such, that the positive current moves from the circumference to the centre. When the collecting wires are away, the discharge takes place through the neighbouring part of the disc; when they are there, it occurs through them and the instrument to which they lead. This is precisely in accordance with the law that a primary, or an existing current, induces a secondary current in an opposite direction in a conductor near it, at the time it is called into existence, or brought near it while it is in existence. Remembering that Ampère's currents circulate round a south pole in the direction of

P

Fig. 5.

the hands of a watch, as at S in fig. 5, if we conceive the P' line P N as one radius of the disc (and what holds good of one radius holds good of all the disc), and that it is moving onward to the right, over the S. pole, a current will be induced in it in a direction the reverse of that indicated by the nearer arrow, a, and therefore in the direction from P to N; and it may be collected at P and led to N through the wires of a galvanometer; and when this is not the case, it will get to N by discharging itself through the substance of the plate. If P'N is another radius, after it has passed the pole, and on its journey away, we shall still get positive electricity at the circumference; for, on leaving the magnet, it has an induced current in the same direction as that indicated by the primary current, b, nearest to it: and this gives a resultant from P' to N, or positive at P' and negative at N, as before. If the disc rotates between two different poles, their action is combined, and a more exalted result is obtained. this arrangement, and a continuous rotation, an uninterrupted current was first obtained. This is the simple form of a magneto-electric machine; but when a U-shaped iron armature is rapidly revolved before the poles of a powerful horse-shoe magnet, each end be

By

comes continually first a north and then a south pole; and if these ends constitute the cores of long bobbins of insulated copper wire, a rapid succession of currents of electricity is produced in the bobbins, and, by suitable arrangements, the currents can be collected and turned to useful account. They produce all the known effects of electricity derived from any of the other sources. CHARLES V. Walker.

Telegraphs.

BY CHARLES V. WALKER.

LESSON L

EARLY EFFORTS.

lightnings, that they may go, and say unto

1. THE Word Telegraph is composed of two Greek words, signifying to write or describe at a distance; and to do so in some way is a part of our every-day existence. The organs of speech enable a man to convey his ideas to a moderate distance; the organs of hearing enable another to receive the ideas. When he has other sounds to contend against, and would convey his ideas in a given direction, he employs the speaking-trumpet, and his object is attained. When he would communicate by word of mouth to greater distances, and many physical obstacles are between him and the object of his desire, he employs a tube, and is able through all the deviations and irregularities of its course to make his words be heard. In most common cases, when he would merely notify his presence and convey but few ideas, he strikes a blow.

2. The blows are sometimes struck at a distance, and bells are rung by aid of wires or ropes. Sometimes they tell their tale merely to our own household; sometimes the village bells resound throughout the vale, and all who hear know but too well whether they call to prayer, or gladden the heart of the trembling bride, or say that one more soul has passed into eternity, or sadly call the mourners on their way to

commit to its kindred dust all that remains of one they held most dear. Sometimes the sounding metal tells of the hidden rock, or calls a slumbering town to be up and doing, for danger is at hand. The trumpet's note is no uncertain sound; it tells the soldier when to charge the foe, and when he must retreat; it rouses him from his slumber, or, with another note, it calls him home to rest. The signal-gun proclaims a ship distressed; the minute-gun that some one great or noble is carried to the tomb; the feu de joie, that 'tis some gala day; the magic twenty-one that England's queen is near, or that some day has come, eventful in the history of our time.

3. These are all but different illustrations of the means we have adopted to convey our wishes afar. The great advantage of sound is that, especially to a man awake, it commands attention, the ear is never closed; and, if loud, it will even awake the sleeper. But it does not penetrate far into space, and its language is limited. Light travels with extreme rapidity, and penetrates to very great distances; for instance, the sun is nearly a hundred millions of miles from us, and yet what a volume of sunlight surrounds us! The distance of the stars is reckoned in millions of millions of miles, and yet the rays of light come to us with great distinctness. The whole coast of this sea-girt England, for instance, sends forth its messengers of light amid the gloom of night to tell the mariner his whereabout; on this point a single steady light proclaims its name afar for many a mile; on this, a light is one time bright and then for a time is hid, and so it speaks; here we have two fixed lights, and here one fixed and one that is often hid. The sound of the seaman's signal-gun will not at all times reach the shore; he then throws up a light, a rocket mounts on high, and tells of his distress. The rays of green or red or white light are the only things well visible to the engine-driver in a pitchy dark night, and yet he drives in safety along a road to him invisible; they tell him, in language unmistakable and easy to him, before he comes upon them, whether he shall stop, or come with care, or boldly drive along. He tells also to others of his coming, by a bright white light, or two or

three, according to his mission; and he warns all followers to keep their distance, by a deep-red light, that speaks as plain as language, that he is on before. By day, when all is light, the distant semaphore, with its arm full up or half way down, or altogether lowered, advises him betimes to stay his course, or slack his speed, or come securely on; a full red disc forbids him to go on, or else it turns its edge to say the road is clear.

4. A few years since, until electricity had proved its goodness, a tall mast was seen on the Admiralty, at Whitehall, furnished with two or three arms, the motions of which were often an object of interest to the passerby; they were seen to move into different relative positions, and for a second or two remain so, and then be changed again in position. These positions corresponded with numbers; and, by reference to an extensive codebook, were convertible into language. When a signal was made it was held up till recognised and repeated by a similar instrument on elevated ground a few miles distant, from which it was repeated by another station, and so on till it reached the coast.

5. This sort of visual telegraph was carried out to a great extent in France by the brothers Chappe. Their semaphore was mounted in like manner on a tall pole, and consisted of a long arm, at either end of which was a short arm. The long arm could be set in a vertical or horizontal position, or inclined 45° from the hori

zontal on either

Fig. 1.

side; each short arm could also be set in one of eight positions in respect to the long arm. Fig. 1 shows the

long arm in the horizontal position, the right arm in the downward vertical, and the left at the left angle of 45° from the vertical upward; the dotted lines show the other three positions of the large arm, and the other seven positions of the left-hand small arm, the same, of course, applying to the righ-thand small arm. The extension with the weight attached is the counterpoise to the small arm. -With the long arm inclined to the right ninety-two positions or signals could be made these referred to the ninety-two pages of a vocabulary; each page contained ninety-two words; and, having in this way first made one signal to indicate the page, a second signal followed to indicate the word; by this simple combination of two signals any one of 8,464 words could be expressed. The MM. Chappe constructed a phrasebook, and a geographical-book of the same extent; and all that was needed was at the commencement of a correspondence to give a previous signal to indicate what book was to be used. The ninety-two signals shown by the arm when inclined to the left indicated certain matters connected with the message or the service; as, urgent, great haste, haste, the destination, the conclusion, leaves of absence, error, delay, fog, rain, accident, wind, fire, &c.

6. These signals ceased to be available when night came on. Dr. Guyot, in 1840, published a plan for attaching lanterns to the arms of the Chappe semaphore; and for so arranging the code that one book should contain all the signals, to have 288 pages, each containing 288 words, phrases, letters, or signals, in all 82,944; but it does not appear that his plans were adopted. The system of signalling by flags is very much of the same nature; a ship makes itself known by hoisting certain flags, to indicate its number, against which, in the signal-book, its name will be found; other flags indicate the class of signals that are to follow, and, by reference to that division of the signal-book, the message given by other flags is read. Sir C. Napier expressed with a few pieces of coloured bunting the words "Well done, Arrogant and Hecla." Night and especially fog are fatal to correspondence by these means.

7. Although sound and light and locomotion have now