SCIENCE FROM AN EASY CHAIR.

Press, Volume XLVIII, Issue 14406, 13 July 1912, Page 9

SCIENCE FROM AN EASY CHAIR.

THE SINKING OF A SHIP IN THE DEEP SEA.

(B7 Sir Rat Lakkestek, X.C.8., 1M1.5.)

(Special rights secured by "The Press.") The dreadful fate of th 0 great steamship, with its human freight, which was driven full speed in the dark.against an [ iceberg, and foundered in water two miles deep, has led to some strange and fanciful speculations with regard to the sinking of such a heavy body in deep water. I have received letters asking mc to say whether the steamship has yet reached the bottom of the Atlantic, whether it is not now flouting sonic thousand fathoius below the surlaca, unable to sink any further, on account of the increased density of the xvater at that groat depth, and whether it is true that it wili never reach the l>ottom. I do not know what is the exact recorded depth of the. Atlantic at the spot where the Titanic sank, but it said to be some- iiOlX) fathoms, or a good'bit over two miles. It is probable that tho ■vessel reached the bottom in less than half nn hour, but may possibly have taken an hour to get there. The reasons for this conclusion 1 will now give. ■V A heavy body falling in a vacuum from a height of two miles to the surface of the earth—that is to say, through a space in which there is neither ■ air nor water (which one can. imagine, although eueh a vacuum does not exist on our earth's surface) — would reach the ground or bottom in about 2o.seconds, and be travelling with enormous velocity; namely, at the rate of about one-sixth oi a mile in a second. It would mako no difference in the time occupied nor in the final velocity attained, whether the falling body were light or heavy, cork or lead. Nor would tho shape of the falling body matter; a feather would fnl! at the same rate as a bullet. A body falling through a vacuum-—that is to say, a truly empty space—would not have to displace any other material as it travelled, nor would it bo subjected to friction. A fairly complete vacuum - can be produced oh a small scale (in Jong glass jars, by pumping out the air),.:and the facts I have stated have been-thus demonstrated by exporiinent and .measurement. ' <"- - '-■* ' _ The rato at which a body starts falling to the earth "in* vacuo" is known, and the increase or acceleration of that rate. It is frwnd that it accomplishes left in the first second, and that tho acceleration of the rato is 32ft for every subsequent second. The "pull" of the. earth's attraction is not merely given at the start, but is poing on all tho time, adding to tho velocity of the falling body. When atmospheric air—the gas consisting of a mixture of nitrogen and oxygen—fills tho spaco traversed by a falling bedy, there is a certain amount of resistance to tho passage of the body. We can readily form some idea of what this is by the resistance which wo feel in walking against the wand or even standing up against it. A big wind offers to us the same resistance as wo should experience in travelling through still air at the rato of fifty miles an hour. Bodios travelling very fast (as fast as a rifle-bullet) through air also aro measurably retarded by surface or "skin" friction, which is sufficient in amou.it to heat them to « high, temperature. It is thus that meteoric stones falling from space »vith enormous velocity (beyond any terrestrial experience) become red or whiio hot as they traverse our atmosphere, and ai*o cither converted into vapour or fall to the ground glowing with heat, and giving "off sparks and smoke. When there is a resisting medium such as tho air—through which a fallins; body makes its way—the shape of tho body becomes very important *as affecting its rato of fall. If it exposes a bread surface to the air. and is itself thin and oi' material, the resistonre may delay the fall <-f the body almost ii-clofmitoly by converting downward into oblique or nearly horizontal moveirent. as in an aeroplane. In a smr.ll experiment: to measure the rate of ii heavy .spherical body-falling.through twenty or even 100 feet of air. the resbtriHc- nf iV air is so small as to be negligible. l<;*t its relative importance increases when a bnrly'fnlls in air from a {treat height, say. two miles, since tho velocity aorelerr.tod by tho earth's attraction through a comparatively long period becomes enormous, i\nd "it is known (as the result of independent experiment) that the retarding effect of resistance is very much more important in lowering tho rate of movement who:i that rato is already very high than when it is lew. Water, being a "liquid." contrasts in important respects, affecting the movement of a body 'through it, with "ga.ses"_ Kirch as air and with "solids." It is "displaced" by a body which is. bulk for bulk, heavier thnn. its?lf. Such a body '•'sinks'' through it. Tao ratu of such pinking is net dependent only on density or voight. Solids arc distinguished from'liquids by tho fact that they resist penetration. nn;l -are not "displaced , ' even by bodits . Iv.wier than their.st'lvrs. Yet many sqli.'ls are lighter than, water. Wood is lighter— that is, less dense—than water, and so is frozen water, or ice. Gases are readily compressible, and readily expanded or rarefied. Their ultimate particles always move apart from one another, so as to attenuate the gas to an indefinite dc-|n - e'_', and make it occupy it'sre and moro space, whilst becoming lighter or less dense, unless iestrainod in a limited chamber, or by somo kind of pre&sure. Liquids flow like gases —that is to say, their particles are freely mobile, and do not cohere and maintain a resisting thape or figure, as 'do those of solids. But'they do not (as do cases) change their bulk although they flow and fit to any shape; thoir particles do not tend to separate, \so as to "-.".Tcfy'' them. On the contrary, excepting for a small expansion caused by increase of temperaturei liquids nnpar orrifaary conditions retain their bulk and their density practically unchanged.

There is no gradual passage, from the liquid state to the gaseous state. When neated to certain '■ degree, varying jui different kinds, liquids bscomo suddenly converted into gases. Water is rapidly converted into truo "steam," a transparent, invisible gas (not the white oiouds of water particles which issue from tho spout of a kettle or the chiniftey of a steam-engine, and commonly but wrongly called '-steam"), when the water is heated to an extent which is registered as 2l2deg. on tho Fahrenheit thermometer, or as lOOde" , . on the Centigrade thermometer. Moreover, small .quantities of the surfacelayer of any mass of water at ordinary temperatures ar y continually changing into the condition oi steam, and are given off into the air, expanding and "diffusing" into it, This is what has happened when, as we say. a pond or an inkpot "dries up." Aiid oven solids sometimes, such as ice, and also cainphorj pass at their surfaces directly from-' tlie solid state to the gaseous state*. Everyone knows liov.- a snowheap 'gradually volatilises during a continuous frost without "melting." The passage, however, from liquid to solid is often quite gradual, and also the passage from solid to liquid, although it is not so in the case of ice and water. Many metals, and also such bodies as wax, become "soft" and pliable when heated, though still retaining their shape. They may then pass gradually to a semi-liquid or "viscous" condition", such as is permanently presented to us (at ordinary temperatures) by treacle or oil. VXsrous or ''sticky" liquids ar-> not necessarily "denser" (that is, heavier, bulk for bulk) tlia : n perfect liquids; nor are dense liquids necessarily more "viscous" than lighter liquids. * For instance, sea-water, which has a quantity of snlt in solution in it. is not appreciably moro viscous than puro water. On the othor hand, the resistance of a viscous liquid to the movement of a body through it is much greater than that of a perfect liquid; it possesses to .some extent the* cohesive quality of a solid. But the influence of viscosity does not come in when the sinking of a sliip in sea-water is under consideration: The, resistance of an ordinary iiquid such as water to the movement of a body.through it is very much greater I than that olFera! by a gas. /Everyone knows how difficult it is to move through Wilbur as compared with moving through air. A much greater force has to be expended to walk, even knee-deep or immersed up to tho chin, in water than in the air. If a heavy body of any kind is allowed to fall into water its path downwards will bo straight and direct only if it is of a simple and compact shape or skilfully "weighted." Movements like thoso of the aeroplane —oblique and horizontal and oscillating movements —arc more.easily set up by water in a flattened and irregularlyshaped body—offering a large surface in proportion to actual weight to the resistance of tho water as it passes through it—than in tho case of air. And this consideration is one of several which we must bear in mind in estimating the length of time which it would take for a great iron steamship to sink through two miles of water and reach the bottom. It is probably wa thin the experience of most of my readers that a white dinner-plate thrown into clear water does not fall in ' a . straight I line through the water to the bottom, but takes curious "aoroplane-like" movements—first to one side and then to the ether. It may topple and turn and then, as it were, "float* , (or "plane," as one may call it) in different directions, until at last it reaches the bottom. Divers will pick up halfpence thrown into clear water whilst the coins are thus zig-zagging through it, but a bullet of the same metal would :go ouickly down and escape them. When engineers desire to ascertain the distance of the bottom of the ocean • from the surface, for the purpose of laying an electric cable, a compact weight of metal attached to a cord (made of steel piano-wire), which unrolls from a great drum with the least possible friction, is dropped from tho side of a ship, and allowed to fall through the water to the bottom. It is of such weight, and offers so little surface to reeistance, that it falls fairly vertically; the lpngth of wire, uncoiled is noted, and tho weight is hauled up again by the turning of tho drum, which is worked by a steam engine. This is the operation called "taking a sounding." Often by a simple contrivance the weight is made with ft hollow or tube-like receptacle, so that on striking the bottom it takes up some of the mud or sand there lying, which is thus brought to the ! ship and examined. I am informed by Messrs Siemens, of Woolwich, the great engineering firm concerned in laying deep-sea telegraph cables, that, according to their experience, tho sounding weight takes twenty minutes to sink to two miles depth in tho ocean. Probably some small allowance should be made for the friction of the piano-wire, in order to estimate tho time which the sounding weight would ( siko were it free and isolated. _ Even if we take this ns. reducing tho time to fifteen or even ten minutes, it is obvious that the rate is much slower I than . that through two miles of air, which for a compact bullet-shaped body would bo but little more than 23sec. On the other hand, the time required for tho sounding weigh-- to sink two miles is not very great. Naturalists have in the last thirty yours sunk their brawls and dredges to this and even greater depths, and after letting them unig and scoop along tho bottom, raised them again, by means of the drum and wheel worked by a donkeyengine, to-the surface. With a hempen ropo and an iron-framed dredge of 3ft opening, I have dredged in the Norway fiords in depths of 300 fathoms. _ Tho buoyancy of the rope, the relatively small siwv of tho dredge, and the strong currents always carried the dredge fnr out of the perpendicular, i'ucl we had to let out as much n3 600 faChoms of cone in order to reach the bottom. The dredge often took ten '.Tiinutc-s u> [ict thoro owing io this ''drifting" on its why down. A matter which affects the- rate of fail oi a heavy body in water is its weight in i;ror>;»rtion to its bulk—what is called its "soccific gravity." Wo state the speoitic gravities of various substances uy taking that of water as 1. Wtf.ifind, for instance, that any solid lump oi iron weighs between seven and oight times as mucii as the quantity of water which occupies the same spi'.cc. Wo therefore .say that tho specific gravity oi' iron is between 7 a-id S. A solid mass oi" tho same specific KJ'av.ty as water would just float in it, h.>mg buoyed uy by the pressure of the water. The piece oi' iron is drawn downwards by a force equivalent to the I difference between its' total weight and that of a bulk of water equal to it- in size, or, as we may say, emial to that of the water displaced by it. In comuar:ng the linking of a. great steamship with that of the heavy weight used in "sounding," wo ], aye " to consider this matter. So long as the iron ship <>"&- t:;ins air shut in, in rooms and com■{jartnifiats, oven wher enough water to Sink tier has rushed :n through her broken side cftor a collision, the weight of tho ship will.be much less than that of a mass of iron of tho same area and measurement. As the water enters and drives out the air her total specific gravity becomes losp and less, and she sinks below tho surface. But proba-o-y a good deal.of air is still held in her various rooms and chambers, and it 15 ? n ;? as tfcfi water faros > ts v;a - v in and tne partitions of the structure are hurst by pressure that the list of the enclosed air escatses, or is compressed i condition. So that at first % n(i <*oes not sink with so great a tio<fn-<j ragging force, displacing the fa-oiyas-.a solid raris of Iron of then *- n S t a?e nnd siso would do. And all, or nearly all, her cava- ,""^. slre ..filled by water she opposes less 6pe W fi c srav i ty water thaa

' were sho eolid iron, and, therefore, sinks more slowly. This lighFer quality of tho whole mass or less specific gravity, makes her more subject to tho "planing" movements and oscillations caused by her shapo -tf&en meeting tho resistance of the water than \vero she of the specific gravity of solid iron. Probably when filled with water, except for a little compressed air left hero and there in cavities, she is about twice, as heaw, bulk for bulk, as tho water. The delay'in sinking two miles would probably not amount to moro than halt an hour as compared with tho twenty minutes taken by tho sounding weight. A notion, I find, exists that the seawater becomes denser as deeper regions are reached, and that before the ship had sunk to the bottom she would be in water so dense —that is, of such high specific gravity—that she would cease to sink altogether, being herself no heavier than an equal bulk of sea-water at that depth. Another erroneous notion is that water becomes "viscous" under great pressure. Both these mistakes are duo to the erroneous belief that liquids are like gases, compressible. It is truo that the pressure increa.ses by 151b to the square inch, or whatsis called an atmosphere—the weight of a column of our atmosphere of a square inch in cross section, for every 30ft which wo descend in tho sea. At 1000 fathims this amounts to about one ton i>rr square inch—and to doublo that at 2000 fathoms. .A gas would be enormously compressed and made "dense" by such a pressure, becoming reduced to half its volume, and therefore of doubled density ''more or lee? exactly), for every ?Oft of water beneath which it is sunk. At a depth of 1000 fathoms it would be coTnnrcssed to l-900th of its bulk, fnd be 200 times as heavy, bulk for bulk, as it was at the surface. But pressure has no such action on liquids. That is one of their characteristics. Atone timo the observations made by early experimenters were held Io pro ro that water is absolutely incompressible.- It has, howovor. now been shown that it loses- 1-I*o 000 th part of its volume when "we double the ordinary atmospheric pressure on its surface. H is not known whether this on for nil i»>crens<? of prosstire. But even if it did. that would only trrve an increase of density at 2000 fnrbonis to a reduction of its bull-, by l-"0t"l\ And that, which is probnbly lively in excess iVs we do rot know that the wrac relation of compression to pressure ia maintained at vcvv iwesure would have prnot->nlly no effect in the s : .nl:i"<r through th" deep water of a iro" ship of a specific srr.ivjty Wrn, i\"\t of ordinfrv vjt.or. "Wbils* iho inrrrsw of <!,-•-, sir-? <iw n to r'T>Tini*'-7s*on would bo ♦"•"lifiMo —tVn Tnrrlocnio.r struchiro called s 'vic. '•osity" rannot bo T>ro<lnr"v) in a. lim-iifl by P7v>s=urp nriv more than can that i called "roliditv." '