The peculiar condition of insulated wires has been closely observed, and many improvements have taken place in the instruments used for telegraphing and for determining force of currents, &c. Beyond all, the grand generalization of HERBERT SPENCER on the nature of electricity sets at rest a theory which, although it had agitated the minds of scientific men for some time past, had not until lately found an exponent of sufficient genius to impress upon it the stamp of genuineness and truth. That theory is in effect, that in the transmission of intelligence along a wire by the aid of electricity, it is not the flow of the electric current, as it is called, which conveys the message, but the impulse given to the molecules of iron composing the wire. In other words, no current of electricity passes along the wire; but a shock or impulse is experienced by the first molecule and transmitted instantaneously to all the succeeding ones, and thus at one end is repeated the signal which was made at the other. In the same manner the shock experienced at one end of a log, when it is struck by a simple blow at the other end, may be the means of conveying a signal, albeit nothing actually passes along the log.

But this is not all. We have learnt many things of importance in telegraphy besides this, which, unlike the great theory on hand already mentioned, have passed into the domain of accepted and practical truths. Since the first Atlantic line was laid, the advance that has been made by the scientific world towards comprehending electrical phenomena is very great. Electric science has passed, since that time, from its childhood to its maturity. So far as the phenomena connected with long electric circuits were concerned, we had in 1858 no knowledge whatever. The instru ments in common use were unsuited to receiving signals through a great length of cable; the necessity of providing for the conductor an insulation so perfect as to approach an absolute condition was inadequately appreciated. The best preliminary test for a long cable had not not been devised, and the old Atlantic telegraph was laid without having been subjected to any searching test on shore. Everybody had advice to give concerning the management of the wire, but no one recommended the precautions which subsequent experience has shown to be necessary. When the signals began to fail the battery power was augmented, and electro-magnetic induction coils, which rapidly helped on the destruction of the conductor, were put in circuit. No one thought of "nursing" the cable-of humoring its feeble attempts at articulate utterance, and of finding out what it said, rather by listening acutely, than by constantly calling on it to "Speak up!' The old cable, however, is dead and gone; part of it has been picked up and applied to ignoble uses, and part of it-the greater part-has been abandoned, and lies where it may rest until the end of time.

THE NEW ATLANTIC CABLE.

The new Atlantic cable now lies on board the Great Eastern. A late letter thus describes the appearance of the big ship:

SCENES ON BOARD THE GREAT EASTERN.

The Great Eastern looks just now more like an engineer's workshop than a seagoing vessel. The vast expanses of her deck are covered with wooden sheds and piles of timber. There are smiths' forges below, and between the decks you might fancy yourself in a machinist's factory. The great engines of the ship, it is true, have lost the bright look of machinery which is in constant use; and the bage dull masses of iron seem asleep, or in a trance. If you descend the ladders which lead to the boilers and furnaces-an expedition which is more like going down a mine than any other to which it can be compared-you find yourself in the midst of darkness, solitude and cold; but in those regions of the vessel where the cable is being shipped and watched there is every sign of keen, vigilant intelligence. When you understand what is being done, you see something more than this-that scientific foresight of the highest order directs every step; and that the thick tarry rope, coarse and rough to appearance, which lies coiled away under water in the tanks of the ship, is manufactured, scanned and tested with as much care as the nicest optical instrument in an astronomer's observatory, or the most delicate apparatus of fragile glass ever applied to the careful experi ments of chemistry.

THE SCIENTIFIC TESTS.

It seems impossible that there can be any fault in the Atlantic cable when the Great Eastern goes to sea. To say nothing of the tests applied to it at the manufactory, it is tested not alone after it has been taken on board, but during its delivery into the ship. As soon as a length is brought alongside, one end is connected with the coils already on board, and the other end with the instru ments in the testing room. The circuit is thus made through the whole extent of the coil, the portion on board and the portion alongside. The process of hauling in then commences, and the insulation is continuously observed. The instruments in the testing room record the smallest deviation from absolutely perfect insulation.

Indeed, from first to last, it has been subjected to a series of the most searching electrical tests, the standard of insulation being fixed at a resistance, per nautical mile, equal to 150,000,000 of SIEMEN's units, at a temperature of 75 degrees-a standard wholly unprecedented in any former work of the kind. In actual practice these tests, great as they were, have been considerably exceeded, and the present Atlantic cable has come out successfully from a series of trials of the most critical character. It will be understood that an insulation which shall be quite perfect, as an electrician understands the word, is not attainable. A piece of metal separated by means of the purest glass, and enclosed in the driest atmosphere that can be obtained, will, if charged with electricity, lose that electricity after a time. In speaking of insulation we must therefore be understood to mean an approximate condition; but the approximation in the case of the new Atlantic cable comes so near to perfection that this rough tarry rope is a scientific wonder.

The last dying pulsation of the old Atlantic cable was forced through it by means of a galvanic battery consisting of two hundred and forty cells. The submarine telegraph from London to Amsterdam is habitually worked with a battery of fifty cells, and such a battery is commonly used for the other submarine lines of Europe. Signals have been repeatedly sent

through more than thirteen hundred miles of the cable now on board the Great Eastern by means of one cell. Galvanic currents so feeble that they could not have been felt by the hand, and might have been passed harmlessly through a circuit completed by the operator's tongue, can be used to convey messages along a length of cable that would very nearly stretch from London to St. Petersburgh. Over needle instruments, such as those in ordinary use for land telegraphy, a current from one cell would be powerless.

To record such faint pulsations of electricity it is necessary to use Professor Thompson's mirror galvanometer. This beautiful instrument consists of a mirror about the size of a fourpenny piece, made of microscope glass, and so thin that it weighs only a grain. On the back of this mir ror a minute magnet is fixed, and thus supplemented it is suspended by a silken fibre in the heart of a coil of wire, so that any current passing through the coil deflects the magnet and the mirror along with it. A ray of light reflected by the mirror falls on a scale, distant about eighteen or twenty inches, and reveals its faintest movements. Different combinations of these movements represent the different letters of the alphabet, and thus the apparently erratic wanderings of a ray of light are made to convey intelligence. An instrument of this kind is constantly used to test the cable, as it is hauled on board; and if any fault had existed it could not have passed without detection. Up to this time no fault has been discovered.

Another of the tests employed was soaking separately each joint of the gutta-percha covering for twenty-four hours in warm water, previous to passing the electric current through. A third, and perhaps the most trying and continuous test, was that of allowing no part of the cable to be out of the water, except merely while in transitu from one reservoir to the other; and in connection with this it may be mentioned that the wrap ping of jute yarn, which forms the padding, around which the external wires are spun, instead of being soaked in tar, as in other cables, which has the effect of temporarily stopping up and concealing minute flaws, has simply being tanned to preserve it decay, thus admitting the water to search out every part of the cable, and keep it, in fact, constantly surrounded with a wrapping of wet yarn.

THE PAYING-OUT MACHINE.

This beautiful machine is an improved and extended copy, as far as general principle is concerned, of that used on board the Agammemnon during the first memorable attempts. Its improvements and modifications, however, are very great. Though stronger and much larger, it is very much lighter, being almost entirely of wrought iron. It has six leading wheels, round which the cable passes in deep grooves, before which it will be finally submerged. Each groove in these six wheels is surmounted by what is termed a jockey-pulley-that is to say, a solid wheel, which keeps the rope down in the groove, and prevents it over-riding or getting out of place. The main drum is about seven feet in diameter, and in payingout will have three coils of cable round it, with a guiding piece of wrougt iron pressing on them sideways to keep them compactly together. As the friction on the guider is great and constant, a duplicate is provided in case of heating, which can be put in gear, with the rest of the machinery with

out the slightest stop on any part. A second drum has also been fitted, in case of anything going wrong with the first. Each of these drums is fitted with a distinct set of simple and most ingenious brakes, invented by Mr. APPOLD for the first expedition. The ordinary condition of these brakes is to maintain a sufficient check upon the drum to keep a strain of say 30 cwt. or 40 cwt. upon the ropes going out, but it constantly happens that a sudden risk of the ship's stern from a wave, gives the cable a strain that requires the instant removal of the brakes to relieve it. This is accomplished by a dyuameter placed on the cable after it leaves the payingout machine and before it passes over the wheel astern into the Atlantic. This dynameter is only a heavy wheel resting on the rope, but fixed in an upright frame, which allows it to slide freely up and down, and on this frame are marked the figures which show exactly the strain in pounds on the cable. Thus when the strain is low the cable slackens, and the dynameter sinks low with it; when, on the contrary, the strain is great, the cable is drawn "taut," and on it the dynameter rises to its full height. When it sinks too low, the cable is generally running away too fast, and the brakes have to be applied to check it; when, on the contrary, it rises rapidly, the tension is dangerous, and the brakes have to be almost opened to relieve it. The simplicity of the arrangement for opening and shutting the brakes is the most beautiful of all. Opposite the dynameter is placed a tiller-wheel, and the man in charge of this never lets it go nor slackens his attention for an instant, but watches the rise and fall of the dynameter as a sailor at the wheel watches his compass. A single movement of this wheel to the right puts the brakes on, a turn to the left opens them. good and experienced brakesman will generally contrive to acoid either extreme of a high or low strain, though there are few duties connected with the laying of submarine cables which are more anxious and more responsible while they last than those connected with the management of the brakes.

THE IMMERSION.

A

The machinery for paying out is being put together at the Greenwich works. The process of immersion will take about a fortnight. The beginning of the shore end will be laid by a small vessel, which will meet the Great Eastern about twenty miles from the Irish coast. The cable will then be passed on board, connected with that in the great tanks, and the big ship will begin her voyage. To the uninitiated this process of cutting and joining the cable appears very mysterious, but the engineers, who are used to the work, face it without any hesitation. The joints do not really endanger the insulation or the strength of the cable, as wherever they are made the external and conducting wires are spliced along a considerable length-sometimes not less than thirty yards-and the gutta percha carefully put on in separate layers, firmly pressed together by means of warm irons. The completeness of the joint is tested by laying it in an insulated metallic vessel containing water, and ascertaining by means of tests applied to this vessel, whether any electricity escapes from the joint as a current is passed along the cable.

In order to guard against any possible sources of accident, every preparation has been made in case of the worst, and, in the event of very bad weather, for cutting the cable adrift and buoying it. For this purpose a

wire rope of great strength, and no less than five miles long, having a distinctive mark at every one hundred fathoms, will be taken in the Great Eastern. This, of course, is carried in case of desperate eventualities arising, and in the earnest hope that not an inch of it will ever be required. If, unfortunately, its services should be wanted, the cable would be firmly made fast to its extremity and so many hundred fathoms of the wire rope according to the depth of water the cable was in measured out. To the other end of the rope an immense buoy would be attached, and the whole then cut adrift and left to itself till better weather. In the experimental cruises which were undertaken before before the starting of the last Atlantic expedition this attempt at buoying the cable was often tried in the deep water of the Bay of Biscay, but never with any great success, and in very deep water it would be a most forlorn hope indeed to try it at all.

THE MERCANTILE ADVANTAGES OF THE CABLE.

We have now to speak of the advantages which commerce will derive from the completion of the Atlantic telegraph. To say that it saves time and economises effort, is to present to the mind a generalization too broad for ready appreciation. But the benefits derivable from this great work are so innumerable, that a mere list of them would not only be difficult to make, but fatiguing to read.

In order, therefore, to get a tolerable idea of the value of the Atlantic cable, let us resort to an illustration. Fill a flat dish with water, and seesaw it. Will the water always follow the inclination of the dish! Of course it will. But will the movements of the one always be identical in point of time with the movements of the other? No. If the dish is see-sawed quickly, the water may be seen running up one end of the dish while the other end is tilted downwards. This phenomenon is caused by the momentum which the water had gained before the direction of the tilting was changed, and until that momentum was overcome by a new inclination of the dish, it continued to impel the water in a direction contrary to that which was exerted by the force of gravitation.

So, in commerce, the products of the world, which, if left free to move naturally, would be attracted towards the most profitable markets, are frequently impelled towards unprofitable ones, because the knowledge of which are the best ones becomes known in most cases only after the product has been already shipped to an unprofitable one.

All this waste of effort-all this source of mercantile loss and embar rassment ceases where the telegraph is established. Goods then go only where they are wanted, and always find a profitable market. The mar kets of the world become equalised, and equalisation is economy and wealth to all mankind.

SOCIAL ADVANTAGES.

Finally, when the social advantages of the Atlantic telegraph are con sidered, it is seen to be a work not only of importance to the world's thrift, but to its morality and social advancement.

The universal postulate is belief; and mankind only war against one another from entertaining opposite beliefs. No agency has ever existed, nor ever will exist, so potent to dispel misunderstandings and to reconcile beliefs, as knowledge; and what is the telegraph but a means of transmit