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770

Solar Telegraphy.

Francisco time when the message arrived there was 11.20 P.M. of January 31-this place being eight hours to the west or earlier than our time. The distance traversed by the message going and coming was 14,000 miles in two minutes. This of course included the time required for working the telegraph at each station.

In closing our short account of the phenomena of this most wonderful of all natural agents, it is curious to reflect that, notwithstanding all the uses and facts of its wonderful workings which have been discovered, its real nature remains as mysteriously elusive as ever. Whether it is a fluid of spiritual fineness, using matter as its vehicle in its rapid course, or whether it is a particular manifestation of that same ethereal medium which manifests itself otherwise as light and heat, or whether it is a purely molecular affection of grosser matter, philosophers have not been able to discover.

SOLAR TELEGRAPHY.

1010. The reflected light of the sun has been used for the purposes of telegraphic communication, and so far as the exchange of signals is concerned, it has been perfectly successful. When the sun's light falls at a certain angle upon a sheet of glass, it is powerfully reflected (Art. 881, p. 650), and if there is sufficient elevation and a clear atmosphere, the reflection may be plainly seen at a distance of several miles. Thus the glass roof of the Crystal Palace, at Norwood, may be seen by reflected light, at several miles' distance, when the palace itself is only dimly visible.

Captain Drummond, the inventor of the lime-light, constructed an apparatus for signalling by flashing the sun's rays by a reflector. He gave to it the name of Heliostat. It consisted of an adjustable mirror as a reflector, worked in connection with a combination of telescopes. In an improved form it is now used in trigonometrical surveys, and, by its aid, triangles, having sides above 100 miles in length, have been formed in Great Britain. Among these may be mentioned that formed by Sca Fell in Cumberland, Donard in Ireland, and Snowdon in Wales, the sides of which are respectively III, 108, and 102 miles in length.

The use of this heliostat in flashing the sun's rays, did no more than convey an arbitrary set of signals. Mr. Mance has lately so improved this method of signalling, as to enable observers at two remote stations to converse with each other. The instrument is called the Mance heliograph, or the sun-telegraph. It consists,

Apparatus employed.

771 in the first place, of a light tripod stand, about four feet long when folded up for transport. On this tripod is screwed a circular mirror, varying in diameter according to the purpose for which the instrument is designed; that is, whether for field or fixed observations. If for the former purpose, the mirror is about four inches in diameter; while, if for the latter, it is about nine inches. The mirror is hung in a frame so as to revolve about a horizontal axis, and it is adjusted to the required angle of incidence with the sun by means of a telescopic connecting rod, having a screw adjustment, the top end being attached to the upper edge of the mirror at the back. The horizontal circular traverse of the instrument is obtained by means of a tangent screw-gearing into a small horizontal wormwheel, with the centre of which the mirror is connected. By means of the tangent screw and the vertical screwed rod, the rays of the sun can be made to fall upon any given point with the utmost precision. The vertical rod behind the mirror is pivoted at the bottom to a lever, the fulcrum of which is on the horizontal worm-wheel, the lever constantly pressing against the lower end of the rod by means of a spring which is placed under it. It will thus be seen that when the rod is depressed, it will depress the top edge of the mirror and draw it slightly backwards, the bottom edge being at the same time slightly raised and thrown forwards. In adjusting the instrument in order to commence signalling, the rays are directed to a point slightly below the distant observer's level, but upon depressing the connecting rod-for which purpose there is a small finger-piece attached to it-the flash is raised to the level of the observer, and he sees it. If now the lengths of these flashes be varied and grouped they can be made to represent letters, and so words composing messages can be spelt out.

In adjusting the instrument for use a light wooden rod, having two brass sliding sights upon it, is employed. This is set up in the ground in front of the instrument, and the operator looks through a small space in the centre of the mirror, from which the quicksilver has been removed, towards the station with which he desires to communicate. The upper sight on the rod is then moved vertically until the centre of the mirror, the sight, and the distant station are truly aligned. Hence when the flash from the mirror is directed on to the sight it is in true line with the distant station, and can be seen by the observer there. This will, of course, be whenever the angle of the mirror is raised; when depressed, or in its normal position, the flash rests upon a cross-piece on the rod, and, according

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Electric and Solar Telegraphy.

as the sun's horizontal and vertical motions cause the flash to deviate from the true line, the signaller is able to see and to correct the error by means of the adjustments on the instrument. The observer at the distant station, having seen the bright starlike appearances, sets his instrument to the point at which they appear and acknowledges the fact, and the parties being thus placed in communication, the interchange of messages proceeds upon the system we have mentioned, namely, the Morse alphabet.

By adopting the Morse system of dashes and dots (see Art. 1003, p. 764) Mr. Mance has been able, on a fine day, to make himselt understood by an observer many miles off, as easily as one electric telegraph operator makes himself intelligible by another.

There are special arrangements for signalling with the sun behind the apparatus, by means of a reflector; for signalling at night, and for signalling either from fixed or variable positions. This sun-telegraph has been for some time employed in India with great success. The signals conveyed have been easily read in ordinary weather without telescopes for a distance of fifty miles, and, under favourable conditions, messages can be signalled and interpreted without telescopes for a distance of from eighty to one hundred miles. Signals by this instrument have been successfully exchanged by observers between the dome of St. Paul's and the Crystal Palace. For military purposes this mode of signalling would be invaluable. In time of war, telegraph wires are easily destroyed, while the heliograph might be so placed as to be out of reach of the enemy. From its inexpensiveness it might also be used in place of the ordinary telegraph wires, in countries where the erection of electric telegraph stations and wires would not be remunerative.

PART V.

ASTRONOMY.

1011. THE word "astronomy," composed of dorp, a star, and ropos, law, is used to signify what can be learned respecting the objects visible in the sky, as to their distances, sizes, motions, and relations to this earth. When the earth is viewed in its totality, and regarded as it would be seen by a spectator at a great distance, it is found to be similar to some of the celestial bodies, so that it has to be considered along with them.

There are two distinct modes of regarding the celestial bodies: First, we may study their distances, shapes, sizes, and motions, as appearing to an observer, without asking any questions as to causes, or seeking any explanation of the appearances. This is the department called Descriptive or Geometrical Astronomy, and includes all the knowledge attained on the subject up to the time of Newton.

Next, we may study the powers or forces that originate and sustain the various movements, and determine the shapes, sizes, and distances of the several bodies. This is to treat the celestial movements exactly as we should treat projectiles or other moving bodies in the earth.

It was through Newton's discovery of universal gravitation that the computations of terrestrial forces could be extended to the moon, the planets, and the other celestial masses! By such means, the various appearances are not merely described, but also explained upon mechanical principles. To this modern department is given the name Physical or Mechanical Astronomy. In the present treatise, only the leading principles of this part of the subject can be indicated.

ANALYSIS OF TIIE SECTIONS.

SECTION I.-This EARTH is in form a globe or ball. Its diameter is nearly eight thousand miles. The globular form is the result of the mutual attrac tion or gravitation of its particles.

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The Earth a Globe or Ball.

At a distance from the Earth, nearly thirty times the Earth's diameter, is the body nearest to it-the MOON.

SECTION II.-The Earth rotates or spins; thereby exposing its surface successively to the Sun, and bringing about the changes of day and night. SECTION III.-The Earth revolves or travels round the Sun in the period of 365 days, which makes our year. From the direction maintained by its axis of rotation, while circling round the Sun, there are produced the changes of the Seasons.

SECTION IV.-The Moon is a smaller globe revolving round the Earth, as the Earth revolves around the Sun.

SECTION V.-The Earth is the third, as to distance from the Sun, and as to length of year, of a series of like globes called PLANETS, which are various in size, some larger, some smaller, than the Earth. There also revolve round the Sun other masses, very peculiar in their characters, called Comets. SECTION VI.-The SUN himself is the nearest to this Earth of innumerable self-luminous bodies, existing throughout illimitable space, which, owing to their distance, appear to our sight very small. These are the STars.

SECTION I.

"This Earth is in form a globe or ball. Its diameter is nearly eight thousand miles."

1012. Canals for inland navigation are now common over the civilized world, and their bottom, in order that the depth of water in them shall be everywhere the same, must be made, not a straight

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surface represented by the line A B C (fig. 297), seen level at B, but as part of a hoop or curved surface, D B E, sinking downward from the straight or tangent level, A B C, taken at the point, B, very nearly eight inches for the first mile from B.

The important fact is to be remarked that the sinking from the tangent line, at double distance from B, is not twice eight inches, as might be expected, but four times eight; at triple distance, is not three times, but nine times as much, and so on for other distances, as the squares. Thus, the distances in miles being 1, 2, 3

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