2^8 



NATURE 



[AiKji'sr 25, 1910 



sand ofl through the open ends of the cell. When this 

 point is reached the cone can no longer grow. A supply 

 of white sand is then poured in, and seen to run down 

 the sloping sides without carrying any of the coloured 

 particles with it. The base has spread out proportionately 

 as the cone increased in height, so that the angle which 

 the sides make with the horizontal shall be 35°. If the 

 sand be wet or damp, this law no longer holds. The 

 addition of sufficient water materially diminishes the 

 friction between the grains. j 



It is often observed when walking along the sea-shore, 

 upon sand left wet by the receding tide, that for a moment 

 the foot, on touching the ground, is surrounded by a com- 

 paratively dry area. This appearance is quickly followed, 

 however, bv one which indicates that the sand has 

 gathered moisture, for on lifting the foot — which has by 

 now probably sunk a little below the surface — the excess 

 of water is particularly noticeable. In order to explain 



this we must have recourse to some ingenious experiments 

 made a few years ago by Prof. Osborne Reynolds. He 

 pointed out' that a number of particles, whether spheres 

 or irregular grains, may fit together in such a way that 

 the size of the spaces enclosed by them is either a maxi- 

 mum or minimum. Figs. 8 and 9 show a sectional view 

 of a cbllection of spheres, arranged in what Prof. Reynolds 

 calls abnormal, and normal pUing respectively. It is 

 evident that the spaces between the spheres are far less 

 in the second than in the first case. Now here is an 

 elastic bag tied upon one end of a glass tube. The 

 arrangement is partly filled with sand and coloured water 

 — the latter standing 2 inches in the tube, so as to serve 

 as an index. If the bag is now tapped, all the particles 

 in it become normally piled. We have seen that any 

 departure from this arrangement will enlarge the spaces 

 between them. It is no longer surprising to notice, there- 

 fore, when the bag is pinched and the grains are thus 

 made to ride up on one another, that the liquid in the 

 tube, instead of rising, actually sinks. 



Returning to the effect observed upon the sea-shore, we 

 NO. 2130, VOL. 84] 



see that the pressure of the foot disturbs the arrangemenj 

 of the sand-particles from one of normal piling to one in 

 which the interstices between the grains become larger. 

 Since these spaces w'ere originally full of water (held up 

 by capillarity), they are now no longer filled, and we 

 obtain a comparatively dry area. Water is rapidly drawn 

 in from all sides, however, by the partial vacuum formed 

 in the interstices, and the internal friction diminishes. 

 The sand feels insecure. On withdrawing the foot normal 

 piling is resumed, the excess of water producing a puddle, 

 until it slowly percolates away whence it came. 

 This brings me to the subject of quicksands. 

 A certain amount of unnecessary mystery seems to 

 surround this matter. I hasten to point out that the grains 

 of quicksands appear to be in no way extraordinary. 

 Nevertheless, the fact remains that sand in certain locali- 

 ties upon the coast readily gives way under a load. 

 Instances are recorded where a cart driven over a wet 

 shore has rapidly disappeared below the 

 surface. The general opinion seems to 

 be that this is due to a soft underlying 

 layer of clay or mud, whicli no doubt 

 in some instances is the true explana- 

 tion. Mr. Carus-Wilson, who is an 

 expert in these matters, pointed out to 

 me recently, however, that another 

 factor may be the imprisoning of gas 

 between the grains, due to decomposi- 

 tion of organic matter. E.xperiment 

 rertainly supports this view, for you 

 see that one of these beakers of wet 

 sand easily sustains a weight which 

 sinks down in the other. Yet both 

 appear similar. The sand in the second 

 beaker, however, was mixed when dry 

 with a powder capable of effervescing 

 if wetted. In the neighbourhood of 

 dangerous bogs, in Ireland especially, 

 it is evident that a quantity of gas is 

 imprisoned in the mud. 



It must also be borne in mind that 

 any surface in so good a contact with 

 wet sand that the air is excluded will 

 be held fast by atmospheric pressure ; 

 .ind further, that an object so situated, 

 and tilted this way and that, will 

 rapidly become embedded and swallowed 

 up. It is by this simple process that 

 the celebrated Goodwin Sands have 

 claimed so many victims. \ large per- 

 centage of the vessels stranded upon 

 them, however, float safely off on the 

 rising tide, but now and then one is 

 caught and doomed. In the past they 

 have been responsible for many a 

 shipping tragedy; and there is a 

 pathetic interest attaching to the fact 

 that ribs and other remains of ships, 

 long lost and forgotten, sometimes 

 reappear for a time above the 

 surface. Since the advent of steam, it is happily a rart 

 occurrence for a vessel to be lost upon a. sandbank. 



In 1849 boring operations were carried out on the 

 Goodwins bv the engineering staff of Trinity House. The 

 Deputy Master and Brethren, whose generous offer of 

 assistance on all matters relating to this. subject I grate- 

 fully acknowledge, have kindly lent a model made at 

 the' time, which shows the nature of the sand found at 

 increasing depths. Solid chalk was reached at So feet 

 below the surface. 



Let us now turn to some experiments upon the flow of 

 sand through a tube. This long glass barrel is filled and 

 ready. I free the nozzle, and collect the powder which 

 flows out during ten seconds. The quantity so obtained 

 is placed in one pan of a balance. When the height of 

 sand in the tube has fallen to only a few inches above 

 the outlet, 1 repeat the operation, placing the second 

 amount collected in the opposite one. You see that the 

 pans again stand level. It is therefore clear that the satid 

 pours (3ut at the same rate, irrespective of its height in 

 the tube. 



