plate of glass, which, when used, is to be kept under the surface of the water.

A very convenient size for the instrument represented in the above figure, is to make the length AC, 3 feet, and the mouth A, where the face is applied, of an irregular oval form, that both eyes may see freely into the tube, with an indentation on one side, that the nose may breathe freely, not throwing the moisture of the breath into the tube. B is a round plate of glass, 8 inches diameter, over which is the rim or edge C; this rim is best formed of lead, of an inch thick, and 3 inches deep; the weight of the lead serves to sink the tube a little into the water. Holes must be provided at the junction of B to C, for the purpose of allowing the air to escape, and bring the water into contact with the glass; on each side there is a handle for holding the instrument. This size and form is very much that of the instrument brought from Norway by John Mitchell, Esq., Belgian Consul, of Mayville, with the improvement for excluding the breath, and allowing the water to get into contact with the glass, which was not provided for in that instrument.

The reason why we so seldom see the bottom of the sea, or of a pure lake, where the depth is not beyond the powers of natural vision, is not that the rays of light reflected from the objects at the bottom are so feeble as to be imperceptible

to our sense, from their passage through the denser medium. of the water, but from the irregular refractions given to the rays in passing out of the water into the air, caused by the constant ripple or motion of the surface of the water, where that refraction takes place. Reflections of light from the surface also add to the difficulty; and before we can with any just hope expect to see the objects distinctly at the bottom, these obstructions must be removed.

This is done to a very great extent by the use of the instrument which forms the subject of this notice; the tube serves to screen the eyes from reflections, and the water being in contact with the glass plate, all ripple is got rid of, so that the spectator, looking down the tube, sees all objects at the bottom, whose reflective powers are able to send off rays of sufficient intensity to be impressed on the retina, after suffering the loss of light caused by the absorbing power of the water, which obeys certain fixed laws, proportionate to the depth of water passed through; for as light passing through pure sea-water loses half its intensity for each 15 feet through which it passes,* we must, from this cause alone, at a certain depth lose sight of objects of the brightest lustre. The perfect purity of the water, and its freedom from all muddy particles floating in it, form an important element in the effective use of the water-telescope; for example, in the Frith of Forth, and similar estuaries, where the influx and reflux of the tide keep particles of mud in constant motion, the instrument is of little or no use; for these act in exactly the same way in limiting our vision through water, as a fog does through the air it is therefore only in the pure waters of our northern and western shores that this contrivance is applied with any advantage; and in such situations we can speak of its powers with confidence, In a trial made with the instrument last autumn on the west coast of Scotland, the bottom was distinctly seen (a white bottom) at a depth of 12 fathoms; and on a black, rocky bottom, at 5 fathoms under water, objects were so distinctly seen that the parts of a wreck were taken up-the

:

Leslie's Elements of Nat. Phil., p. 19.

exact place of which was not known previous to its use. In these experiments a lenticular form of glass was made use of at the bottom of the tube, having a plane surface to the water, but no great or marked advantage was observable from this construction. With respect to the history of this contrivance for viewing the bottom of the sea, we are unable to assign any particular date: so far as our information goes, it has been in use from a very remote period. We are informed that it is in general use in seal-shooting along our northern and western islands, where, sometimes in the form of an ordinary washing-tub, with a piece of glass fixed in its bottom, the shot-seal was looked for, and the grappling hook let down to bring him to the surface. It may not be generally known, that in seal-shooting, the shot or wounded seal always seeks the bottom, from which he never rises after death, till washed ashore by the action of the sea: it is only when the fatal ball deprives him of the power of diving that he is ever found at the surface. In such employments, therefore, the use of this instrument, however modified, must form an important auxiliary to the best rifle. Throwing oil over the surface of the water is used in the same pursuits; but this only so far stills the ripple, leaving the reflections. Our eminent engineer, Mr Robert Stevenson, made use of the water-telescope more than 30 years ago, in works connected with harbour improvement in the north of Scotland; it has also been used to examine the sand-banks, &c., at the bottom of the River Tay, but in this case the mud prevented its use in any considerable depth of water. To obviate this difficulty, the construction was modified thus: by making the tube of considerable length, and placing the glass at the lower end, this tube was thrust through the water till within a few feet of the bottom, acting as a cofferdam to set aside the dirty water, and enable the bottom to be seen; but in this method of application it was found very difficult to hold the tube down in the water from its buoyant power, and we are informed by Mr Thomas Stevenson, C.E., that, he understood from this cause its use had been discontinued. He suggested a simple remedy; viz., to fill up the empty tube with pure water. We are indebted to Mr Mitchell, the gentle

man already mentioned, for having brought this instrument into notice in the public prints, under the name of Norwegian water-telescope, on the shores of which country it is stated to be much used in fishing-in particular, that of the herring; but the herring-fishers on the east coast of Scotland inform us, that they require no such auxiliary, as, from the surrounding elevated grounds, they can tell the position of the shoal, and, from their motions seen from such situations, they know where they are to be found when they go out a-fishing.*

*Norwegian Water-Telescope.

The water-telescope is thus noticed in a very promising periodical, the American Annual of Scientific Discovery, just published, of which a copy reached us a few days ago.-ED. Phil. Journal.

The water-telescope is an instrument which the people of Norway have found of so great utility, that there is scarcely a single fishing-boat without one of three or four feet in length, which they carry in their boats with them when they go a-fishing. When they reach the fishing-grounds, they immerse one end of this telescope in the water, and look through the glass, which shews objects some ten or fifteen fathoms deep as distinctly as if they were within a foot of the surface. When a shoal of fish comes into their bays, the Norwegians instantly prepare their nets, man their boats, and go out in pursuit. The first process is minutely to survey the ground with their glasses, and where they find the fish swarming about in great numbers, they give the signal, and surround the fish with their large draught-nets, and often catch them in hundreds at a time. Without these telescopes their business would often prove precarious and unprofitable; as the fish, by these glasses, are as distinctly seen in the deep, clear sea of Norway, as gold-fish in a crystal jar. This instrument is not only used by the fishermen, but is also found aboard the navy and coasting-vessels of Norway. When their anchors get into foul ground, or their cables warped on a roadstead, they immediately apply the glass, and, guided by it, take steps to put all to rights, which they could not do so well without the aid of the rude and simple instrument, which the meanest fisherman can make up with his own hands, without the aid of a craftsman. This instrument has been lately adopted by the Scotch fishermen on the Tay, and, by its assistance, they have been enabled to discover stones, holes, and uneven ground, over which their nets travel, and have found the telescope answer to admiration, the minutest object in twelve feet of water being as clearly seen as on the surface. We see no reason why it could not be used with advantage in the rivers and bays of the United States.

Experimental Investigations to Discover the Cause of the Change which takes place in the Standard Points of Thermometers. By JOHN ADIE, F.R.S.E., F.R.S.S.A. Communicated by the Author.

It has long been known to experimentalists that, in thermometers constructed with the greatest care, a change takes place after a lapse of time in the standard points, as given by the melting of ice and boiling of water under a fixed pressure; on this account it has been recommended by most writers, where the employment of thermometers is treated of, that they should from time to time be compared one with another, and also at the freezing point. This change is a rising of the mercury in the tube, so that, after a length of time, the mercury will not sink to the point laid off in the construction of the instrument. To investigate to what cause this change was due, formed the object of my experiments: Was it a change in the glass of which the bulbs are formed, or in the mercury with which they are filled? I was aware that thermometers filled with alcohol were not subject to this change, which would lead to the inference, that the change was in the mercury and not the glass; but then, in the spirit-thermometer, air is left above the column of spirit, whereas, in those constructed with mercury, the air is expelled, and there is a vacuum above the column; consequently, the bulb is pressed together with the force of an atmosphere on all sides; might not this force, acting for a length of time, cause some small alteration in the arrangement of the particles forming the glass of the bulb ?

This is the explanation accepted by most of the Italian and French writers on this subject. Some suppose that the mercury may contain air and moisture within its particles; but such a hypothesis I think inadmissible, as in the case of a vacuum over the mercury, these particles would seek the void, and cause rather a depression than a rising of the freezing point. Mr Daniell, in his Essay on Climate, adopts the same view; and Sir John Herschel, in his article "Heat," in the Encyclopædia Metropolitana, says: "The freezing point upon the mercurial thermometer has been supposed to undergo