was proportionally diminished. At about half-past four o'clock, in the evening, the gases at the throat were peaceable and their flow feeble. A minute before the explosion, a projection took place at the tuyere (the embrasure had always remained open;) the gases which enveloped the hot air apparatus detonated, and a current of gas issued from the tympe. The projections then began from the throat, and lasted for nearly two minutes. The furnace was almost entirely emptied. At this time, also, the flames issued from the crevices and interstices of the masonry.

Upon an examination of the interior of the furnace, there was found no trace of any permanent obstruction, by semi-fused masses cemented to the inner walls of the furnace.

Both these furnaces were similar in their dimensions; both worked easily fusible ores in fine grains, yielding about forty per cent. of cast iron; both used as the combustible, a mixture of charcoal and baked wood (or rather dried wood, for it rarely loses more than thirty per cent of its weight.)

M. Sauvage, the Engineer, to whom we owe this account, attributes these explosions to the evolution of gases from the wood, within the cavities which form in the furnaces during the irregular action described. The intense heat finally gives to these gases sufficient tension to burst suddenly the opposing barriers, and thus the explosions take place.

An account of a similar accident which occurred upon the 24th of December, 1840, at the furnace of Vanvey, Department Côte d'Or, is given by M. de Nerville. It would seem that in this case, the effect was due to the very irregular action of the hot-air blast, which was heated only by the combustible gases conducted from the furnace throat. The heat given to the blast must consequently vary materially with the quantity of gas issuing from the furnace. The explosion took place from the tympe.

"The examination of the facts which preceded and followed this unfortunate accident, does not permit us to doubt, that the projection of the matters contained in the crucible was solely due to the fall of a large mass of the ore not yet deprived of water, upon the liquid iron and slag. A vault had formed in the furnace, and that there existed a large empty space below it, is evident from the fact that after the accident, the ore and charcoal contained in the furnace was so thrown down, as to present at the throat a depression of nearly six feet. It may easily be conceived that the fall of such a mass into the crucible would force out iron and slag, and cause the instantaneous formation of steam."

* The apparatus, arranged at the tunnel-head, to collect the gases, presented, moreover, the disadvantage of preventing the proper arrangement of the ore in the furnace, which instead of being evenly distributed, was piled up around the walls.

FOR THE JOURNAL OF THE FRANKLIN INSTITUTE.

Strength of Iron Wire at a low Temperature.

The following experiments were made with iron wire inch in diameter, subjected to direct strain:

Notice of a Spiral Magnet, by which Secondary Currents may be demonstrated in the body of the Magnet. By CHAS. G. PAGE, M. D., Washington, D. C.

More than three years since, while investigating the action of closed secondary currents, I was led to the conclusion that these currents must be developed in the body of the magnet itself, as well as in the coils of metal surrounding the magnet. (See Silliman's Journal, vol. 35, No. 2, page 255.) Prof. Henry, engaged about the same time in a similar train of investigations, had arrived at the same conclusion. b

We had both, however, been anticipated by the conjectures of Prof. Ettinghausen, of Vienna. Since that time the existence of these currents has been regarded only as a matter of reasonable inference, and I know of no attempts to elicit and demonstrate them by the common tests. In the experiments above alluded to, I failed to detect these currents, from the want of a delicate Galvanoscope; but the magnet about to be described, affords currents of sufficient magnitude to be appreciated by shocks and bright sparks.

2.

e

d

The magnet m, fig. 1, consists of a long sheet of very thin iron, rolled up in the form of a cylinder, and covered with three layers of insulated copper wire, the ends of which are soldered to the single wires a, b, for the purpose of communication with a galvanic battery. A sheet of paper is rolled up with the sheet of iron to insulate the several turns of the spiral. The wire c is soldered to the middle of the outer edge of the sheet; the wire d is also connected with the outer edge. The wire e is connected with its inner edge. Fig. 2, is an end view of the magnet exhibiting its spiral turns, with a fold of paper between. When the wires a and b are connected with the poles of a galvanic battery, the inclosed sheet of iron becomes a strong magnet. When the galvanic connexion is broken, a brilliant spark occurs, showing a power in this respect, superior to that of a solid bar of iron. If the ends of d and e be joined, a momentary current is obtained both at the completion and the rupture of the galvanic circuit. Ife and c be joined, the current is much stronger than from d and e. It appears from this, that the wires d and e conduct the currents from only a single transverse section of the magnet, while the currents from e and c are constrained to deviate from their natural course, viz. at right angles to the axis of the magnet, and take an oblique direction from one angle of the sheet to the middle of the opposite edge. The direction of these currents I have ascertained is in conformity with the law of secondary currents, as follows:-The initial secondary is in opposition to the primitive current, the terminal secondary in the same direction with the primitive.

S

Description of a new plate, or quantity, Helix, for Electro Magnetic Apparatus. By CHAS. G. PAGE, M. D., Washington, D. C. The design of the new helix, is firstly to obtain a maximum of magnetic influence, by surrounding an iron bar with the greatest possible number of circumvolutions of conducting metal within a given space, and to bring those conducting circuits as near as possible to a direction at right angles to the axis of the magnetic bar.

Secondly, the plate helix affords a means of modifying magnetoelectric currents, so as to obtain the maximum of their magnetizing influence. I have therefore called this a quantity helix, to distinguish

it from helices of wire, in which the magnetizing property of the currents diminish in a certain ratio to the extent of wire employed.

When a magnetic bar is to be covered with copper wire wrapped with silk or cotton, it is evident that the double thickness of the insulating material, increases the obliquity of the wire to the axis of the magnet, besides occupying space which should be given to the conducting metal. The round form of the wire also, necessarily involves much waste room. The conditions requisite to the full economy of magnetizing power seem to be well answered by the plate helix. a, fig. 2, represents one of the plates of which the helix is composed. It is an annular plate of copper, about three inches in diameter, the opening in the centre being from three quarters to an inch in diameter, to admit the magnetic bar. The plate is cut open at e, and one of these cut edges is soldered to the edge of a similar plate, and thus the series continued to any extent desired. Upon the upper surface of each plate a piece of thin paper of the same size and form is fastened with varnish, for the purpose of insulation. b represents the helix with the copper wires c and d attached to its extremities. I have not yet fully ascertained the value of this helix, but from the experiments already made with it, am confident it will answer my expectations. It magnetizes powerfully, gives brilliant sparks, and will be a valuable instrument for exhibiting the magnetic effects of mechanical electricity, as it can be stretched open to an extent which will not allow the electricity to pass from one plate to another, except in the direction of continuity.

Franklin Institute.

The Annual Meeting of the Institute was held at their Hall, January 20th, 1842.

THOMAS FLETCHER, Vice President, in the Chair;

GEORGE W. SMITH, Recording Secretary, P. T.

The minutes of the last meeting were read and approved.

Donations were received from the Hon. Charles Brown, Member of Congress; the Young Mens' Mercantile Library Association, of Cincinnati, Ohio; Prof. Alex. Dallas Bache, Jno. D. Koecker, John White, Charles B. Trego, Prof. Walter R. Johnson, Charles Ellett, Jr., A. D. Chaloner, M. D., Henry R. Campbell, the Select and Common Councils of the City of Philadelphia, Andrew Fountain, A. Pardee, Zebeelon Parker, of Newark, Ohio; Calvin Olds, of Marleboro', Vt., and from the Estate of John Ronaldson, Esq., deceased.

The Corresponding Secretary laid on the tables the periodicals received in exchange for the Journal of the Institute.

The annual report of the Board of Managers was read and accepted, and referred for publication.

The Treasurer presented his report of the funds for the last quarter, and also a statement for the year ending December 31st, 1841— which were read and accepted.

Mr. Henry R. Campbell, from the Committee of Tellers of the annual election for Officers and Managers of the Institute, for the ensuing year, (appointed at the preparatory meeting this day,) presented their report of the result of the election, when the Vice President declared the following gentlemen duly elected:

SAMUEL V. MERRICK, President.

ISAIAH LUKENS,

THOMAS FLETCHER,

}

Vice Presidents.

ISAAC B. GARRIGUES, Recording Secretary.

ALEX. DALLAS BACHE, Corresponding Secretary.

FREDERICK FRALEY, Treasurer.

Eighteenth Annual Report of the Board of Managers of the Franklin Institute, of the State of Pennsylvania, for the promotion of the Mechanic Arts.

The Board of Managers beg leave to present their eighteenth annual report:

VOL. III. 3RD SERIES.-No. 3.-MARCH, 1842.

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