Dec. 30, 1881.] 



• KNOWLEDGE ♦ 



181 



bow late in life some persons have been in detectinfj ench inequality 

 of yision. Indeed, there is a case mentioned by the youiifrer 

 Herschel, of an individual not knowing of complete blindness of 

 one eye until advanced in years. ^Vhcn there is a weak one and a 

 stroup, it is generally the we.ik one that is most liable to disease, 

 from its unconsciously straining to share the labour of rcading, 

 painting, or engraving with the other. 



EARTH TREMORS. 



(Feom the Tunes ) 



ONE day during the past summer, at the end of a long uphill 

 beat after the partridges, I threw myself breathless on the 

 i,Tound, and on my back waited for the others to come up. As they 

 drew near, five or six strong, tramping heavily through the turnips, 

 1 was strnck by an apparent tremor of the earth beneath me. It 

 •ivas shivering like a jelly — or I was; for a moment I was in doubt 

 '.vliich. Spreading out my hands upon the surface, and lying as 

 « I'lsp and flat as I could, I was soon made sure that the tremor was 

 1 ;dly in the eai-th and not in me. It grew more and more distinct, 

 keeping time with the tramp of the walkers. When at last they 

 reached me I told them of their Neptunian feat, and, making them 

 jump altogether a few yards off was gratified to find that I could 

 Ihu- liing about a very respectable earthquake at will. The 

 nii ; 1. was very peculiar, and I can well believe that a queasier 

 siMiiiiKjli than mine would soon be conscious of something very like 



'iia? do terrc. We examined the structure of this skipping hill, 



ut found nothing that helped us much to an explanation. It was 

 !iiainly made up of a thick |cap of gi-avel on a base of red sand- 

 stone, and so was not likely to contain anything like a high-arched 

 hollow or concealed morass within. 



This vivid little experience made me readier, perhaps, than some 

 to accept the striking statements about earth-shaking made by the 

 brothers Darwin at the York meeting of the British Association. 

 Especially was I prepared to give credit to what they quoted from 

 the Astronomer Boyal about Greenwich Hill and the Observatory. 

 He wrote : — 



" In the old times of Greenwich Fair, some twenty years ago, 

 when crowds of people used to run down the hill, I find the 

 observers could not take reflection observations for two or three 

 hours after the crowd had been turned out. . . We do not have 

 anything like such crowds now, even on Bank holidays, and I have 

 not heard lately of any interference with the observations." 



There is as little foundation for the calumnious hypothesis that 

 the observers whose reflections were thus agitated had been visiting 

 the fair themselves as for the suggestion that the above experience 

 of my own took place after luncheon. No, the truth is, the solid 

 rtirth is a very elastic solid after all, and Greenwich Hill and the 

 '"servatory and all that it contained were trembling like my High- 

 md knoll. The howdah of the Atlas-elephant that stands on a 

 tortoise is a rather ricketty structure, and quakes with every jog of 

 the Titanic beast. But is it not being tugged at by every petty 

 planetoid, pullod from its path by every planet, heaved all awry 

 through its yielding bulk by sun and moon in their cotirses P It is ; 

 but over and above these longer, graver motions, there are incessant 

 tremblings and quiverings in quick periods measured by seconds or 

 less. This unlooked-for sensitiveness to small stresses, this inces- 

 sant vibration when all obvious disturbing causes axe eliminated, 

 ■ire the new facts that the Darwins have so strikingly brought out. 

 How solid rock and massive piers of stone warp under heat and 

 cold hke unseasoned wood, how a wide stretch of ground may swell 

 and rise for hours together after a little water has been poured on 

 it. how the passage of a train miles away, or the pressing of a finger 

 on the ground near at hand, may be enough to deflect the plu>nb- 

 line to a visible degree — these and many other new phenomena are 

 detailed in the full and most interesting preliminary report on the 

 Lanar Disturbance of Gravity handed in to the Section of Physics 

 by the ingenious brothers. 



The title reminds us that, as so often in science, it was in looking 

 for one thing that they found another. Every one knows that as 

 the earth pulls the moon round in its monthly orbit, so, too, the 

 moon puUs the earth and everything upon it. If a plummet be 

 hnng up right under the moon, so to speak, the earth is drawing 

 the bob downwards, the moon verj- much more feebly pulls it 

 upwards. The result is that the bob weighs a trifle lighter than if 

 the moon were abolished. Thanks to the moon, the string is less 

 severely strained. If the moon be not right overhead, but down a 

 little towards its rising or its setting point, the bob will be a little 

 drawn aside out of the straight and the plummet will no longer 

 give a true plumb-line. As the moon rises, crosses the sky, 

 and sets, then the direction of the plumb-line will change 

 through a small angle. Of course, even when no moon 

 la seen, its sUent influence must be felt, and the plumb-line 



will return to its position by the time the moon is ready to rise 

 again. Uow small the change really is wc may gather from the 

 fact that with a pltimmet 300 yards long the travel to and fro of 

 the bob could scarcely in this country reach a thousandth of an 

 inch. This is what is meant by the lunar distttrbanco of the direc- 

 tion of gravity ; and there must, of course, be a solar disturbance 

 also, the .same in kind, but naturally very much smaller in amount. 

 To investigate these disturbances experimentally clearly calls for 

 refined skill and very delicate apparatus ; but Sir William Thompson, 

 to whom instrumental difficulties are always but child's play, in 

 suggesting the investigation three years ago, had in view the detec- 

 tion of an influence still more recondite and refined. 



I have said that the moon, in pulling asido the bob of the 

 plummet, pulls also on the earth beneath it. If the earth were 

 peifectly stiff and unyielding, this pull could have no effect on the 

 deflection of the plumb-line. But if, as we have reason to believe, 

 the earth yields like a great viscous mass to great stresses as well 

 as to small ones, a hump of solid earth — a land-tidc-^will travel 

 round the globe in obedience to the moon's attraction. This hump 

 in its course will pass under the suspended plummet, and the actual 

 deflections of the plumb-line as obser^-ed will no longer agree with 

 those reckoned on the supposition that the earth is rigid. If we 

 had an instrument, then, by which the minute aberrations of a 

 carefully-suspended pendulum, isolated as far as possible from all 

 local disturbance, coilld be magnified up to the point of visibility, 

 we should have it in our power to settle some very pretty points in 

 the physical theory of the world. Such an instrument, after various 

 trials and failures, the Darwins have erected in the Cavendish 

 Laboratory at Cambridge. 



A massive stone, weighing three-quarters of a ton, is bedded in a 

 pit upon the native gravel. It is surrounded by a trench, a foot 

 wide, to isolate it completely from the floor and the building. The 

 pendulum is a massive cylinder of pure copper, hung, by a brass 

 ■wire about a yard long, inside a hollow cylindrical copper support, 

 that rises from the stone. - A tiny galvanometer mirror is hung by 

 two fine threads, one of which is fastened to the bob, and the other 

 to a projection of the fixed support. This suspension is so arranged 

 that any movement of the bob displaces the mirror to a much 

 greater degree. A ray of light is sent from a distant lamp on to 

 the mirror, and thence reflected to a scale seven feet away. The 

 magnification resulting from this double process is something like 

 50,000 times. To stiU and quench accidental tremors, the hollow 

 copper cylinder is filled up with a mixture of spirits and water. It 

 is a fact, made out by physicists that a boiled mixture of gin and 

 water is much more viscous and clogs the motions of bodies im- 

 mersed in it much more effectually than either the neat gin or the 

 simple water. Further, to ward off the effects of external changes 

 of temperature, the whole instrument is immersed in a tank of 

 water resting on the stone ; and lastly, after the precedent of the 

 Tishbite, the surrounding trench is also filled up with water. Tims 

 protected, the apparatus might seem sufficiently cut off from local 

 influences, but as a fact its sensitiveness is 'now so great, that 

 the observation has to be carried on in another room by means 

 of a window and a telescope. Standing in the room itself 

 IG feet away, it is enough to shift your weight from one foot to 

 the other to cause the speck of light to run along the scale. The 

 same restilt follows if you press steadily with your fingers on the 

 stone edge of the trench, but you may strike a good sharp blow 

 even on tho stone base without effect. It is the distortion of the 

 soil by slight, steady pressure that is transmitted through solid 

 gravel and stone, and shows itself as a microscopic deviation of the 

 pendulum. Such being the case, the instrument should be delicate 

 enough, in all conscience, to determine lunar and even solar dis- 

 turbances in the direction of gravity; but, unfortunately, having 

 got so far we seem almost to have done too much. When 

 regular series of observations are made it is found that the pen- 

 dulum is hardly ever steady. The image on the scale dances about 

 incessantly. "The ground is never really still. Some days it may be 

 quieter than others and generally there is evidence of distinct diurnal 

 periods, but the minor zigzags constantly interrupt, and at times 

 reverse for an hour together, the slower march northwards or 

 southwards. These tremors have been hitherto so persistent and 

 so wildly irregular, that for the present, at least, the prospect of 

 unravelling from them the perturbations due to the moon does not 

 seem very near. Mr. George Darwin talks of the probable neces- 

 sity of building a gravitation observatory at the bottom of a mine. 

 There, it may bo hoped, the railway train and the market cart will 

 cease from troubling, and the plummet, save for the steady paces of 

 the moon, wiU be at rest. The work of examining and observing 

 these tremors of the surface is, however, still going on at Cam- 

 bridge, and already several sharp seasons of microscopic earth- 

 quake unsuspected outside have been noted. Sometimes a very 

 storm of tremor breaks out, for which no sufficient local cause can 

 be traced. 



