LAND TIDES.

Dunstan Times, Issue 2481, 17 May 1909, Page 2

LAND TIDES.

(By Professor Garrett P. Service.)

Modern science has been severely criticised for spending its time calculating the orbit of distant planets and studying remote heavenly bodies while it confesses ignorance of the most elementary facts in regard to the phenomena of our own earth which are of vital importance to mankind.

There is some truth in this indictment of science, but recent terrestrial happenings have turned the attention of the scientific world more closely to the study of earthquakes, and we now are beginning to understand the cause and effect of -earth disturbances. As an illustration of the recent and very encouraging progress in this direction, 1 may mention the fact that I’rofessor Perret did, beyond doubt, predict with surprising accuracy the Messina earthquake of last December, and Professor Emile Marchand, of the Pic du Midi Observatory, early in February, predicted an earthquake at Saint George, a village near Grenoble, France—this earthquake did, :n----deed, occur on the very date and at the nlace predicted by the French astronomer. Camille Flammarion, the French astronomer, announces an “almost incredible discovery’’ made by the German savant, Xieckcr —viz: that twice every day the earth heaves with a great tide, the fluctuation of level amounting to about eight inches. The idea that that there may be such a tide is not new. But if M. Hccker has succeeded in measuring its amount and demonstrating its reality be has undoubtedly achieved a great advance of knowledge. The cause of such a tide can only be the differential attraction of the sun and the moon. It must arise from the same forces as those which produce the familiar tides in the waters of the ocean. Moreover, if it exists, it must play an important part of the production of volcanic eruptions and earthquakes. Therefore, it is of vast importance to mankind. There must be critical times when such disturbances arc more likely to occur. The foreknowledge of such times would be imnortant as affording a means of forecasting impending disasters. M. Meeker says that this tide runs around the earth’twice every day. Every twenty-four hours the surface is heaved up eight inches. Because of its relative nearness the tidal power of the moon is about two and a-half times that of the sun, hence the principal wave must follow tlio moon. When the two bodies—the sun and the moon—act together, as happens at new and full moon, the tide is accentuated, for then both pull together. It was long ago noticed that seismic disturbances are rather more frequent at just those conjunctures. But, as we shall see, there are ether causes of irregularity in the force of tides. Of course, such a tide would tend to open cracks and lines of weakness in the earth’s crust, thus inducing earthquakes and volcanic outbursts. The location of these linos is well known, and they are lines of danger. One of the greatest runs along the eastern coast of Asia, passing through Japan, where the shell of the earth is constantly more or less disturbed, and then down, in a curving course, through the Philippines and the East Indies. There is a great branch of this fault traversing Java and Sumatra ,where the stupendous explosions of Krakatoa occirred in 1883. The rrreat series of faults seems to extend on, by way of New Guinea and New Zealand, into the south polar regions, where the famous volcanoes Erebus and Terror smoke and vomit flames and lava on the ice-covered Antarctic continent itself. In order to understand what is meant bv a tide in the body of the earth we must first see what an ordinary/tide is. Everybody knows that all bodies attract all other bodies with a force that is measured by two things—first, the mass {i.e., the amount of matter in the attracting body), and second, the distance between the bodies concerned. To illustrate this, suppose wo have four bodies. Each pulls upon the others, but for simplicity we will suppose that only the large body acts. Since the force varies with the distance, it will pull the nearest body more powerfully than the middle one, and the middle one more powerfully than, the most distant one. The consequence is that the first body will be moved more than the next one beyond it, and that more than the one which is most distant. The result will be that the distance between

the attracted bodies will be increased. Now, let us increase the size of the middle body until the other two just rest on its surface. Then the nearest body will be drawn away from the central one, audi the latter in turn will bo separated from the other small body behind it. This brings us at once to the conditions which produce tides, for if we imagine the two smallest bodies to be reduced to drops of water lying on the surface of the earth, then it is evident that the attraction of the large body on the right, which we may call the sun, will tend to separate them, the drop nearest the sun rising away from the earth, and the earth as a whole moving away from the drop Oi. the other side. In. place of_ the two drops put oceans, and the result is a tide raised by the differential attraction of the sun. The tide rises on both sides at once because the tendency of tho attraction on bodies at different distances, but free to move, is to separate them along a line running to the attracting body. The differential attraction of the moon, ns already remarked, is much greater than that of the sun because of its greater nearness. The result of the tidal action of the sun and moon shows in the ocean surface, which is drawn out on each side of the earth so as to give it an ellipsoidal figure. The next thing that we have to understand is that the differential attraction of the moon or the sun, or both together, tends to draw the whole earth out into an ellipsoid, the reason being the same as that alveadv given—viz., that the nearer •particles are attracted more powerfully than the more distant ones. As a mattei of fact, the earth is solid, at least on itssurface, but all the same the tendency of • the attractions to which it is subjected is to deform it in a line pointing toward the source of tho attractive force. Now, this is what M. Hecker means by his great tide heaving the earth’s surface. The whole framework of tho globe feels the tidal strain, and, according to his measurements, yields enough to cause a swelling of the entire earth twice every day to the amount of eight inches. If the earth were liquid, it would yield more, and the height of the tide would be greater, but being rigid, it yields slightly—so slightly that until this alleged measurement by Hecker no one has known how much it was, or whether it was measurable at all. But then, it may be asked, Why does the tide run around the earth twice every For the same reason that the oceanic tides run thus. If the earth stood still on its axis the tides would stand still also, or would only move slowly so as to keep in line with the moon or the sun. But each twenty-four hours the earth turns once on its axis, in consequence of which the part of the surface presented toward the moon, for instance, is continually changing as the earth turns under the moon. It is this fact that makes the deformation of the rocky shell disastrous in its possil.'le effects, because tho particular portion affected is constantly Now, the centre of the tidal swell is in Europe, a few hours later it is under the Atlantic Ocean, and later still it lifts the continent of America. Thus, it runs round the globe in twefity-four hours. On its heels, twelve hours liehind, comes the counter tide, and again, the shell of the earth is heaved by the resistless force. Imagine an eggshell thus periodically deformed by the force of tides within the egg and one would say that the shell would be broken to fragments. Just so the crust of the globe would ho broken if the tidal swell were sufficiently great. But eight inches is a small amount in comparison with the 8000 miles of the earth’s diameter. Wo must remember that the wave in the earth is as broad as tho earth itself, and the eight inches represent the height at tho centre, not the difference of elevation between two adjacent parts of the crust, or two adjoining rocks. Yet, even this is enough to have, under favoring circumstances, a decided effect on the condition of the crust on which we dwell, for that crust is not a homogeneous shell, but consists of layers of rock which, in past times, have been broken and tipped up in such a manner that in many places they are ready to slide one upon another, thus producing earthquakes. Hero the question of the condition of the interior of the globe comes into play. It used to bo thought that the core of the earth was liquid on account of tho excessive heat prevailing there. But Lord Kelvin showed that while the heat is probably sufficient to make it liquid,’ yet in consequence of the pressure it must be really extremely rigid—more rigid than steel. Nevertheless, it is elastic, and consequently may yield to deforming forces. Bub' while wo no longer think that we live on top of a tremendous ocean of liquid tire, separated from us by a shell of cooled rock, yet there are many phenomena, such as volcanic eruptions, which seem to prove that there is molten rock at certain points beneath -as. Tnis gushes out when pressure is brought to bear upon it, and the lava flows to the surface. It would be difficult to imagine a more efficient means by which such pressure could be brought into play than a deformation of the crust by such bodily tides as Hecker declares exist. Why, then, is not every passage of the earthtide accompanied by earthquakes and eruptions? In times past it may have been so, but now the crust has so far settled and solidified that disastrous effects are produced only when the force d the deforming tide reaches a critical point. Minor points of that kind are reached at full and new moon, when the sun and the moon unite their forces deforming the earth, and, as. already said, it has been noticed that seismic disturbances are somewhat more frequent at such times. But there are special crises when the forces are still greater, and when the earthtide must rise considerably higher than its average. Such special period are present whenever full or new moon coincides with the time of nearest approach of the moon to the earth and of nearest approach of both to the sun. They can, of course, be predicted from a knowledge of the motions of the earth and the moon in their orbits. The distance of the earth from the sun varies to the extent of 3,000,000 miles, and that of the moon from the earth to the extent of 31,368 miles. But the variation in the case of the moon’s orbit is proportionately three times as great as in that of the earth, because the moon’s orbit is so much smaller. More than that, the form of the moon s orbit is constantly changing, and with it the amount of variation of distance. Inns it will be seen the problem of determining tho precise periods when the sun and the moon, both being at their least distance, s'multaneously exercise the greatest deforming force upon the earth is somewhat , complicated. When those periods occur the earthtide must continually vary slightly in height. The eight inches of M. Flammarion can only represent its average amount. These facts give a vivid idea of the constant agitation of the apparently immovable surface of our globe. In the eye of science it is no more immovable than tho surface of a lake over which ceaseless breezes are playing. Since the invention of the seismograpli it has been known that, as Professor John Milne says, "the ground on which we dwell is , incessantly in a state of tremulous motion.” It is not surprising, then, that earthquakes are continually occurring somewhere; the only wonder is that they are not more numerous. Fortunately for us they assume a shattering intensity only when they occur along distinctly marked lines of weakness in the crust,“and these lines are well known, to geologists. It is easy to imagine that a passing wave of the great earthtide, when that tide is at its maximum, may strain and open these cracks, thus inducing both volcanic eruptions and earthquakes.