the letters, numerals, &c., are indicated. When the operator at B has completed his message, the handle H' being raised by the spring under it to the position in which H is shown, a message can be received at B.

I have in the figure and description assumed that the current from either station acts directly on the magnet which works the recording style. Usually, in long-distance telegraphy, the current is too weak for this, and the magnet on which it acts is used only to complete the circuit of a local battery, the current from which does the real work of magnetising м at a or m' at B, as the case may be. A local battery thus employed is called a relay.

The Morse instrument will serve to illustrate the principle of the methods by which facsimiles are obtained. The details of construction are altogether different from those of the Morse instrument; they also vary greatly in different instruments, and are too complex to be conveniently described here. But the principle, which is the essential point, can be readily understood.

In working the Morse instrument, the operator at в depresses the handle н'. Suppose that this handle is kept depressed by a spring, and that a long strip of paper passing uniformly between the two points at a prevents contact. Then no current can pass. But if there is a hole in this paper, then when the hole reaches a the two metal points at a meet and the current passes. We have here the principle of the Bain telegraph. A long strip of paper is punched with small and long holes, corresponding to the dots and marks of a message by the Morse alphabet. As it passes between a metal wheel and a spring, both forming part of the circuit, it breaks the circuit until a hole allows the spring to touch the wheel, either for a short or longer time-interval, during which the current passes to the other station, where it sets a relay at work. In Bain's system the message is received on a chemically-prepared strip of paper, moving uniformly at the receiving station, and connected with the negative pole of the relay battery. When contact is made, the face of the paper is touched by a steel pointer connected with the positive pole, and the current which passes from the end of the pointer through the paper to the negative pole produces a blue mark on the chemically-prepared paper.1

We see that by Bain's arrangement a paper is marked with dots and lines, corresponding to round and elongated holes, in a ribbon of paper. It is only a step from this to the production of facsimiles of writings or drawings.

1 The paper is soaked in dilute ferrocyanide of potassium, and the passage of the current forms a Prussian blue.

Suppose a sheet of paper so prepared as to be a conductor of electricity, and that a message is written on the paper with some nonconducting substance for ink. If that sheet were passed between the knobs at a (the handle н being pressed down by a spring), whilst simultaneously a sheet of Bain's chemically-prepared paper were passed athwart the steel pointer at the receiving station, there would be traced across the last-named paper a blue line, which would be broken at parts corresponding to those on the other paper where the non-conducting ink interrupted the current. Suppose the process repeated, each paper being slightly shifted so that the line traced across either would be parallel and very close to the former, but precisely corresponding as respects the position of its length. Then this line, also, on the recording paper will be broken at parts corresponding to those in which the line across the transmitting paper meets the writing. If line after line be drawn in this way till the entire breadth of the transmitting paper has been crossed by close parallel lines, the entire breadth of the receiving paper will be covered by closelymarked blue lines except where the writing has broken the contact. Thus a negative facsimile of the writing will be found in the manner indicated in figs. 8 and 9.1 In reality, in processes of this kind, the papers (unlike the ribbons on Bain's telegraph) are not carried across in the way I have imagined, but are swept by successive strokes of a moveable pointer, along which the current flows; but the principle is the same.

It is essential, in such a process as I have described, first, that the

Sir W. Thomson states, in his altogether excellent article on the electric telegraph, in Nichol's Cyclopædia, that the invention of this process is due to Mr. Bakewell.

recording sheet should be carried athwart the pointer which conveys the marking current (or the pointer carried across the recording sheet) in precise accordance with the motion of the transmitting sheet athwart the wire or style which conveys the current to the long wire between the stations (or of this style across the transmitting sheet). The recording sheet and the transmitting sheet must also be shifted between each stroke by an equal amount. The latter point is easily secured; the former is secured by causing the mechanism which gives the transmitting style its successive strokes to make and break circuit, by which a temporary magnet at the receiving station is magnetised and demagnetised; by the action of this magnet the recording pointer is caused to start on its motion athwart the receiving sheet, and moving uniformly it completes its thwart stroke at the same instant as the transmitting style.

Caselli's pantelegraph admirably effects the transmission of facsimiles. The transmitting style is carried by the motion of a heavy pendulum in an arc of constant range over a cylindrical surface on which the paper containing the message, writing, or picture is spread. As the swing of the pendulum begins, a similar pendulum at the receiving station begins its swing; the same break of circuit which (by demagnetising a temporary magnet) releases one, releases the other also. The latter swings in an arc of precisely the same range, and carries a precisely similar style over a similar cylindrical surface on which is placed the prepared receiving paper. In fact, the same pendulum at either station is used for transmitting and for receiving facsimiles. Nay, not only so, but each pendulum, as it swings, serves in the work both of transmitting and recording facsimiles. As it swings one way, it travels along a line over each of two messages or drawings, while the other pendulum in its synchronous swing traces a corresponding line over each of two receiving sheets; and as it swings the other way, it traces a line on each of two receiving sheets, corresponding to the lines along which the transmitting style of the other is passing along two messages or drawings. Such, at least, is the way in which the instrument works in busy times. It can, of course, send a message, or two messages, without receiving any."

In Caselli's pantelegraph matters are so arranged that instead of a negative facsimile, like fig. 9, a true facsimile is obtained in all respects except that the letters and figures are made by closely-set

It is to be noticed, however, that the recording pointer must always mark its lines in the same direction, so that, unless a message is being transmitted at the same time that one is being received (in which case the oscillations both ways are utilised), the instrument works only during one half of each complete double oscillation.

dark lines instead of being dark throughout as in the message. The transmitting paper is conducting and the ink non-conducting, as in Bakewell's original arrangement; but instead of the conducting paper completing the circuit for the distant station, it completes a short home circuit (so to speak) along which the current travels without entering on the distant circuit. When the non-conducting ink breaks the short circuit, the current travels in the long circuit through the recording pointer at the receiving station; and a mark is thus made corresponding to the inked part of the transmitting sheet instead of the blank part, as in the older plan.

The following passage from Guillemin's "Application of the Physical Forces" indicates the effectiveness of Caselli's pantelegraph not only as respects the character of the message it conveys, but as to rapidity of transmission. (I alter the measures from the metric to our usual system of notation.')"Nothing is simpler than the writing of the pantelegraph. The message when written is placed on the surface of the transmitting cylinder. The clerk makes the warning signals, and then sets the pendulum going. The transmission of the message is accomplished automatically, without the clerk having any work to do, and consequently without [his] being obliged to acquire any special knowledge. Since two despatches may be sent at the same time— and since shorthand may be used-the rapidity of transmission may be considerable." "The long pendulum of Caselli's telegraph," says M. Quet, "generally performs about forty oscillations a minute, and the styles trace forty broken lines, separated from each other by less than the hundredth part of an inch. In one minute the lines described by the style have ranged over a breadth of more than half an inch, and in twenty minutes of nearly 10 inches. As we can give the lines a length of 44 inches, it follows that in twenty minutes Caselli's apparatus furnishes the facsimile of the writing or drawing traced on a metallised plate 44 inches broad by 10 inches long. For clearness of reproduction the original writing must be very legible and in large characters." "Since 1865 the line from Paris to Lyons and Marseilles has been open to the public for the transmission of messages by this truly marvellous system."

It will easily be seen that Caselli's method is capable of many important uses besides the transmission of facsimiles of handwriting.

It seems to me a pity that in the English edition of this work the usual measures have not been substituted throughout. The book is not intended or indeed suitable for scientific readers, who alone are accustomed to the metric system. Other readers do not care to have a little sum in reduction to go through at each numerical statement.

For instance, by means of it a portrait of some person who is to be identified-whether fraudulent absconder, or escaped prisoner, or lunatic, or wife who has eloped from her husband, or husband who has deserted his wife, or missing child, and so on-can be sent in a few minutes to a distant city where the missing person is likely to be. All that is necessary is that from a photograph or other portrait an artist employed for the purpose at the transmitting station should, in bold and heavy lines, sketch the lineaments of the missing person on one of the prepared sheets, as in fig. 10. The portrait at the receiving station will appear as in fig. 11, and if necessary an artist at this station can darken the lines or in other ways improve the picture without altering the likeness.

But now we must turn to the greatest marvel of all-the transmission of tones, tunes, and words by the electric wire.

The transmission of the rhythm of an air is of course a very simple matter. I have seen the following passage from "Lardner's Museum of Science and Art," 1859, quoted as describing an anticipation of the telephone, though in reality it only shows what everyone who has heard a telegraphic indicator at work must have noticed, that the click of the instrument may be made to keep time with an air. "We were in the Hanover Street Office, when there was a