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a good working electric current-to have a perfectly insulated line of wire, which, by their united action, would supplant the letter-carrier and take the place of the Semaphore, equally regardless of tide, storm, or hazy weather. The labours of Bright, Walker, and Wheatstone have resulted in a degree of excellence which can scarcely be surpassed; one has only to enter the nearest telegraph-office and have his message transmitted to the most distant county of England in a few moments, in order to be apprised of this fact. Further economy in the working, and still greater celerity, appear, however, to be the aim of the telegraph engineer. In one respect these endeavours must command our liveliest sympathy. The most simple system of train-signalling will assuredly be found to be the best not only in the working but in the less liability to get out of repair. Tyer exhibits an instrument which can be worked by the most unskilful hands, and by means of a gong, a bell, a red indicator and a black indicator, gives all the signals having reference to the safety of the train, and cannot by any possibility be mistaken. One condition necessary to be observed, and which, if strictly adhered to, would prevent numerous accidents, is that "no signal is to be considered complete until the reply has been received." Allan exhibits his Automaton Telegraph, in which several novelties are introduced. In the first place, any person sending a message intended to be confidential, and having the cipher in his possession, punches the dots separately on a ribbon of paper at the signalling-office, and this paper being passed over the tooth of a small rotating spur wheel, electrical contact ensues at the different punctures, which is duly answered by a similar puncture on a similar ribbon of paper at the receiving office. By this method it will be seen. that the signalling or recording clerk is no longer necessary,the slips of paper are put into the machine at one office and are instantaneously reproduced on the paper at the other extremity. The punched hieroglyphics are delivered to the person for whom they are intended, and the telegraph clerk is completely ignorant of the purport of the message. Both the positive and negative electric currents are used for marking, thus causing a saving of 30 per cent. in the number of currents sent through the wires. In Allan's composing-machine the Morse system of dots and dashes is altogether rejected, and the system of dots alone made use of-the number of dots forming any letter being punched on the ribbon at a single operation by merely touching one of the thirty keys of an instrument arranged like a piano. One of the particular advantages which the system possesses is that the ribbon of paper at the sending end may be passed through the apparatus several times, always reproducing a similar one at the receiving extremity, and may thus be a

source of economy where numerous copies of the same despatch are required to be delivered at the same time-as, for example, at various newspaper offices. The various letters are, however, sent as rapidly by Henley's more simple machine, which prints by dots and dashes. Probably, Hughes's American printing telegraph, in which the letters are printed in Roman type at once, will eventually come into general use.

Of all the numerous trophies in the nave of the Exhibition, the holophotal light-house revolving apparatus of Messrs. Chance is the most dazzling, and one regrets that there is not an opportunity of witnessing the flashes of light which it would send forth were the building in a state of darkness, and the great lamp at its centre duly burning. How different from the first attempts at light-houses-where a fire of flaming coal or wood warned the mariner of his approaching danger, or even from the feeble light of a few candles, without either metalreflector or "bull's-eye," which for half a century surmounted Smeaton's noble structure. Each of those curved and annular prisms and lenses are ground and polished with mathematical exactness in order to reflect and refract, and so economize every ray of light proceeding from the four concentric-lighted wicks, and send them far and wide round the horizon. With one of those massive glass cages the light may be seen at a distance of thirty miles. The dioptric light-house apparatus, although often thought of, has only been successfully realized within comparatively few years, and that mainly through the exertions of Fresnel. The combination of metal reflectors and lenses proposed by that eminent philosopher, who busied himself as much with the humble improvement of the lamp itself as with the deeper researches on the form and arrangement of the lenses, is, however, here much simplified, and glass is made to perform the whole duty. The perfection to which the manufacture of this material has been carried, and the improved plans introduced by Mr. Stephenson in the arrangement of the prisms by which the divergent rays of the flame are gathered up and distributed in one uniform horizontal beam, have fully proved the excellence of the change. The lantern revolves on its axis by means of clockwork machinery, which keeps an uniform rate of motion, and by the number of flashes emitted the sailor is able to tell the name of the particular lighthouse to which he is near. Among other articles exhibited by this firm, we might point attention to the discs of glass of twenty-five inches in diameter, prepared for optical purposes, and from which it is to be hoped that, if perfect, they may speedily be worked up into the object-glass of a large refractor.

When magnetic observatories were first established, it was

THE EXHIBITION OF 1862.

necessary for an observer to be constantly in attendance, in order to register the different fluctuations of the magnets. Mr. Brook hit upon the happy idea of applying the photographic process to this end, and at the last Exhibition his apparatus for this purpose was in full work. Similar machinery, as arranged by the late Mr. Welsh, and now in use at the Kew Observatory, is here exhibited, and it will be seen with what simplicity the light coming from one fixed, and another hemispherical mirror, attached to the magnets, impresses two images on a waxed paper photographically prepared-one of which gives a fixed line, and the other, of course, moves according to the oscillations of the magnets, the distance between the two showing the amount of the latter. This principle has also been applied to registering the fluctuations of the barometer and thermometer, and is preferable to any other, -in the former case, being much more safe and simple than the numerous self-registering mechanical apparatus here exhibited, for showing the height of the mercury during the twentyfour hours, in all of which friction takes place more or less.

The ozonometer of Dr. Lankester is an ingenious adaptation of the same self-recording principle, only that, in this case, it is the life-giving properties of the atmosphere (and not solar or artificial light) which leave traces on the prepared paper. The effect of this element is shown in a large diagram, drawn up by the same authority, representing the various tints of slips of paper exposed during different states of the weather; and to the meteorologist this is as useful, and should be as strictly attended to, as the weight, temperature, or humidity of the atmosphere. To those who have used the ordinary maximum thermometer, with an index floating on the mercury, which is only too liable to get out of order, the self-registering maximum thermometer of Professor Phillips, consisting of a broken column of mercury, will be a great boon. The mercurial minimum thermometer, of Casella, is another great improvement;

in this case advantage is taken
of the adhesive property of mer-
cury for glass in vacuo, and also
of the fact, that when two tubes
are joined to one bulb, the mer-

d

cury will rise in the larger by expansion, and fall in the smaller by contraction. He therefore makes a branch, d, to the ordinary thermometer, having a flat diaphragm at b, the inlet to which is larger than the bore of the long tube, c. The mercury in cooling withdraws the fluid in the indicating stem only, whilst on expanding, it flows into the passage where it finds the least resistance; in other words, the mercury in the long tube cannot

rise, but will fall. Mr. Johnson shows a novel instrument for registering deep-sea temperatures, where advantage is taken of the different degrees of the expansion of metals. Riveting two thin pieces of brass and steel together, he has contrived means to register the effects of the small curvature which the combined metals must undergo by a change of temperature. By this method the pressure of the sea on the bulbs of self-registering thermometers is got rid of, which is quite destructive of delicacy at immense depths. The same gentleman shows his instrument for recording deep-sea soundings, in which the fact that water is slightly compressible is ingeniously applied.

MM. Negretti and Zambra exhibit a deep-sea sounding thermometer on quite a different construction to that of Mr. Johnson. In their instrument the bulb of the thermometer is enclosed in a second cylinder partly filled with mercury in vacuo; the pressure of the water at great sea depths exercises its influence on the latter the internal bulb being quite free from pressure on this account.

Of the ordinary instruments of science-microscopes and microscopic preparations-the collection shown in the French and English departments is excellent. It would be invidious to point out any one maker among so many known to fame. Single and binocular and museum microscopes are here-from the cheapest to the most expensive and complicated forms. We may say the same of all the ordinary surveying, nautical, and drawing instruments, such as theodolites, sextants, compasses, and levels. Among the newly invented microscopical apparatus, the visitor will perceive, in Ross's case, the hemispherical condenser of the Rev. J. Reade, from which, by means of double stops and hemispheric lens, one, two, or three apertures may be obtained, and the linear markings, no matter at what angles, on test-microscopic objects, obtained by means even of an ordinary microscope. The application of aluminium to small telescopes and other instruments intended to be held in the hand, renders them very commodious; a sextant here exhibited, only weighs one-half of a similar one constructed of the ordinary metal. A contrivance for facilitating the approximate solution of problems in spherical trigonometry (exhibited by Moore) will be found useful to the seaman. The apparatus for spectrum-analysis invented by Crookes has now become one of the necessities of the laboratory and the observatory, and replaces the simple prism and theodolite originally made use of by Fraunhofer. We trust that the inventor may soon set at rest the disputed question of the spectra of stars which has within the last few months so much engaged the attention of astronomers.

In some departments and countries the chronometers are

ranged under the head of Philosophical Instruments. This is by no means a mistake, for to the weatherwise, the star-gazer, and nautical man, a good chronometer is as essential as scales, weights, and measures to the chemist. Two of our greatest living astronomers, Professors Hansen and Bond, were originally in this profession.

Whilst the English department is full of the admirable mechanism and inventions of our Dents, Bennetts, Frodshams, Losebys, &c., we perceive that other countries have not been behindhand. Germany has adopted Hartnup's excellent balance. Hohwn, of Amsterdam, exhibits a compensation of his own invention, whilst a small pocket chronometer, of exquisite workmanship, exhibited by Baume and Lezard, which has been compared at the Geneva Observatory, appears to have kept its rate with extraordinary accuracy, and reflects great credit on the manufacturers. The ships' chronometers of Grandjean also show the progress which this branch of manufacture has been making in Switzerland, where goodness and cheapness are combined. The nautical astronomer here finds plenty to engage his attention in every department of his occupation.

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