is no trigger, and therefore it is impossible that the gun can go off by accident. The needle which ignites the explosive compound in front of the powder, passes into the screw which forms the breech, being kept back in its normal position by a spiral spring: it abuts against a thin elastic plate that closes the aperture in the screw, and does not project beyond the general surface. When the gun is to be discharged, the thumb of the right hand is pressed on the elastic plate. This forces in the needle, and ignites the detonating compound.

IMPROVEMENTS IN GALVANIC BATTERIES.-If the nitric acid in the Bunsen battery is replaced by an aqueous solution of picric acid, the evolution of disagreeable and unwholesome gases will be prevented, while the efficiency of the battery will not be injuriously impaired. The dilute sulphuric acid may be replaced by a solution of sea salt. The addition of picric acid also to a battery containing but one fluid greatly improves its action. The resistance to the current caused by the porous vessel of a Daniels battery is removed by a slight modification of its details. Within the outer vessel, which may be made of glass or porcelain, is placed a cylinder of copper much smaller than the outer vessel, but having attached to its lower end a disc of copper that just fits on the bottom of the outer vessel. Between the latter and the copper cylinder is the diaphragm, a cylinder of glass or ordinary porcelain, having on the outside, at the distance of one-third of its height from its lower extremity, small projections for supporting a cylinder of zinc. This battery is charged by placing siliceous sand in the interior of the diaphragm, and on this sand crystals of sulphate of copper; then pouring a solution containing about five per cent. sulphuric acid gradually into the outer vessel, until it reaches the crystals of sulphate. The electricity passes directly from the zinc to the copper disc, without being retarded by passing through a porous vessel. If the exterior vessel is glass, any deposit of copper on the zinc can be perceived at once and prevented. The greater the number of times per day the battery is to be used, the more permeable ought to be the sand, that the sulphate may be supplied with sufficient rapidity. The stratum of dissolved sulphate must never be allowed to rise high enough to come in contact with the zine; if it is becoming too high, sand is to be added, or some of the liquid within the diaphragm is to be removed with a syphon-which will cause the sulphate to be driven back, on account of the greater height of the liquid in the external vessel.

IMPROVEMENTS IN PLATING AND GILDING.-The danger to the workman from contact with mercury is entirely prevented in gilding and plating, by dipping the article to be gilt or plated in the solution of a basic salt of mercury while in connection with the positive pole of a galvanic battery, and, when it is covered with mercury, immersing it in a strong solution of the gold or silver salt; then plunging it a second time in the mercurial solution, and afterwards evaporating the mercury in a furnace. Or the article which is to be gilt may be dipped into sodium amalgam, the surface of which has been covered with a little water, the portions which are not to be

gilt having been protected with varnish; after which an amalgam of gold is to be applied, and the mercury is to be evaporated by heat. Amalgam containing only the one two-hundredth part sodium will be sufficiently active to amalgamate tarnished metals, or iron and platinum, which, in ordinary circumstances, have no tendency to become wetted with mercury.

IMPROVEMENT IN HOROLOGICAL MACHINES.-It is scarcely too much to assert that the rate of a well-constructed clock or watch would be invariable, but for alterations of temperature. With an increase of temperature the pendulum is lengthened, and consequently its vibration is rendered slower; with a decrease, the contrary takes place. Analogous effects are produced by changes of temperature on watches and chronometers: and as, during summer and winter, night and day, the temperature of the air is perpetually changing, it may well be supposed that without the adoption of some means for counteracting the effects of changes of temperature, the rato of clocks, watches, and chronometers would be subject to perpetual alteration; and such is the case with ordinary horological instruments. But ingenious means have been devised for compensating the changes produced by temperature. These means, however, are subject to two considerable drawbacks: they are more or less complicated and therefore expensive, and they are rarely as perfect in their action as might reasonably be desired. A new mode of compensation for clocks, watches, and chronometers has been invented by M. Menon, which is extremely simple, and therefore inexpensive; and very effective, because calculated exactly to counterbalance the effect of changes of temperature, by bringing into opposite actions two precisely equal forces. In the compensations hitherto in use, the expansions and contractions of different substances are made to counteract each other; and thus, from the difficulty of making them exactly equal in their operations, a serious source of error is introduced. With the gridiron pendulum, for example, the rods which raise the bob of the pendulum may not expand to the same degree as the rods of a different metal which lower it and the raising and lowering may thus not be equal-that is, the length of the pendulum may vary. With the mercurial pendulum, the centre of gravity of the mercury may not be altered in position so as exactly to counterbalance the alteration in the length of the pendulum rod produced by change of temperature. In M. Menon's contrivance, the two portions of the compensation are exactly the same in every respect, and therefore when their expansions or contractions are made to neutralize each other, the effect must be zero-that is, the length of the pendulum must remain unchanged. In the construction of a compensation pendulum on this principle, two rods of the same metal, and of the same dimensions, are used; the pendulum is attached to one, and the other is coiled up-merely for convenience, the alterations produced by change of temperature being exactly the same whether the rod is in the form of a right or curved line. The rod, in the form of a spiral, is used to suspend the bob of the pendulum, or the pendulum itself. For this purpose, one end of it is fixed to the bob, which

is easily moveable on the rod, and the other is connected with the rod or one end is fixed to a support, and the upper end of the pendulum is attached to the other the arrangement being such that when the spiral rod expands or contracts, it raises or depresses the bob on the pendulum rod, or raises or depresses the pendulum itself through the fork: the effect being a practical shortening or lengthening of the pendulum exactly to the extent that the change of temperature has lengthened or shortened the pendulum rod; the acting length of the latter, therefore, never alters. In applying this principle to the watch or chronometer, the spiral spring and the compensator are made of the very same material and dimensions: and it is so arranged that, when change of temperature alters the length of the spiral spring, the same change alters that of the compensator which acts on the "regulator;" and thus the length of the acting part of the spiral spring is kept without change.

The

IMPROVEMENT IN THE SAFETY VALVE.-Without an effective and reliable safety-valve, the use of a steam-boiler must be constantly attended with the most imminent danger, Whatever care may be bestowed by the manufacturer on the construction of the safety-valve, it may be rendered nugatory by the ignorance or temerity of the person in charge of the engine: since he may overload the valve, and thus create a pressure within the boiler which it was not constructed to endure, and which it may not be capable of bearing. The evil may indeed be prevented by the use of two safety-valves, one of which is beyond the power of the engine man. But ingenuity has devised a still more simple remedy; one that not only prevents the production of steam at too high a pressure, but which actually causes every attempt to produce it to be accompanied by a reduction of pressure, and thus removes all temptation to tamper with the valve. new form of safety-valve differs little from the ordinary kind, and is extremely simple. In the ordinary kind the fulcrum of the lever is absolutely immovable: in the new kind, it is fixed; in ordinary circumstances, being kept down by a spiral spring. But attempting to overload the valve brings the lever down on a stud, which is at the side of the valve most remote from the fulcrum, and which comes into action as a new fulcrum by supporting the lever, changing the latter from the third to the first order. The former fulcrum yields to the additional weight, the spiral spring being compressed, and is raised up, the safety-valve being at the same time opened, or allowed to open; and thus the steam is permitted to escape, though at a pressure too small to raise the valve when weighted as it should be. In its normal state the fulcrum of the lever is at one end, the weight at the other, and the power-that is the tendency of the safety-valve to rise-between the fulcrum and weight. When the valve is overloaded, the weight-that is the resistance of the spiral spring is at one end, the power-that is the weight with which the lever is loaded-is at the other, and the fulcrum-that is the stud on which the lever has been brought down by the overloading-is between the power and weight: the effect of the latter being aided by the tendency of the steam to raise the valve.

A NEW GLASS FOR OPTICAL PURPOSES.-The density imparted by

thallium, as illustrated in thallic alcohol, suggested the application of that metal to the production of a very dense, and therefore highly refractive glass; and experiments made on the subject have been extremely successful. Difficulties of a serious character were encountered at first, the glass produced being fibrous, and not very transparent; but these have been overcome by the assistance of M. Feil, a glass manufacturer of great experience, and a thallic glass in every respect suited to optical purposes has been obtained. When carbonate of thallium is employed in its manufacture, its tinge is yellowish; but if this is considered an objection, the substitution of sulphate for the carbonate affords a perfectly colourless result. Thallic glass is the most dense, and the most highly refractive and dispersive known.

MAGNETISM AND DIAMAGNETISM OF GASES AND VAPOURS. A simple and effective experiment by which the magnetism and diamagnetism of gases and vapours may be rendered visible to a large audience has long been sought for, and has at length been discovered. It consists in blowing a bubble at the end of the stem of a clay tobacco-pipe, the gas used for inflation being oxygen, and the fluid a solution of soap mixed with glycerine, which affords a bubble that lasts a considerable time. The bubble is placed above the poles of an electro-magnet, and at very small distance from them. When a current is sent through the coil of the electro-magnet the bubble is attracted, and if the circuit is completed and broken several times in succession, a very striking oscillation of the bubble will be produced. If magnesium is burned over the pole of an electro-magnet to which a conical form has been given, when the circuit is complete, the smoke will be divided laterally, and assume the form of the letter U.

LITERARY NOTICES.

THE CONSTELLATION-SEASONS: An Easy Guide to a Knowledge of the Stars. Exhibiting, in Twelve Planispheres, the Appearance of the Heavens at any Hour of the Night, all the Year round. By RICHARD A. PROCTOR, B.A., F.R.A.S. (Longmans.)-Mr. Proctor has produced one of the most useful series of maps we have seen for the purpose he had in view, namely, enabling students to learn the position of the chief stars and constellations at various periods of the year, or to recognize them at any time. "Each map," he tells us, contains the whole of the visible heavens at the hour and date mentioned beneath it; the centre of the map is the point over the observer's head at the time named; the outline of the map is the observer's horizon; each star is placed in its proper direction as respects the compass-points (marked in round the map); and each star is placed at its true proportionate distance from the centre." For the teaching of young people, and for adults who wish to know the position of particular stars at certain dates, these maps will be

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found very handy. Mr. Proctor has devoted great labour to their production, and he deserves the sincere thanks of all who wish to see the elementary facts of stellar astronomy simplified. They will be of important value to amateur telescopists.

A HANDY BOOK TO THE COLLECTION AND PREPARATION OF FRESH WATER AND MARINE ALGE, DIATOMS, DESMIDS, FUNGI, LICHENS, AND MOSSES, AND OTHER OF THE LOWER CRYPTOGAMIA, WITH INSTRUCTIONS FOR THE FORMATION OF AN HERBARIUM. By JOHANN NAVE. Translated and Edited by the Rev. W. W. SPICER, M.A., F.R.M.S. (Hardwicke.)-This is really what it calls itself, a "Handy Book," with twenty-six plates, and elaborate directions for collecting and preserving the objects to which it alludes. It will be found of much use to beginners. The information is conveyed in a very clear and simple manner.

A TREATISE ON PUNCTUATION, AND ON OTHER MATTERS RELATING TO CORRECT WRITING AND PRINTING. By an OLD PRINTER. (Pitman.) -Good punctuation is an immense aid to the intelligibility of a book, and many works of merit are quite puzzling to read for the first time, because their punctuation has not been made upon a good system. The little book before us is well conceived and judiciously executed. We recommend it to all who desire to learn a simple but necessary art.

NOTES AND MEMORANDA.

NEPTUNE'S GROTTO.-Under this title some ingenious person has brought out a pretty chemical toy, which we saw in operation at Messrs. Horne and Thornthwaite's. A white solid is put into a clear solution, and thereupon beautiful arborescent and other forms are seen to grow in the course of a few minutes. If the experiment is performed on a small scale in a little zoophyte trough, under the microscope, and viewed by reflected light, with a 14-inch objective, the effect is singularly beautiful. A chemist will easily recognize the materials employed. They give rise to a double decomposition, and the precipitation in crystalline form of an insoluble salt of lead.

VOLCANIC ACTION IN THE AZORES.-M. St. Claire Deville read to the French Academy a letter, dated Angra, June 7, 1867, published in the journal "A Persuaso," at St. Michel, in the Azores, stating that, from May 26, they had experienced strong earthquakes, and that in the night of June 1-2, a volcanic vent was formed nine miles north-west of Serreta, which continued active, and occupied a zone of more than 24 miles in a westerly direction. The writer states, "It was in the sea, lat. 38° 52', and long. west of Greenwich 27° 52', in a line from Tercera to Gracioza. It constantly emits great stones and enormous masses of lava, the accumulation of which may produce a new islet, likely to be dangerous. Jets of vapour and boiling water appear in different positions, and for a considerable distance a strong odour of sulphur is noticeable. From time to time noises are heard like repeated discharges of artillery. The Intendant of the Marine, various civil and military engineers, and others, have gone off to survey these phenomena, but the danger has kept them at a considerable distance.

MEDICINES THROUGH THE NOSE.-M. Raimbert recounts to the French Academy various experiments in administering medicines in the form of a snuff, to act by absorption through the membrane of the nose. Sugar powdered with hydrochlorate of morphia, and taken in this way, he found useful in violent