ONE day during the past summer, at the end of a long uphill beat after the partridges, I threw myself breathless on the ground, and on my back waited for the others to come up. As they drew near, five or six strong, tramping heavily through the turnips, I was struck by the apparent tremor of the earth beneath me. It was shivering like a jelly—or I was; for a moment I was in doubt which. Spreading out my hands upon the surface and lying as dose and flat as I could, l was soon made sure that the tremor was really in the earth 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, them jump altogether a few yards off, was gratified to find that I could thus bring about a. very respectable earthquake at will. The motion was very peculiar, and I can well believe that a queasier stomach than mine would soon be conscious of something very like a ^l de terre. We examined the structure of this skipping hill, hut found nothing that helped us much to an explanation. It was mainly made up of a thick cap of gravel on a base of red sandstone, 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 Royal 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 1 have not heard lateiy of any interference with tile 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 earth is a very elastic solid after all, and Greenwich HiU and the Observatory and all that it contained were trembling like my Highland knoll. The hOWdah of the Atlas elephant that stands on a tortoise, is a rather rickety structure, and quakes with every jog of the Titanic beast. But is it not being tugged at by every petty planetoid, pulled from its path by every planet, heaved all awry through its yielding bulk by sun and moon in their courses? 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 incessant vibration when all obvious disturbing causes are eliminated, are the new facts that the Darwins have so strikingly brought out. How solid rock and massive piers of stone warp under heat and cold like unseasoned wood; how a wide stretch of ground may swey 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 plumb line to a visible degree—these and many other new phenomena are detailed in the full and most interesting preliminary report on the Lunar 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. pulls the earth and everything upon it. If a plummet be hung up right under the moon, so to speak, the earth is drawing the bob downward, the moon very much more feebly pulls it upward. The result is that the bob weighs a trifle lighter than if the moon were abolished. Thanks to the moon, the string is leM severely strained. If the moon be not right overhead, but down a little toward 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 is seen, its silent influence must be felt, and the plumb line will return to its position by the time the moon is ready to rise again. How small the change really is we may gather from the fact that with a plummet three hundred yards long, the travel to and fro of the bob could scarcely II this country reach a thousandth of an inch. This is what is meant by the lunar disturbance of the direction 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 Thomson, to whom instrumental difficulties are always but child's play, in suggesting the investigation three years ago, had in view the detection of an influence still more recondite and refined. I have said that the moon, in pulling aside the bob of the plummet, pulls also on the earth beneath it. If the earth were perfectly 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 tide—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 observed 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, could be magnified up to the point of visibility, we should have it in our power • “ On an Instrument for Detecting and'Measuring flmall Changes in the Direction of Gravity.” By George H. Darwin. M.A., F.B S., late Fellow of Trinity College, Cambridge, and Horace Darwin, M.A., A.M.I.l:.lS. British Association Report. 1881.to settle some very pretty points in the physical theory of the world. Such an instrument, after various trials andfail- ures. the Darwins have erected in the Cavendish Laboratory at Cambridge. A massive stoue, 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 hoRow 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 hob 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 50,010 times. To still and quench accidental tremors, the hollow copper cylinder is filled up with a mixture of spirits and water. It is a fact, ^^e out hy Poiseuille, that a boiled mixture of ginand water is much more viscous and clogs the motions of bodies immersed in it much more effectually than either the neat gin or the simple water. Further to ward off the effect' 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 Tisbbite, tile surrounding trencb is also filled up with water. Thus protected, tbe apparatus might seem sufficiently cut off from local influences, but as a fact its sensitiveness is now so great that tbe observation has to he carried on in another room by means of a window and a telescope. Standing in the room itself sixteen feet away, it is enough to shih youi' weight from one foot to the other to cause the speck of ligbt to run along the scale. The same result follows if you pre^ steadily with your fingers on the stone edge of the trench, but you may strike a good sharp blow even on the stone base without effect. It is the distor- sion of the soil by slight steady pressure that is transmitted through solid gravel aud stone and shows itself as a microscopic deviation of the pendulum. Such being the case, the instrument should he deiicate enough, in all conscience, to determine lunar and even solar disturbances 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 pendulum is hardly ever steady. The image on the seale dances about inceiMantly. The ground is never really still. Some days it may bequieter 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 northward or southward. These tremors have been hitherto so persistent and so wildly irregular that for the present, at least, the prospect of unraveling from them the perturbations due to the moon does not seem very near. Mr. George Darwin talks of the probable necessity of building a gravitation observatory at the bottom of a mine. There, it may be hoped, the rail way train and the market cart will cease from troubling, and the plummet, save for Ihe steady paces of the moon, will be at rest. The work of examining and observing these tremors of the surface is, however, still going on at Cambridge, and idrcady several sharp seasons of microscopic earthquake, unsuspected outside, have been noted. Sometimes a very storm of tremor breaks out for which no sufficient local cause can be traced-. Even so far, the outcome of these experiments may prove of high value to practical astronomers. The piers on which their great telescopes turn are built of solid stone, hitherto regarded as the material most insusceptible to change or disturbance. The Darwins have shown that such piers are really most sensitive to inequalities of temperature and to small stresses. They yield and warp to a most unexpected degree. Tbeir had conducting power is responsible for this in part, and it is fruitfully suggested that it might be well to plate the piers with coppenand to swathe them wi*h flannel. Astronomers, who, to their vexation. have to redetermine the level of their instruments from hour to hour, and who have long suspected the occurrence of microscopic earthquakes, will take note of this practical hint.. They will make ready use, too, of the observation here recorded as to the effect of the observer's own weigbt. They will think more of the drainage of the soil around their instruments after the observation on the irregular and long-continued swelling of ' the ground that results from the percolation of water. Meanwhile the British Association and Cambridge may be congratulated (in the new and valuable, field of work thus opened out under their au.;pices, and especially on their having enlisted the services and energies of two workers who so worthily keep up the tradition of an honored name. —London Times.
This article was originally published with the title "Earth Tremors" in s , , 5007-5008 (March 2013)