under water, but that it travels faster in that medium than in air; yet such is the case.

The Philosophical Transactions' contain many accounts of experiments made with a view to determine the action of water in this respect. Mr. Anderson, about ninety years ago, tried in the first case how far persons under water could hear sounds produced in the air; and in the next place, whether persons above water could hear sounds produced in the water. He caused three people to dive at once into water, and remain for a few seconds about two feet below the surface; he then spoke to them as loud as he was able, and on their coming up they said they had heard him, but that his voice sounded very low. He then caused them to dive to a depth of twelve feet below the surface, and fired a gun immediately above the water; on coming up, they said they had heard it, but that the sound was exceedingly faint. The converse of many of these experiments was next tried. A diver contrived to “ halloo" under water, and produced a sound which was heard faintly above.

The Abbé Nollet descended to various depths beneath the water, for the purpose of determining whether he could hear the sound of a bell rung above water; the sound was faint, but always audible to him. Franklin, on one occasion, plunged his head below water, and caused a person to strike two stones together beneath the surface; at more than half a mile distance he heard the blows distinctly.

In the year 1826 this subject was experimentally

ended in the capture of the town, was the occasion of | serious detriment to it. In the fifth century Bononia is said to have been unsuccessfully attacked by Attila, king of the Huns; and in the ninth century it was laid waste by the Northmen, who landed just by. From the discovery of a ring to which the cables of vessels were fastened, it is thought that the sea flowed up as far as the present upper town of Boulogne, in which case Gesoriacum must have been at the bottom of a small bay. Several Roman antiquities have been discovered at Boulogne; among these are medals and tombs. During 1823, 1826, and 1827, several tombs were discovered. Those discovered in 1823 were close to the sea; those discovered in 1826 and 1827 were a little out of the town, on the right of the road to Paris. The coffins in these last-mentioned tombs were ranged in regular order, and the bones (some of which bore the marks of deep wounds) were in good preservation. Several wells, a Roman road, and the foundations of what was considered to be a votive altar, were discovered at the same place; also many vases of different forms, and a great number of medals. Similar discoveries had been made before. On a cliff near the entrance of the port there stood a tower, of which the remains are represented in the cut at the head of our article, from a sketch recently taken, which tower D'Anville considers to be one built by Caligula, as mentioned above. It was an octagon, and each side is said to have been about twenty-four or twenty-five French (equal to twenty-five and a half or twenty-six and a half Eng-tested in a remarkable manner on the Lake of Geneva, lish) feet (at the base, we presume), and it rose to the height of one hundred and twenty-five feet. It had twelve stages or floors, and the diameter of the tower appears to have diminished three feet at each stage, so as to form so many external galleries of a foot and a half in width, going all round the tower. On the top of the tower lights were placed, so that it served as a lighthouse to vessels navigating the Channel. The tower was built in a manner somewhat similar to that of the Palais des Thermes, a Roman edifice at Paris. It is built with iron grey-stone, three tiers together, succeeded by a double tier of a yellow stone of a softer texture, and on this a double tier of very hard and red bricks. At the time of its erection it stood more than a bowshot from the sea, but the cliff was so much excavated by the waves, and fell in so far, that the tower was at last undermined and overthrown in the year 1644. It had been repaired by Charlemagne in the early part of the ninth century; and when the English were in possession of Boulogne they surrounded this tower with a wall and towers, so as to convert it into a donjon, or keep of a fortress. These walls and towers shared the fate of the original Roman work in being overthrown by the advance of the sea. The tower was named in the middle ages Turris ordans | (supposed to be a corruption of ardens, burning) or ordensis; and is still spoken of as the Tour d'Ordre. There were in the middle of the last century some ruins of the Roman walls, built of the same materials as the above-mentioned towers.

ON THE PRODUCTION OF SOUND UNDER
WATER.

EXPERIMENTS of a remarkable kind have at different
times been made on the power of water to transmit
sound, and on the comparison between it and the air
as a medium for sound. Under ordinary circum-
stances, we know but very little of the conveyance of
sound under water; our sound-producing instruments
and our auditory apparatus being equally exposed to
the open air. It would perhaps excite surprise in
many to be told that sound can not only be conveyed

by M. Colladon. One point which he wished to determine was, the duration and quality of sound in water. He found that the sound of a bell struck under water, and heard at some distance, had no resemblance to that of a bell struck in the open air. Instead of a prolonged sound, there is heard under water a short and sharp noise, which M. Colladon says he can compare to nothing better than to that of two knife-blades struck against each other; and on retiring from the bell, the sound always preserves this character, diminishing only in intensity.

M. Colladon provided a curious kind of apparatus for making these investigations. It consisted of a thin tin cylinder about eight or nine feet long, and eight inches in diameter, closed at one end and open at the other. This was plunged into the water, leaving the open end above the surface; and the ear, applied to this end, could hear any sonorous effects which might be the object of examination. With such a contrivance, M. Colladon, applying his ear to the open end of the tube, while the closed end was immersed in the water of the lake of Geneva, could hear the sound of a bell struck under water, when the bell was so far distant as two thousand, six thousand, and in one instance, fourteen thousand metres (about nine miles). This latter distance was across the whole breadth of the lake, from Rolle to Thonon. The spot was particularly well calculated for such an experiment, the water being very deep, without a trace of any current, and of the most transparent purity. The signals were made by the inflammation of gunpowder, which being performed by the same blow of the hammer by which the bell was struck, all loss of time was effectually avoided. The lapse of time, in those experiments whose object was to determine the velocity of sound in water, was reckoned by a quarter-second stop-watch, and was computed from the appearance of the flash to the arrival of the sound.

M. Colladon found that the power of hearing sounds produced in the water, when the head of the listener was out of the water, and no tube employed, depended greatly on whether he was nearly over the spot where the bell was placed. At a distance of two hundred

metres he heard the bell very distinctly, while at four or five hundred metres distance he could not hear the slightest sound, even when the car was almost close to the water. When, on the contrary, the head was immersed for a few seconds beneath the water, or the hearing-tube was employed, the sound could be heard distinctly at from ten to twenty times this distance. The employment of the tube had a remarkable effect in bringing the sound to the ear of the experimenter. M. Colladon remarks:-" The agitation produced by the waves does not alter the duration nor the velocity of sound, when a tube is used for hearing. The last of the three experiments mentioned above (i. e. two thousand, six thousand, and fourteen thousand metres) was made in storiny weather. The wind, which at first was weak, increased to such a degree, that several anchors were necessary to hold the vessel. Notwithstanding the noise of the waves, I could still distinguish pretty well the sound of each stroke, and the duration of its transmission was not altered."

To ascertain the effect of screens or obstacles on the intensity of the sound, M. Colladon chose two stations, at no great distance apart, and so situated that the straight line which joined them grazed the extremity of a thick wall which rose above the level of the water. He then caused a bell to be struck regularly, in the water, with strokes of equal intensity; and on listening to the sound with the tube alternately on either side of the line which grazed the extremity of the wall, he found that there was a marked difference in intensity, according as this extremity was or was not interposed between the bell and the tube-the screen sensibly diminishing the intensity of the sound.

Several years afterwards, viz. in 1837, Professor Bonnycastle, of the United States, performed some experiments, at the instance of the American Government, in furtherance of the inquiry into the transmission of sound in water. The American Government placed at his disposal the brig Washington,' in which he prosecuted his inquiries. He provided a small petard (a species of small cannon), about five inches long by two and a half in diameter, with adjustments suitable for discharging it under water. As a sound-receiver, he provided a tube of tinned iron, eight feet long by an inch and a quarter in diameter, terminated at one extremity by a trumpet-shaped mouth twenty inches in diameter. He also had a cylindrical tube, similar to that employed by Colladon, closed at one end, and capable of being immersed to half its length in the water. He provided likewise a very delicate chronometer or time-measurer, capable of measuring fractional parts of a second of time. The ship's bell was removed from its place, and adjusted so as to be rung under water.

With these instruments Mr. Bonnycastle sought to determine how far distant a sound could be heard, when produced under water, and listened to with the aid of either of the two tubes. He found that the trumpet-shaped tube, being open at both ends, admitted water into its interior, which effectually interfered with the success of the experiments. With the cylindrical tube, he heard the sound of the bell at a distance of a quarter of a mile, but at the distance of a mile the sound was wholly inaudible, thus presenting a marked contrast to the results obtained by M. Colladon; a contrast due, probably, to the existence of a current in the one case, but not in the other. He then modified the trumpet-shaped instrument, so that the mouth should be at right angles with the stem, and thus directed towards the bell; and he also covered the mouth with thin metallic plate. These alterations being made, he found that the trumpet-tube conveyed the sound much more distinctly than the cylindrical, the difference being more and more marked as

| the distance was increased. The results, however, were not on the whole so satisfactory as those of M. Colladon.

Mr. Bonnycastle then entered upon the experiments which were the main objects of his attention, and for which the American Government had thought fitting to assist him. These were, to determine whether the depth of the sea could be found by the echo of a sound from its sandy bottom. It is known that in the open air the interval which elapses between the production of a sound and the return of its echo depends exactly on the distance of the echoing surface, and these quantities have been determined with very great exactness: thus, if a sound is echoed from a wall, and returns to the sound-producing instrument exactly one second after it was produced, then the wall is known to be about five hundred and sixty-five feet distant. It was an analogous mode of calculation which Mr. Bonnycastle sought to obtain in the sea. The ship was moored at a considerable distance from the land; the hearing tube was placed vertically in the water; the petard was lowered; and the observers prepared themselves to listen for the echo. When the petard was fired, two distinct blows were heard, at an interval of about one-third of a second apart; the two shocks were also heard at the ship, and at the same interval apart. If the one was the echo of the other, then the echoing surface must have been about one hundred and sixty fathoms distant; whereas on sounding, the bottom was found at five hundred and fifty fathoms. On the following day the experiment was repeated very close to the shore, when the interval of one-third of a second was still perceived between the shocks: this showed that the second could not have been an echo of the first from the bottom of the sea; and Mr. Bonnycastle considers that he has failed in his object, at least so far as present modes of experiment are concerned.

Still more recently, M. Colladon has stated that he has renewed his experiments, with a view to follow out the attempts made by Mr. Bonnycastle. In a letter to M. Arago, a year or two ago, he gives several new results which he had obtained by his apparatus, which led him strongly to think that a useful mode of maintaining correspondence by submarine transmission of sound may one day come into use. On one occasion, M. Colladon had placed at his disposal a bell belonging to one of the churches at Geneva, weighing five hundred kilogrammes (eleven hundred pounds). This bell was suspended to an apparatus placed on a vessel, by means of which it was easy to sink the bell in the water and draw it up again. It was sunk to the depth of three metres (about ten feet), in a place where the water was about fifteen metres deep; and to strike the bell he used a hammer weighing ten kilogrammes, fixed to a long iron handle, the upper part of which was above the water and was bent at right angles. With this apparatus he made many experi ments, and found that he could hear the sound of the bell under water distinctly at a distance of thirty-five thousand metres (considerably above twenty miles). M. Colladon states that the noise of a chain moving under water is so distinctly perceptible, that it may be known when a vessel, three thousand or four thousand metres distant, raises her anchor; and he hints that this may be found advantageous in time of war.

Should the transmission of sound under water be hereafter applied to a useful purpose, it will be owing mainly to the circumstance that the intensity of sound dies away less rapidly in water than on land. The possibility of applying this method to the determination of the depth of the sea, seems to be a problem yet to be solved; for the experiments hitherto made have not afforded satisfactory results.

THE GLACIERS OF THE ALPS.

WITHIN the last few years the subject of Glaciers has engaged the attention of scientific men to a very marked degree. The principal appearances presented by these vast masses of ice among the Alps, as well as certain facts concerning their movements and the effects which they produce, have been long familiar to scientific travellers in Switzerland. But M. Agassiz of Neuchâtel having broached a theory of a very bold and original kind, to account for their formation, a new zest has been given to the subject, and expeditions are now made every summer from all the countries in Europe to the Alps, by persons desirous of testing the new views by actual observation. How some of these tourists fare while on these expeditions, we briefly noticed in a recent number (747). We shall now endeavour, without hazarding any opinion whatever as to the soundness or unsoundness of M. Agassiz's theory, to state the broad features of the subject so far as we can in a popular form.

Saussure, one of the most successful of Alpine travellers, gives an imaginary bird's-eye view of part of that range as a means of showing the nature and position of the glaciers. He says that, if a spectator could be imagined at such a height as to embrace within his view a large group of the Alps, he would see a mass of mountains intersected by numerous valleys, and composed of several parallel chains, the highest in the middle, and the others decreasing gradually as they recede. The central and highest chain would appear to him bristled with craggy rocks, covered throughout the year with snow and ice in all those places that are not absolutely vertical; but on both sides of the chain he would see deep and verdant valleys, well watered and covered with villages. When he looked more in detail, he would see that the central range is composed of lofty peaks and smaller chains, covered with snow on their tops, but having all their slopes that are not very much inclined covered with ice, while the intervals between them form elevated valleys filled with immense masses of ice, extending down into the deep and inhabited valleys which border on the great chain. The chain nearest to the centre would present to the observer similar appearances, but on a more limited scale, beyond which he would see very little more snow or ice.

The masses of ice here alluded to are the glaciers. They occupy two different positions: in one case they are on the sloping sides of lofty mountains; and in the other they occupy the depressions of elevated valleys. Of these glaciers there have been reckoned about four hundred between Mont Blanc and the Tyrol; and they vary in size from three to fifteen miles in length, from one to three miles in breadth, and from one hundred to six hundred feet in depth or thickness. The surface of these glaciers is very unequal. Sometimes, when the ground on which they lie is but slightly inclined, the surface of the glaciers, though rough and granulated, is tolerably even, presenting but few crevices; but if the bed be inclined so much as thirty or forty degrees, the ice breaks into fragments, and these fragments get displaced and heaped together in the most fantastic form, having among and between them chasms of a hundred feet or more in depth. In some instances the surface of the glacier is purely white; but this only occurs in the upper valleys, where few rocky fragments can fall into it. In the lower valleys, and on the gently sloping sides of mountains, the glacier is generally covered with large blocks of stone, or with mud and sand resulting from the abrasion of those blocks. The overlying stones give rise to very fantastic appearances.

During all parts of the year in a greater or less de

gree, but especially in summer, there are torrents of water flowing out from beneath the glaciers, occasioned by the partial melting of the ice, either by solar heat or by the internal heat of the earth. These streams give origin to the Rhine, the Rhône, the Danube, the Po, and many other important rivers; and in their progress through the body of the ice, they scoop out large and lofty caverns, which often present very remarkable and picturesque appearances.<

The glaciers descend slowly a little every year, varying in distance according to the declivity of the ground and the warmth of the season. The ice appears to adhere pretty closely to the sides and bottoms of the valleys during winter; but when the warmth of summer heats the soil all around, and thaws the ice at its surface and edges, the liberation of the glacier ensues, aided by the action of the currents flowing beneath, and by the friction of masses of ice and of stone. It often happens that the vast field of ice slips down very slowly till it comes quite close to the green cultivated patches of ground attached to the cottages of the peasants. In the valley of Chamouni, Ebel found that the glaciers advance about fourteen feet in a year; in that of Grindelwald the glaciers move rather faster, being at the rate of twenty-five feet in the year. Besides this descent, it is found that the glaciers are subject to other minute changes. If the glaciers are observed for a few years in succession, it is found that they recede occasionally in position, so as to keep a kind of balance in position for a long period.

One of the most remarkable points connected with the glaciers is the existence of ranges of stones in certain definite positions with respect to their length. Along the edges of some of the glaciers, where they spring from or adjoin the rocky soil, are masses of stones accumulated in the form of long parapets, walls, or dykes, to which the name of moraines is applied. Some glaciers have a moraine on each side, some have a moraine on one side only, while others are without them. These moraines sometimes attain a height of more than a hundred feet. Not only in the glaciers themselves, but in various other parts of the high mountain-valleys these moraines, or vast walls of loose stones, are found. Besides the moraines at the margins, there are long and high ridges formed of fragments of rocks, boulders, sand, and earth, on the middle of the glaciers, and at a considerable distance from the margins, but parallel to them. In some cases these ridges are thirty or forty feet in height, and several of them occur on one glacier.

These being some of the chief features presented by the Alpine glaciers, we may now notice the customary mode of explaining them, previous to the publication of M. Agassiz's opinions.

On many of the Alpine elevations snow falls for the greater part of the year. This snow accumulates in immense masses, which are precipitated in the form of avalanches from the ridges into the upper valleys. By spring-time these masses have become heaped up into an enormous aggregate; and during the summer the heat of the sun melts a good deal of the snow, and produces streams and torrents which form the sources of considerable rivers; but as the mass is more than can become wholly melted, the remainder is frozen into the icy field which we call a glacier. The nature of this ice is very different from that of the compact and transparent ice of ponds and lakes; for the rains which occasionally fall, and the water resulting from the partial melting of the snow during the summer, percolate the mass, and, while confined partially within it, become frozen in the ensuing winter. The water, in filtering through the mass, being unable to expel all the air lodged in the interstices, this air. together with that which is freed during the subsequent

coming warmer and the ice receding farther towards the pole.

The descent of the glaciers has been thus explained: -During the winter, when the half-snowy, half-icy glacier becomes hardened and fixed to the ground, fresh accumulations of snow are formed at its upper extremity, derived from all the mountain-peaks in the vicinity; and this mass, which increases enormously by the spring, pressing on the upper part of the glacier, forces it irresistibly downwards into the valleys of the Alps, and that this ice, having upon it, and leys. Sometimes this descent, though slow, is so forcible, that the glacier has been carried not only into a valley, but quite across it, and has even ascended some distance on the opposite side.

In Switzerland, at a height of nine thousand feet among the Alps, there is a kind of boundary or limit, below which there are repeated instances of moraines or ridges of loose stones, grooved and scratched rocks, and polished rocks; whereas above this boundary the peaks do not exhibit these appearances. M. Agassiz hence concludes that this height marked the upper level of the ocean of ice which once filled all the valbeside it, and beneath it, fragments of rock, and melting as the hemisphere became warmer, furrowed, scratched, and polished the surfaces of the rocks which it met with in the descent, the ice itself sometimes producing the mechanical effect, and at other times the stones which it bore along with it. According to the nature of the rock which composed the valley and the flanks of the mountain, so would it be acted on more or less by this kind of friction.

The formation of the moraines is thus explained :— When the rocks bordering the glaciers are themselves bare of snow or ice, in consequence of the rapidity of the slope, and are stratified, they are easily disintegrated by the alternate action of wet and frost, heat and cold, and the fragments thus detached roll down There are immense blocks of stone on and among to the side edges of the glacier, where the greater part the Jura mountains of Savoy, placed at such an elevaare stopped, while some isolated blocks are urged tion as has puzzled geologists to explain how they got further towards the middle. The general inclination there. M. Agassiz assumes that when the whole of the glacier and its downward motion are the means Alpine district, except the higher peaks, was enveloped of collecting a quantity of these fragments at the in ice, fragments of rock became broken off from lower edge of the glacier, so that in some cases the these peaks, and falling upon the ice, were by it transwhole glacier is surrounded by a moraine. The ported, in proportion as it melted or gave way, to conparallel ridges of stones on the glacier itself have been siderable distances, where they obtained lodgment on accounted for thus:-The glacier, slipping down gra- solid ground in various positions. There are in Scotdually upon the inclined bed of the valley, recedes from land some curious parallel terraces on either side of the sides, carrying part of the lateral moraine along two or three glens near the Caledonian Canal, the with and upon it. This retreat always leaves a conterraces being strictly on a level, and following the siderable space, particularly in the wider valleys, be-windings of the glen with great uniformity. These tween the foot of the mountains and the edge of the terraces have obtained the name of the "parallel roads glaciers, which space during the succeeding winter of Glenroy." Some have thought that in early times becomes filled up with fresh snow, which becomes these were roads artificially formed; in later times it again converted into ice, and on which a new moraine has been supposed that they are the sedimentary deis collected. This recedes like the first, and so on, posits on the banks of what were once lakes; but the whereby the surface of the glacier becomes covered with parallel ridges of stones.

glacial theory" of M. Agassiz has recently been brought to bear upon them, and it is supposed that these valleys were once filled with ice, the parallel roads being consequences of the descent of the glaciers at a later period, when the ice was about to dis

appear.

M. Agassiz, as a means of explaining these and a great many other phenomena observable in mountain valleys, directs attention back to a remote period when, as he supposes, a large part of what is now Europe was one sheet of ice; and he then conjectures that the To follow out the details of this remarkable theory present Alpine glaciers are merely the remains of is not our object: but as the "glacial theory" is now that ice. In many parts of Europe there are rocks becoming a matter of prominent interest in scientific exhibiting singular furrows in their surfaces in a works, and as men of science have to a certain extent parallel direction, and other rocks whose surfaces have divided themselves into 'glacialists' and 'non-giabeen polished by some kind of friction. No circum-cialists,' according as they do or do not agree in stances at present observable seem to afford an explanation of these effects, and therefore some writers have referred them to some sort of current acting at a former period; but M. Agassiz thinks that, whether occurring in the Alps, in France, in Scotland, or in Sweden (for they have been observed in all these places), these furrows and abrasions have been occasioned by the movement of ice at some remote period.

M. Agassiz assumes, as the basis of his views, that at one time the polar ice extended as far towards the equator in the north as it now does in the south hemisphere; and thinks that all the effects connected with glaciers, &c. may be deduced from such a state of things. There is a belt of stones running across the centre of Russia at about 50° lat.; and many persons have supposed that these must have been brought there by a current or flood of some kind from the north. But M. Agassiz thinks they once marked the southern margin of an immense glacier or sea of ice, extending thence northward. There is another belt of stones farther north; and these, he thinks, formed the glacierlimit at a later period, when the hemisphere was be

opinion with M. Agassiz, we have thought it right to give a slight idea of what the term means, and what venient, then, for those who may meet with discusis the subject under consideration. It will be consions on the subject, to bear in mind that the "glacial theory" supposes a time to have existed when many of the countries of Europe were enveloped in ice nearly to the tops of the highest mountains; that this ice melted as the northern hemisphere gradually became warmer; that fragments of rock became transported by the ice to great distances; that the ice and the fragments furrowed, scratched, or smoothed the rocks over which they passed; that all the lower valleys and plains have become so warm that the ice has disappeared from them; that the higher valleys and the sides of mountains in the Alps still exhibit remnants of this ice in the form of glaciers; and that the boulders and other masses of stone observable in particular situations have been brought thither while ice was yet in or near those parts.