, a 

 small stream of water within a very short distance of the top of Pedro 

 tallagalla, the highest mountain in Ceylon. I have reason to 

 think that there is no great intrinsic improbability in the notion of a 

 spring even at 9000 feet, and my son says that in Freshfield's article he 

 mentions that De Saussure noticed a stream of pure water issuing from 

 the base of the glacier des Tetes Rousses. It is a very rare thing 

 indeed for the meltings of a glacier, which have passed any distance 

 under the glacier, to be clear when they issue. You know what the 

 Arve is at Chamonix in summer ; I am told that in winter, when its 

 volume is enormously diminished, it is clear, which may and probably 

 does mean that it has a nucleus of spring water, which is what flows 

 in winter. 



But whatever the source of the sub-glacial accumulation, I think 

 two things are clear : (I) That those large tanks, so to speak, are due 

 to the motion of the glacier and not to the mere action of water. 

 Water woidd, no doubt, do some little melting at the sides and on the 

 floor of its receptacle, but in so confined a space tlie melting would 

 be trivial ; aiifl as the water was accumulating and not changing, the 

 action of melting ice round it would be to lower its temperature 

 greatly. Such sheets of water as the Miirjelen See hollow out great 

 vaults underneath the ice. I once saw there a block of many hun- 

 dreds of tons, on which it was the merest chance that four of us were 

 not standing at the time, break off and fall into the lake, undermined. 

 But then there is a lake nearly a mile long, above the freezing tem- 

 perature, and with the surface warmed a good deal by the summer 

 sun and the surface water between 32" and .39° sinking so as to keep 

 the whole mass from getting down to the freezing point. In the 

 cavities, in the present instance, the sun can have done nothing to 

 warm the water after it got into them. (2) It is clear, I think, 

 that the natural and u.sual drainage must have got blocked. As a 

 rough general rule, where you have any very marked feature in 

 glacial structure at any one time, you have it always. The seracs 

 of the Col du Geant are constant. Particular passages over berg- 

 schrunds or through great mazes of crevasses, with a good deal of 

 local variation, have yet a great amount of general constancy, and I 

 suspect that whatever circiunstanccs in the bed of the glacier, or 

 what not, make a chasm, such as that in my glacier d'Orny, one 

 year make it in a great many other years ; and yet this accumulation 

 i.s a new thing, or with the perpetual shifting of the ice wliich takes 

 place in motion some such hurst would almost certainly have taken 

 place before. Tliat there was a channel between the two cavities, I 

 should have very little doubt, especially considering that the lower 

 cavity is at a place where the incline of the glacier becomes much 

 steeper than before. Generally speaking, no doubt, the cracks below 

 the lower cavity give abundant outlet for the water, but some move- 

 ment of the glacier mu.-.t have blocked them up — a thing which one 

 knows perfectly well does take place in numerous spots from time to 

 time — and then the accumulation became formidable. The roof of 

 either cavity was, in the course of glacier motion, bound to 

 come down some day, and that it should break suddenly was 

 to be expected. The ice avalanches that one lias to look out 



for on the Petit Plateau, for instance, always happen without 

 warning. That is to say, you see that the masses are getting 

 dangerously near the edge of the cliff, but whether they will 

 fall within'five minutes or five days no human being can do anything 

 more than guess. Wlien the roof once did fall, being as it was very 

 large the pressure on the containing system would be very great. In 

 the instance 1 mention of the Marjelcn See I was immensely struck 

 with the commotion produced in the water, and yet there was an open 

 lake a mile long with a long flat margin round a considerable part 

 of it. It was half an hour Ijefore the recoil waves sent back from 

 the end of the lake ceased to boom with ominous sounds against the 

 ice cliffs, aim the earlier recoils brought down fresh falls of ice. I 

 am not the least surprised at the dam at the end being burst, and 

 at the rush of water from above sweeping the lower hole pretty clear 

 of water. 



An illustration of the fact that drainage through the lower strata 

 of a glacier sometimes gets blocked for long periods together oi'curs 

 to me. When I first knew the Mer de Glaee well, in 18.57, there was, 

 at the foot of the Taeul, in the angle between the glacier du Geant and 

 the glacier del'lCchaud, a large lake certainly more than a quarter of 

 a mile long. I have visited the same spot in fourteen dift'ercnt years 

 since. I ha\e never seen it so large again, and many years it lias nol 

 existed at all. The difference is chiefly due to drainage. 



I think your paper on the St. Gervais disaster, as far as I can 

 judge from what has so far been ascertained, is quite right as to the 

 modus operandi. If all be well, I hope to go and see the course of 

 the torrent next autumn, and it is fairly certain that one year will 

 make very little dift'erence in the physical appearances. It is the 

 second time I have known the broader valley below St. Gervais 

 covered with dJI/ris. In 1852 there was a wonderful fail of warm 

 rain over the whole range of mountains for nearly three days. I got to 

 Chamonix on the tliird of these days, having come down the Great 

 St. Bernard and through Martigny, and liaving met with floods which 

 gave us some very long detours to make. Parts of the valley of 

 Chamonix, but very much more the embouchure of the stream 

 which comes from the Col du Bonhomme and receives all the lateral 

 drainage from the Mont Blanc chain, were covered at least three or 

 four feet deep with dehrix and glacier mud. Now, I fancy, the depth 

 of deposit is nearly ten times what it was then. I am sure what I 

 saw then was terrible enough. In one place we saw the actual birth 

 of a torrent of mud, saw it burst out of the mountain side and spread 

 over the cultivated patches above the hamlet of Le Tour. 



From Charles Tomlinson Esq., F.R.S., &c. 



It may be thought presumptuous in me to form any opinion ou 

 the subject, seeing that I have not visited the spot, nor studied the 

 phenomena in situ as you have done. 



But I venture to remark that I miss from the account any 

 reference to the hydrostatic force of water, which is suflicient of itself 

 to do any amount of mechanical work without subsidiary aid of any 

 kind. 



A cubic foot of water weighs u|)wards of 62 pounds, and in con- 

 sequence of the perfect distribution of fluid pressure in all directions 

 the pressure on a square foot of water at the depth of ten feet is 

 upwards of 623 poinids. 



But the cavity containing the pent-up water which caused the 

 disaster is estimated at a vertical depth of 130 or 1 60 feet, which 

 gives a pressure on the lowest foot of water of 8060 and 9920 pounds, 

 which pressure is repeateil on every cubic foot along the lowest line 

 but diminishes upwards to the surface cubic foot pressure of 62 

 pounds. Hence, we have a force acting day and night all the year 

 round, which in time must break through any icy barrier that la 

 likely to be opposed to it. The fall of ice from the roof could not 

 act as a piston or lend any eflicient aid to the result. There are also 

 objections to the word " suction." My conclusion (subject to the 

 objections noted at starting) is that hydrostatic pressure alone was 

 the only moving originating cause of the aalamity. 



There are, however, a few other considerations which I take the 

 liberty of adding to this proof which the Editor has been so good as 

 to forward to me. 



The warm weather that preceded the catastrophe would tend to fill 

 u]illic sub-glacial reservoirs, and also to thin the outer wall of ice. 

 If an opening had been made at the base of the deeper or 160 feet 

 cavity (the velocity of the outflow being as the square root of the 

 depth) the rush of water would have been thirteen times greater than 

 at the top. But I imagine there was no point or line of least 

 resistance at or near the base of the cavity but was spread over its 

 whole surface, so that as the water accunudated, a uioment arrived 

 when the resistance was not equal to the prcssui'e, and the whole 

 ex])Osed fat'C of the icy barrier was burst open at once, aud the 

 contents of the reservoir, thus set free, proceeded rapidly on the 

 work of destruction. 



