graph. I recollect your telling me that the principles upon which the electric telegraph was based were those of electromagnetism, but I cannot comprehend how words can be conveyed by electro-magnetism, or by wires.

Fa. I dare say not; but I think this difficulty may soon be got over. It need scarcely be told you that the wires merely convey an electric current, and not sounds. The words are conveyed by signs mutually agreed upon before-hand by the persons stationed at each end of the wires; and these signs consist of deflexions of a magnetic needle produced by the electric current.

Ch. I begin now to have some idea of the manner in which the communication might take place. For it is evident that if each letter of the alphabet were signified by a certain number of deflections of the needle, there would be no difficulty in the matter.

Fa. You are quite correct; and the plan you have suggested is that usually adopted. But the process has been much simplified and abbreviated by attention to certain circumstances. Thus, the deflection of a magnetic needle varies according to the direction in which the current of electricity passes through the coil of wire, and by changing this direction, the needle may be deflected to the right or the left at will. Hence, two distinct signs may be made with a single needle. Again, by using a number of needles and wires, which can be worked simultaneously, the communication is still further simplified.

Ch. Are there then half as many needles in use as there are connecting wires?

Fa. No; the returning electric current is conveyed by the earth, which has been found to answer the purpose sufficiently well, so that each of the wires which you see on the side of a railway belongs to a single needle.

Ch. But how are the currents conveyed across the sea? Fu. By wires, as on the land. The great difficulty to be overcome consisted in insulating the wires when immersed in so good a conductor as water. This has been effected by enclosing them in tubes made of gutta percha, which is a very bad conductor of electricity, and possesses the valuable property of being easily moulded or its surfaces united by means of heat. Long tubes of this substance enclosing the

wires are laid upon the bottom of the sea, and extend from one telegraph-station to the next.

Ch. Then I suppose that the pieces of glass and earthenware by which the wires are supported upon the poles at the side of the railway-lines also act as insulators.

Fa. Exactly so; if these were absent, when the poles became wet, the electric current would descend the poles, and return by the earth, and so the communication with the distant station would be interrupted.

Ch. But it must be very fatiguing for any one to sit and watch constantly whether the needle or index hand of the telegraph moves or not.

Fa. This would certainly form a difficulty; for whilst the attention of the telegraph-worker were withdrawn for even a short period, several of the signals, denoting letters and words, might have been made and not seen. This is obviated by the attention being drawn when the signals are about to be made by the ringing of a little bell, which is instantly heard by an attendant.

Ch. But how is this effected?

Fa. By means of an electro-magnet. I told you in a former Conversation, that a piece of soft iron surrounded by a coil of wire through which a current of electricity is passed, becomes temporarily magnetic; hence, if the piece of iron were in the form of a common horse-shoe magnet, for instance, and the keeper were connected with a little bell, either by means of a lever or in some other way, as soon as the current was transmitted through the wire the magnet would attract the keeper and set the bell in motion. As soon as the worker of the telegraph has heard the bell, and is ready to attend to the signals, he rings by the same means at the distant station to signify that this is the case. Various modifications of the electric telegraph are in use at different places, to enter into which, we have not at present time; but I hope enough has been said to render intelligible to you the general principles upon which they are based.

Ch. Will you now explain to me the manner in which the Stereoscope produces such remarkably deceptive appearances, for they seem to me very puzzling?

Fa. Certainly; and in so doing I must recal to your mind what I said in a former Conversation regarding the

manner in which the interpretations of the impressions made upon the eye are controlled by experience; and when this experience has not been obtained, the simple sense of sight is very deceptive as to the form and distance of objects. So much so, that in certain instances which have occurred of persons born blind, and whose sight has been subsequently restored when their reason has become matured, the most efroneous ideas have been entertained as to the form and distance of objects; thus, the latter, although distant, have appeared to be close to the eye, and it was found impossible to decide whether the sense of touch or of sight was to be trusted in determining the form of objects. And in certain engravings in which the shadows which would have been formed by the figures were very exactly represented, the idea has been conveyed to the mind that these figures were really solid; a fallacy only to be detected by the sense of touch.

In the stereoscope which we are considering, the fallacy is connected with the judgment formed from the perspective view of objects. When we look at any solid body with each eye separately, i.e., closing one with the finger, then closing the other while the first eye remains open, two distinct perspective views of the object are obtained. Thus in the instance of the cube represented below, Fig. 1 would repre

sent the view as seen with the left eye only open, and Fig. 2, that with the right eye open. Now under ordinary circumstances, these two images of the cube being seen simultaneously, and depicted upon corresponding parts of the retinæ of the two eyes, the idea conveyed to the mind is that of a single object; and as previous experience has taught us that all single objects which present two perspective views are solid, we naturally conclude that the cube is solid, even without taking it into our hands.

It is evident, however, that the two drawings of the cube do not appear to the eye as a solid body, although they

clearly present the proper perspective views; and this because the two perspective views do not appear to emanate from one object, the rays from each figure impinging upon different parts of cach retina, as if from two different objects or bodies.

But when the drawings are viewed through the stereoscope, the rays, a, proceeding from each drawing are refracted outwards, at the same angle as each set of rays would have formed had they proceeded from a single object, d, placed in the centre of the box. Hence we obtain the requisite conditions for the production of the impression upon the mind of the existence of a single solid object, i.e., two perspective views appa

a

a

rently emanating from one object, and impinging upon corresponding parts of the retina of the two eyes.

Ch. But how is this refraction outwards produced?

Fa. By two slightly magnifying lenses, inclined outwards, and situated one in each eye-piece at b b.

Ch. Then if I were to turn round either of the eye-pieces, I should lose the solid appearance of the figures?

Fa. Certainly; and you would see two distinct plane or flat figures, for the reason stated above.

Ch. The stereoscope then shows a use of our having two eyes instead of one which never occurred to me; and I suppose that a man who had lost the sight of one eye would be unable to distinguish whether an object were solid or not?

Fa. To such a man, solid objects would doubtless not appear to be so, but by changing the position of his head he would easily obtain two perspective views, and so might conclude as to the solidity of a body.

Ch. Would it be possible to reflect the images of two plane figures in such manner as to make them represent a single solid body?

Fa. Certainly; I omitted to tell you that the stereoscope mentioned above, containing the lenses, is called the Lenticular, or Refracting Stereoscope; but the first one made was

upon the reflecting principle, and is called the Reflecting Stereoscope.

Fa. The Aneroid Barometer was invented by M. Vidi, of Paris; and although it is not so perfect a philosophical instrument as the mercurial barometer, yet it possesses the great advantage of extreme portability, for it is not more than about five inches in diameter, and two in thickness. In form it resembles a watch of the above dimensions. Upon its face is a curved graduated scale, the degrees of which correspond to the altitude in inches and fractional parts of the mercurial barometer; a thermometer is also affixed to the face, and the indications are made by an arrowshaped hand.

Ch. I think then I have seen the aneroid barometer in a shop-window, with the case made of brass.

Fa. Very probably, for they are everywhere to be seen in London. Inside the case is a circular flat metallic box, of about two-thirds the size of the outer case, the front and back surfaces of which are corrugated, so as to render them flexible and elastic. The air is exhausted from this box, which is afterwards hermetically sealed. The box is firmly fixed in the case, and to its front surface one end of a broad lever is connected by a socket, whilst the other end of the lever rests upon a spiral spring, which by resisting to a certain extent the pressure of the lever, keeps the surfaces of the box in a state of tension. By means of another lever and a chain, the movements of the first lever are transferred to the hand, which indicates them upon the scale.

Ch. Do you think then that the aneroid will supersede the mercurial baremeter ?

Fa. Most probably not, because the small movements required to be indicated by the barometer must, to a certain extent, be interfered with by transference through a system of levers; although some careful researches by Mr. Belville, of the Royal Observatory, have shown that this result occurs to a considerably less extent than might have been anticipated.