after them a great number of ingenious individuals, constructed various arrangements of machinery to be set in motion by magnetic attraction and repulsion.

At that period the expectations that electro-magnetism would ultimately supersede steam, as a motive force, were very sanguine. There seemed to be nothing to prevent an enormous velocity of rotation, and consequently an enormous power, except the resistance of the air which it was easy to remove, the resistance of iron to the induction of magnetism which I succeeded in overcoming to a great extent by annealing the iron bars very well, and the inertia of the electric fluid.

We are indebted to Professor Jacobi for the exposition of the principal obstacle to the perfection of the electro-magnetic engine. He has shewn that the electric action produced by the motion of the bars operates against the battery current, and in this way reduces the magnetism of the bars, until, at a certain velocity, the forces of attraction become equivalent to the load on the axle, and the motion in consequence ceases to accelerate. The Russian philosopher had not, however, given precise numerical details concerning the duty of his apparatus, nor had he then determined the laws of the engine; I was therefore induced to construct an engine adapted for experimental purposes, in order in some measure to clear up these points. The experiments which I instituted two years ago with this instrument, were fully detailed in the Annals of Electricity, vol. 4, p. 474; I need not, therefore, allude to them in this place further than by stating some of their principal results.

I found, 1st, That the counter electric action, or, in other words, the magnetic electric resistance to the battery current, is proportional to the velocity of rotation and the magnetism of the bars.

2nd, That the economical duty at a given velocity of rotation is a constant quantity, whatever the number of similar pairs in series (provided that the resistance of the battery is kept the same).

3rd, That at small velocities great advantage is obtained by reducing as far as possible the resistance of the battery, and by arranging the coils so as to facilitate as far as possible the transmission of the current.

And 4th, That the economical duty at a given velocity, and for a given resistance of the battery, is proportional to the mean intensity of the several pairs of the battery.

With my apparatus, every pound of zinc consumed in Grove's battery produced a mechanical force (friction included), equal to raise a weight of 331,400lbs. to the height of one foot, when the revolving magnets were moving at the velocity of eight feet per second.

Now the duty of the best Cornish steam-engines is about 1,500,000lbs. raised to the height of one foot by the combustion of each pound of coal, or nearly five times the extreme duty that I was able to obtain from the magnetic engine by the consumption of

a pound of zinc. This comparison is so very unfavourable, that I must confess that I almost despair of the success of electro-magnetic attractions as an economical source of power; for although my machine was by no means perfect, I do not see how the arrangement of its parts could be improved so far as to make the duty per pound of zinc much superior to the duty of the best steam-engines per lb. of coal. And even if this were attained, the expense of the zinc and exciting fluids of the battery is so great, when compared with the price of coal, as to prevent this class of magnetic engine from being useful for any but very peculiar purposes.

NEW CLASS OF MAGNETIC FORCES.

A few weeks ago an ingenious gentleman, of this town, suggested to me a novel form of electro-magnetic engine. He was of opinion that a bar was increased in length by receiving the magnetic influence; and that, although the increment was perhaps very small, it still might be found valuable as a source of power on account of the great force with which it would operate. At that gentleman's request, I have entered into experiments to ascertain whether his opinion was correct, and if so, into a calculation whether the new source of power might be advantageously adopted for the movement of machinery.

After some preliminary trials, I adopted the following method of experiment. A length of thirty feet of copper wire, one twentieth of an inch thick, and covered with cotton thread, was formed into a coil twenty-two inches long and one-third of an inch in interior diameter. This coil was secured in a perpendicular position, and the rod of iron, of which I wished to ascertain the increment, was suspended in its axis so as to receive the magnetic influence whenever a current of electricity was passed through the coil. Lastly, the upper extremity of the rod was fixed, and the lower extremity was attached to a system of levers which multiplied its motion three thousand times.

A bar of rectangular iron wire, two feet long, one-quarter of an inch broad, and one-eighth of an inch thick, caused the index of the multiplying apparatus to spring from its position, and vibrate about a point, one-tenth of an inch, in advance, when the coil was made to complete the circuit of a battery capable of magnetizing the iron to saturation, or nearly so. After a short interval of time the index ceased to vibrate, and began to advance very gradually in consequence of the expansion of the bar from the heat which was radiated from the coil. On breaking the circuit, the index immediately began to vibrate about a point, exactly one-tenth of an inch, lower than that to which it had attained.

By multiplying the advance of the index by the power of the levers, we have of an inch, the increment of the bar, which may be otherwise stated as of its whole length.

Similar results were obtained by the use of an iron wire, two feet long and one-twelfth of an inch in diameter. Five pairs of the nitric acid battery produced an increment of the thirty thousandth part of an inch; and when only one pair of the battery was employed, I had an increment very slightly less, viz., the thirty-three thousandth part of an inch.

This increment does not appear to depend upon the thickness of the bar; for an electro-magnet made of iron, three feet long and one inch square, was found to expand under the magnetic influence to nearly the same extent, compared with its length, as the wires did in the previous experiments.

I made some experiments in order to ascertain the law of the inTheir results proved it to be very nearly proportional to the intensity of the magnetism and the length of the bar.

crement.

Trial was made whether any effect could be produced by using a copper wire, two feet long and about one-tenth of an inch in diameter; but I need scarcely observe that the attempt was unattended with the slightest success.

A very good way of observing the above phenomena is to examine one end of an electro-magnet with a powerful microscope, while the other end is fixed. The increment is then observed to take place with extreme suddenness, as if it had been occasioned by a blow at the other extremity. The expansion, though very minute, is indeed so very rapid, that it may be felt by the touch; and if the electromagnet be placed perpendicularly on a hard elastic body, such as glass, the ear can readily detect the fact that it makes a slight jump every time that contact is made with the battery.

When one end of the electro-magnet is applied to the ear, a distinct musical sound is heard every time that contact is made with, or broken from, the battery; another proof of the suddenness with which the particles of iron are disturbed.

With regard to the application of this new force to the movement of machinery, I have nothing favourable to advance. An easy calculation on the basis of the modulus of elasticity of iron will shew that an electro-magnet, consisting of a bar of iron one inch square and three feet long, will exert a force of about ten pounds over the space of the twenty thousanth part of an inch, every time that contact with the voltaic battery is made or broken, providing the transmitted current is capable of saturating the iron. If, therefore, contact be made, and broken a hundred times per second, for an hour together, we shall have only fifteen pounds raised to the height of one foot. This force is, therefore, far too minute for the movement of machinery; and the duty per pound of zinc is vastly less than that of the common electro-magnetic engine.

We shall now examine the bearing of this new property of the electro-magnet on magnetic theory. We shall consider it in connection with the hypothesis of Ampère and pinus.

The theory of Ampère refers the phenomena of magnetism to the

attraction and repulsion of currents of electricity moving in the same or contrary directions. Fig. 1, plate 4, represents the section of six particles of a magnetized bar of iron, according to (perhaps) the best modification of that philosopher's theory. The black circles represent atoms of iron, and the shades around them represent atmospheres of electricity moving in planes at right angles to the axis of the magnet.

This theory affords a good explanation of most cases of magnetic attraction. But the physical conditions which are demanded by it are impossible, and contrary to the analogy of nature, for it must necessarily suppose motion, or at least an active force to be continued against antagonist forces, for an indefinite length of time, without loss, in order to explain the phenomena exhibited by a hard steel magnet.

The only way in which it can account for the fact that iron, after receiving a certain quantity of magnetism, is incapacitated from receiving a further supply, or becomes saturated, is by supposing that the electricity which revolves around each atom of iron has a centrifugal tendency. The velocity of the electric currents around the atoms of iron will tend to be proportional to the inductive influence which urges them, and if the electricity be not endowed with centrifugal force, it is difficult to say why it should refuse to travel beyond a certain velocity; and in that case the phenomenon of saturation is left unexplained. If, however, the momentum of electricity, and its consequent centrifugal tendency when rotated, be supposed to exist, the currents will be prevented from going beyond a certain velocity by their interference with one another.

These principles are, however, less successful in accounting for the increase of length which we have noticed in a bar of iron when under the magnetic influence; for as the electricity is supposed to revolve in a plane at right angles to the axis of the bar, the divergence of the fluid from each atom of iron by centrifugal force would have the effect of shortening the mass of iron, which is directly opposed to experience.

If we now turn to a corrected theory* of Epinus, we shall see that it explains very well, facts which are unaccounted for by the former theory.

Let the black circles in Fig 2 represent six atoms of iron; the shades around them atmospheres of magnetism; and the white rings over these still rarer atmospheres of electricity. Further, let the space between the atoms be supposed to be filled with calorific ether in a state of vibration, or otherwise, to be occupied with the oscillations of the atoms themselves. Such a state of things will probably give a good idea of part of a bar of unmagnetized steel or iron. Now, if an inductive influence be applied to the atoms represented in Fig. 2, the magnetism is supposed to accumu

See Dr. Roget's Treatise on Magnetism (s. 133).

late on one side of the atoms of iron, as represented by Fig. 3, and the bar is rendered magnetic.

Such a theory seems to me to afford a natural and complete expression of facts. It supposes nothing which we cannot readily comprehend, except the existence and elementary properties of matter, which are necessarily assumed by every theory, and which the Great Creator has placed utterly beyond the grasp of the human understanding.

When all the magnetism of each atom of iron is accumulated to one side, such atom may be said to be saturated with magnetism.

It is obvious, that when the magnetism is accumulating at one side of each particle, the bar will increase in length in the direction of its polarity, and decrease in a direction at right angles to this. The former fact we have proved by experiment, and there can be little doubt that a very delicate apparatus would exhibit the diminution of the thickness of a bar of iron in consequence of the communication of magnetic virtue.

Our theory will also account for the fact, that at a certain degree of heat all the magnetic power of iron is destroyed. I have before observed that the space left between the magnetic particles in Fig. 3 represents the room taken up by their vibration. This vibration is called heat, and will, of course, increase in violence and extent with the increase of the temperature of the bar. Now, it is natural to suppose that the atoms of iron have far greater weight and inertia than the atmospheres of magnetism and electricity which surround them; therefore these atmospheres will be in a state of vibration, while the atoms of iron remain in a state of comparative quiescence; and when their vibration has reached a certain extent, the inductive influence will not be able to arrange the magnetism in any definite direction with regard to the atoms of iron.

The retentive power may be explained by supposing the magnetism to adhere to the atoms of iron to a certain extent. And if we make another supposition, viz., that an atom of iron, on combining with an atom of carbon, loses its attraction for magnetism on that side which is next the carbon, the superior retentive power of steel in comparison with that of iron is explained.

Ladies and gentlemen, I have had the honour of laying before you a theory of magnetism, first originated by Epinus, and somewhat modified by myself. In my opinion it affords a simple and comprehensive expression of facts, which is the chief value of any theory. Whether or not such a state of things as I have assumed really exists in nature, it will require further experiments to demon

strate.