two extremities of A'B' are united by a conductor, the negative electricity of a' unites with the positive of h', and at the same time the contrary electricities of each of the particles of a', b', c', d', e', f, g' combine, and thence there results a current which progresses in the conductor from B' to A' and from A' B' in the wire A'B' itself. Thus A'B' is traversed, in this case, by a current directed in the same direction as the inducing current. The state of electric tension which exists in the wire A' B' whilst the current traverses A B is that which Faraday has called electro-tonic; and the cessation of that state produces the second current of induction, while its creation produces the first. * * * In reality in the above theory which we have given, the production of the two instantaneous induced currents is altogether similar to that which passes in the charging and discharging by cascade of several consecutive Leyden jars, in which the inner coating of each communicates with the outer coating of the preceding one." De la Rive then shows how this hypothesis is applicable to the explanation of the production of induced currents, by the approach and recession of a current in reference to a closed circuit; to the production of induced currents by magneto-electric currents; and to all the facts embodied in Lenz's laws.

*

It appears to me that the truth of the above hypothesis can be tested in the following manner. A closed metallic circuit (which we will call the secondary wire), is in proximity to a wire carrying a voltaic current, (which we will call the primary wire), which produces in the former powerful induced currents, on making and breaking battery contacts. While in these circumstances, we pass through the secondary wire a direct current, and measure with precision its intensity; then an exactly equal current is passed in an inverse direction, and its intensity measured. Now, if as De la Rive supposes, the secondary wire be in a state of polarization like in character but inverse in direction to that produced in the primary by the passage of the current, there will be a diminution in the intensity of the current which traverses the secondary wire in a direction opposed to this polarization, while that traversing it in the direction corresponding to this polarization, will have the same intensity as when it traverses the secondary wire, when the primary current is not in its proximity.

Four conditions have to be fulfilled in the above experiments. First, a strong inductive action must be brought to bear upon the secondary wire; 2dly, this action must be produced by a constant current; 3dly, the secondary wire must be a good conductor, and therefore cannot have in its circuit a liquid battery, wherewith to propagate the current in this wire; 4thly, a means must be devised of producing at will, in the secondary wire, a

current or wave of electricity which shall always have the same intensity. The following apparatus combines these requirements in an eminent degree. Twenty-five spirals (such as are described in my papers in the Sept. and Nov., 1870, Nos. of this Journal) were placed between twenty-five similar spirals, so that each alternate spiral belonged to the same circuit. The terminals of the first series of spirals were connected with a galvanic battery, while those of the second combination were connected with a spiral similar to one of those described in the paper "on the measurement of electrical conductivities," in the Nov., 1870, No. of this Journal. This spiral was placed on a magnet, about 3 ins. distant from its end, so that by suddenly slipping it off a current could be produced in the secondary wire without the interposition of a liquid. By interchanging the connections of the terminals of this spiral, an inverse current could be produced on sliding it off. A reflecting-galvanometer, reading deflections to 15", was also placed in the above circuit.

If the spiral is always removed from the same length of magnet with the same velocity, it necessarily follows-the circumstances in each case being alike-that successive currents of equal intensity will traverse the secondary wire. This constancy of velocity in all the experiments can be produced in the following manner. The magnet was placed vertically, N. pole downward, and the terminals of the spiral, which were 18 ins. in length, were connected with the ends of a cylinder of copper, transversely divided by an insulating disc of ebonite. Each end of this horizontal cylinder rested on a V, and a close contact was ensured by amalgamation. The spiral was now allowed to fall off the magnet, and as it always made this movement with the same velocity, it followed that currents of equal intensity were produced during each fall. It was however found that currents of such equality of intensity were produced, by quickly slipping off the spiral, that the range of their differences did not exceed 20′′ in the deflection of the needle of the reflecting-galvanometer. In the experiments which follow, I used the currents produced by quickly removing the spiral from the magnet. This ready method of producing at will an electric current or wave of any required intensity, seems to me will be of value in many electrical researches, for I have found that any current, produced by chemical means, fails in constancy when its intensities are measured by a reflecting-galvanometer, though by the ordinary instrument it may appear so.

In the apparatus thus arranged we have a combination of 25 spirals, containing 367 feet of inch copper wire, in proximity to a combination of 25 similar spirals. A current was passed through the first spirals, of such a strength that the resistance in the battery equalled the outside resistances in the spirals, &c.

After the violent action at the galvanometer produced on making contact at the battery had subsided, and the needle was at rest, its position was determined. The spiral on the magnet was now slipped off, and the deflection of the needle precisely observed. The battery current was now broken; the connections of the terminals of the spiral interchanged, and contact at the battery formed. The spiral was now again removed from the magnet, and the deflection measured. It was found that the deflections of the needle in the first and second experiments gave (for the mean of 6 experiments of each) angles sensibly equal, and that with a galvanometer reading with precision, a deflection to 15", and whose needle made about three vibrations per minute. These experiments were many times repeated with the current of the battery reversed in the primary spirals as well as in the magneto-current in the secondary spirals, and always with the same results.

Therefore this fact (irrespective of any theoretical consideration) is established:-that a definite electric current, traversing a metallic circuit in proximity to another traversed by a powerful voltaic current, has the same intensity whether passed in the same direction as the latter or in a direction opposed to it. There is, however, no doubt, a diminution in the velocity of this current similar to that observed in submarine cables; and it will be interesting and important to ascertain whether that velocity is the same in a direct as in an inverse direction. At another time I propose solving this question, and it would be well to reserve until then any hypothesis as to the real condition of a closed circuit contiguous to another carrying a voltaic current. It has always, however, appeared to me that the explanation of dynamic induction given by Prof. W. A. Norton (in this Jour., Jan., 1866,) in his paper on "Molecular Physics" affords a simpler and more consistent explanation of these phenomena than any heretofore framed.

Faraday (Exp. Res. 20 and 33) has also attacked the problem discussed in this paper. He introduced a small voltaic arrangement in the circuit of the secondary wire so as to produce a deflection of 30° or 40° in the galvanometer needles, and then a powerful battery was connected with the primary wire. He found, that after the deflection produced by the induced current had subsided, the needle resumed its former position and such he found was the case whichever way the contacts were made. For the following reasons I do not consider these experiments sufficiently refined to have solved so delicate a problem. First, from many experiments, which I have made, I have shown that no voltaic combination, however constant in the usual acceptation, can be formed so as to hold a galvanometer-needle even approximately in one position for only a minute, when it is ex

amined by the method of reflection. Secondly, a needle already deflected 30° or 40° has lost its delicacy in showing any minute increase or diminution in the intensity of the current; thirdly, the greatest objection to this method is this very feeble induced current that can be sent through the great resistance of the battery in the circuit of the secondary wire; finally, the manner of reading the galvanometer indications is entirely too gross.

Faraday also (Exp. Res. 3186) approximately showed that when a magnet is quickly introduced into a wire loop or quickly withdrawn from the same that the currents induced in the latter, as measured by an ordinary galvanometer, are of the same intensity; but a deflection on such an instrument cannot be read closer than 15', while in my experiments I have shown with more refined apparatus that magneto-electric waves can be evolved not differing more than 20" in the deflections which they produce.

Though ignorance of science excuses one no more than ignorance of the law, yet I may remark that this apparatus was devised and the results obtained before I was aware of Faraday's work in the same direction.

Those who would use this means of obtaining magneto-electric waves of equal intensity must guard against changes of temperature; for both the intensity of the inducing magnet and of the galvanometer needles vary with the temperature, while the resistance of the wire of the spiral and of the galvanometer rises and falls with the same. Indeed if a method could be devised of keeping the wires and the galvanometer needles at a constant temperature while we altered that of the inducing magnet a refined method would present itself for the determination of the variation of the force of magnets with the temperature. Probably the effect of the heat upon the wire circuit could be tabulated and eliminated from the results of such experiments, thus leaving a residual which will express the effect of temperature on the magnet. Or, a differential apparatus might be devised, consisting of two magnets with two wire circuits passing through a differential galvanometer. Having brought the induced currents of the two magnets to equal actions by an interposed resistance, we could then, by altering the temperature of one of the magnets, ascertain its effect with great precision; and it could be expressed in the length of resistance required again to balance the disturbed equality of the magnets.

November 7, 1870.

ART. V.-Abstract of the Programme for the Observation of Stars to the Ninth Magnitude, undertaken by the German Astronomical Society.*

THE Astronomical Society undertakes the construction of a complete catalogue for the northern heavens of the stars of the first nine magnitudes, upon the basis of the Bonn Durchmusterung. The region to be observed lies between -2° and +80° of declination. The region around the pole is not included, as the labors of Carrington, the Kasan and the Hamburg observatories render a repetition of this work superfluous on the part of the Society. The work will be distributed as follows: Kasan 80° to 75°; Dorpat 75° to 70°; Christiania 70° to 65°; Helsingfors 65° to 55°; Cambridge, U. S., 55° to 50°; Bonn 50° to 40°; Chicago 40° to 35°; Leipzig 35° to 30°; Cambridge, Eng., 30° to 25°; Berlin 25° to 15°; Leipzig 15° to 10°; Mannheim 10° to 4°; Neuchâtel 4° to 1°; Palermo + 1° to -2°.

The limits of these zones refer to the equinox of 1855 0, which is that of the Durchmusterung. For purposes of comparison, the limits above given will be exceeded 5' to 10', and in very northerly regions even more.

In particular the stars to be observed are as follows: All stars of the D. M. within the prescribed limits having a magnitude of 90 or brighter; all stars fainter, which also occur in the Histoire Celeste (marked in the D. M., L), or in the Königsberg zones (marked K), or in the Bonn zones (marked A); a part of these latter, which have been recently determined at Bonn, (marked B). A comparison of the older collections of observations is consequently necessary for completeness.

Every star will be observed twice. Whenever necessary to discover disagreements, a third observation should be made. The observations will be differential in their character, and will depend upon a catalogue of 539 zero stars, to be determined at Pulkova.

To facilitate reduction as well as observation, it is recommended to the observers to divide the zones into subzones. It is deemed inexpedient to observe zones of a length of more than one hour and a half, both on account of the physical fatigue and also the too wide separation of the fixed points of reference. Two zones of one and a half hours each are preferable to a single zone of three hours.

The clock error and pole point will be determined altogether by means of stars from the Pulkova catalogue, mentioned above. At least two zero stars must immediately precede and follow

* Prepared for this Journal by A. N. SKINNER, of the Naval Observatory, Washington.