velocity of transit due to the elasticity of the rocks, which was also experimentally obtained, was extinguished thus by their want of continuity, &c.

Now, from these different rates of wave transit in diverse materials, it results that if an impulse be given at a single point, it may be perceptible several times in succession by a person so situated as to receive it through different media.

Let, for example, one stand near a line of railway, and a heavy blow be delivered upon the iron rail; it will be heard first, through the iron rail; almost directly afterwards a second sound will be heard through the air; and almost at the same time the person will feel the pulse of the blow reach his feet through the ground. While, if another person had his head immersed in the water filling a side drain along the line, he would have heard the sound through the liquid at a moment different from the arrival of any of the other waves.

Such waves, only on a larger scale, constitute an Earthquake shock. An originating impulse (something of the nature of a blow, or having the effects of one) there must be for every shock, but we are not here concerned with the source from which that impulse may be produced. It may be an explosive production or condensation of high-pressure steam in heated cavities, deep beneath the surface, or sudden increase or decrease of its tension, or sudden fracture or fall, or forcing up or down or against each other of great rocky masses, or if (in near propinquity to active volcanoes), it may be any of their throbs or throes, or explosive ejections, or the recoil from these; it matters not as respects the physical theory of Earthquake-motion, and the explanation this renders of Earthquake-phenomena, what or which or whether any of these be the cause of the blow, so long as some sort of impulse be given, and the seat of this be more or less deep beneath the earth.

Then in all directions outwards from this centre of impulse, there will be transmitted an elastic wave. The form of the wave, if originated at one point, would be that of a spherical shell concentric with the centre of impulse, if the medium were quite homogeneous; but in nature, the wave assumes ellipsoidal and various other more complex forms, and rapidly gets broken up into smaller and still more complex waves, by dispersion, by interference, refraction and reflection, in consequence of the shattered and varying nature of all the superficial formations through which it is transmitted.

The wave starts from the origin with one normal and two transversal vibrations, i. e. every particle vibrates not only to and fro, in the radial direction from the centre, but also at right angles to this, in two directions at once. The former is the larger vibration and the more important to attend to, so that we may often, in investigating Earthquake-phenomena, altogether pass over the transversals. These vibrations constitute the proper motion of the wave as contradistinguished from its motion in transit.

A plumb line passing from above the surface of the earth and through the centre of impulse is called The Seismic Vertical. The wave or shock passing outwards from this centre, reaches the earth's

surface vertically, and soonest in this Vertical, which is the shortest distance between any point below and the surface, and here it only produces (neglecting transversals) a rapid movement up and down. The surface of the ground actually rises and sinks again to its previous place, with great rapidity, and through a range that may be several inches or perhaps feet, dependent on how great and how near the blow is given below, and what is the intervening material.

For all points around the Seismic Vertical, the wave emerges at slopes, called emergent angles, which become more and more nearly horizontal as the distance on the surface is greater. The spherical or quasi-spherical shell wave-form at any given distance outwards when cut by the earth's surface, intersects it as a closed curve, more or less circular, elliptic, or oval, and the crest, so to say, of this surfacewave, called a coseismal line, because all bodies situated in it are shaken at the same instant, travels along the surface of the earth with a real, though not large, and with a constantly diminishing undulation, like a roller at sea, constantly enlarging the curvilinear area within it; and as it passes outward, objects in succession are disturbed or overthrown, not by the transit of the wave-form, but by the wave itself, that is, by the movement of the particles in motion in the wave.

There is a certain distance outward upon the earth's surface, all round the Seismic Vertical, at which it may be proved that the overthrowing power of the shock is a maximum, greater than anywhere, within or without it—within, because there the direction of normal movement in the wave is more nearly vertical, and hence less calcuated to upset objects standing on the ground-and without, because the further the shock has travelled away from the Seismic vertical, the more its power (to speak loosely) has decayed. This is the Meizoseismal circle or curve. The angle made with the Seismic vertical by a line drawn from any point in this curve at the surface down to the centre of impulse, is for the same conditions constant.

If the impulse or blow has been accompanied by rending or fracture, or the striking or grinding together of hard or rocky masses, or by the rush of vapours or gases, then the wave of shock will be accompanied by waves of sound. But these latter may or may not travel just at the same rate, or by quite the same wave-paths to the ear of a person upon the surface, as does that of the shock which he feels. Hence there may be Earthquake shocks, with or without sounds, and the shock may be perceived before any sound is heard, or the sounds may precede and herald the shock, as the awful "bramidos" generally do the Earthquakes of Mexico.

But to hearers remote from the Seismic vertical, the sounds, if any, will reach their ears not only through the earth, but through a longer or shorter intervening range of air, and hence at very different times and with very different amounts of repercussion and reverberation, although originating in one sound only, as of a single rend, or grind, or explosion.

A remarkable use has been made, for the first time, of the differences in the character of the sounds heard nearly simultaneously, and at about equal distances all round the Seismic vertical, in the Report

addressed to the Royal Society of the examination made on the facts of the Neapolitan Earthquake of 1857, by employing them to determine approximately from their varying character the form of the focal surface or cavity, or of the subterranean locus of the centre of effort, --and the method will no doubt hereafter, when more largely and completely applied, yield very important results. Space forbids us, however, here to do more than mention it, and refer to the Report in question.

These, then, are the waves produced by a single impulse, and constituting an Earthquake whose origin is inland. But should the origin be under the sea, then at the point passed through by the Seismic vertical and around it, the sea-bottom is, as on land, suddenly upheaved, and again dropped down; or it may be, as by submarine volcano, actually broken up altogether, and steam, lava, and floods of lapilli, and so forth, may be then belched forth under water. In either case there is forced up a volume of water upon the sea's surface just above, or several of these in succession, and as each mass falls again it assumes the horizontal form of a circular liquid wave of translation and these are propagated outwards over the surface of the sea, like the circles or ring-shaped waves on a pond, when a pebble is dropped into it. The altitude and breadth of these waves depend mainly upon the magnitude of the disturbance of the bottom, and on the depth of water above it; the rate of their propagation outwards has nothing to do directly with elasticity, it is dependent simply upon the square root of the depth of the water traversed by the wave on its surface. If the ocean continued everywhere of the same depth, and the original impulse came from a single point, or circular disc, then the horizontal plan of the crest of any one of these waves would always remain a circle; but the depth varies-and as that part of the expanded circle which is over a deeper part moves on much faster than portions moving over shallow water, or approaching shores-so the circles soon get distorted into various other closed curves, and the original radial direction of translation outwards gets changed to any extent so that a wave might, without any reflection, even double back upon its original line of progress.

When the long flat swell of such waves, as they are originated on the deep sea, approaches the shores and reaches shoal water, their fronts become steeper and steeper, and they finally roll in upon the shore, as the great sea waves of South American and other Earthquakes, so much dreaded wherever they have been once experienced. They are often so large that they only topple over as breakers after they have rolled in unbroken masses far inland.

Such was the wave that swept, in one unexpected deluge, thousands of people off the Quay at Messina, and which in some South American Earthquakes have inundated devoted cities like Valparaiso and Callao, with a frowning crest 80 feet in height. Not that the wave while it was far out at sea possessed anything like this altitude,--but just as the Atlantic tide wave,-when constricted in the Bay of Fundy, or in our own Bristol Channel reaches 70 or 40 feet; so does the Earthquake sea-wave rise and get steep in the narrow and shallow waters.

Thus, we see that in an Earthquake whose origin is beneath the sea, there may be a series of waves, all arriving in the following order, differently, and at different times, to an observer standing on the land. 1st. The great Earthquake wave of shock.

2nd. The forced sea-wave (of which we have as yet not spoken); it is the roll of water forced up by, and carried along with, the earth-wave, which raises the sea-bottom, and with it the water upon its back as it were, and at its own rate of motion, after it has got into shallow water. This is but occasionally perceptible, and only in great Earthquakes. 3rd. The sound-wave through the earth, which may or may not bo before.

4th. The sound-wave through the sea.

5th. The sound-wave through the air.

All these except the second are elastic waves.

6th, and lastly. The great sea-wave, or wave of translation, rolls in and completes the catastrophe, often hours after the shock has done its work of destruction; or portions of it may roll in upon shores that have felt no shock at all. Thus in the great Earthquake at Japan, which a few years ago wrecked a Russian frigate in one of the harbours there, the great sea-wave produced in the deep seas, near those great Islands, hours afterwards, reached the opposite shores of the Pacific, at St. Diego and Francisco, and gave the first intelligence at those places of the disaster that had occurred at the further side of that great

ocean.

Space forbids us now to pursue the subject further. At some future opportunity we may be enabled to revert to it; and to develope the relations between the movements of the elastic-wave particle and the wave's transit to which we have in the preceding pages almost confined our remarks. It remains also to be shown by what methods the position and depth, and even the form and magnitude of the deep-seated focus of an Earthquake, may be ascertained by deciphering, with the help of science, the terrible handwriting left by the destroyer upon the country it has overthrown. To these should be added some description of the secondary effects of Earthquakes, in moulding anew the features of the lands they pass over, and how those affect and modify the shocks that reach them. Something, too, might be said as to the distribution of Earthquakes in time and in space upon our Earth's surface; what are the conditions originating within our planet; the impulses on which their existence depends; and, lastly, what is the function of Earthquakes, and what uses they fulfil as parts of the great cosmical machine.

LIGHTHOUSE ILLUMINATION BY MAGNETO

ELECTRICITY.

By J. H. GLADSTONE, Esq., Ph.D., F.R.S.

ANYONE who, on a tolerably clear night, has crossed the channel between Folkestone and Boulogne, and remained on deck, must have noticed on the French coast what appeared a brilliant star, now waxing, now waning. It was the light of the far-famed Pharos, on Cape Grisnez. But if he has made the passage within the last eighteen months, his gaze will have been attracted by a still brighter star on the British coast, of a bluish tint, steady and brilliant. This is the Magneto-electric Light at Dungeness, the brightest spark in the world, and one which unites a rare scientific with a practical interest, and may prove only the first lighted of a multitude of similar beacons. I propose to say a few words on the history, production, and merits of this Light.

HISTORY.—If we ask the parentage of the Magneto-electric Light, Mr. Frederick Hales Holmes is certainly its father, but, like other beings, it has had two grandfathers-the philosopher who first showed the conducting power of charcoal, and the brilliancy of the light between charcoal terminals of an interrupted galvanic current; and Professor Faraday, who discovered that when a piece of soft iron, surrounded by a coil of metallic wire, was made to pass by the poles of a magnet, an electric current was produced in the wire, which revealed its existence by effecting chemical decompositions, or by giving a spark. This spark, it is true, was barely visible as at first obtained, but it has been exalted into the present Magneto-electric Light.

It appears that in 1853 some large Magneto-electric machines were erected in Paris for producing gas by the decomposition of water, the object of the proprietor being to use this gas for the purposes of combustion; but the scheme failed, the Company that was being formed came to nothing, and the machines were pronounced by leading scientific men to be only expensive toys. Mr. Holmes, however, who was one of the referees, proposed to turn them to account for electroplating and gilding, and thought it possible that the Electric Light might be produced advantageously by their means. "My propositions," he says, in his evidence before the Royal Commission on Lights, Buoys, and Beacons, "were entirely ridiculed, and the consequence was, that instead of saying that I thought I could do it, I promised to do it by a certain day. On that day, with one of Duboscq's regulators or lamps, I produced the Magneto-electric Light for the first time, but as the machines were ill-constructed for the purpose, and as I had considerable difficulty to make even a temporary adjustment to produce a fitting current, the Light could only be exhibited for a few minutes at a time-say ten or twenty minutes-when the adjustments were entirely displaced by the friction; the rubbing surfaces were worn away. From this time I directed my attention more particularly to the reconstruc