of a cubic inch, for the weight of an additional atmosphere. Now as the body consists of solids and liquids of almost incompressible matter, and there is only a small part of the volume consisting of elastic fluid that is compressible, no material change of volume can take place, but on the sudden transition from one atmospheric pressure to another; and unless a change of volume take place, we cannot feel any pressure, either inward or outward. The phenomena of the water hammer shew, that the particles of water are hard, as they strike each other like flint and steel; and it is exceedingly probable that other bodies, solids as well as liquids, are constituted in like manner. A general pressure on the system, then, only increases in a small degree the attraction of the ultimate particles, and it is met by a corresponding increase of repulsion from the atmosphere of heat; so that the system remains as nearly as possible the same, and unaffected by such pressure.

I can scarcely forbear observing on the present occasion the absurdity of those who remark, that all people might swim, and that it is only from fear or ignorance of the art that some fail in the attempt. When we see that some persons are heavier than water, and others only .8 of that weight, it would be just as plausible for a piece of deal to upbraid a piece of lignum vitæ with the inability to swim from fear, or from want of skill in the art, which the deal considered of easy acquisition.-Manchester Memoirs, vol. v. New Series.

Chemical Analyses of Spinel, Gahnite, and Chrome Ore.
By Mr HERMANN ABICH.

Constituent Parts.

Massive Crystalliz.

Blue Spinel, Red Spinel, Pleonaste, Pleonaste, Gahnite, Gahnite, Chrome Chrome
Sweden.
Ceylon. Ural. Vesuvius. Sweden. America. Ore.

Ore.

On the Uniform Permeability of all known Substances to the Magnetic Influence, and the Application of the fact in Engineering and Mining, for the Determination of the Thickness of Solid Substances not otherwise Measurable. By the Rev. WILLIAM SCORESBY, F. R. S. Lond. & Edin., Correspondent of the Institute of France, &c. &c. Communicated by the Author. Concluded from p. 334 of preceding Vol.

INVESTIGATION OF THE LAW OF THE DIRECTIVE POWER OF BARMAGNETS, AND THE EXTENT TO WHICH THE MAGNETIC INFLUENCE MAY BE CARRIED FOR MEASURING DISTANCES.

2. THE law of the directive power of bar-magnets, at different distances, was the next subject of investigation.

Coulomb, I believe, was the first to establish, by the test of satisfactory and consistent experiments, the previously assumed law, that the force of magnetic attraction and repulsion is in the inverse ratio of the squares of the distance. The application of this law to the investigation in hand served at once to verify the law, and to render the results of my experiments of general application. In regard to the comparison of distances, it appeared to me to be of considerable advantage to estimate all distances in lengths of the bar made use of, by which the results for any one bar became applicable to all other bars of a proportional form and quality, and state of magnetic energy. And such, therefore, with a certain modification, afterwards found to be necessary, was the measure constantly adopted.

Placing now the magnet in the direction of the east or west point of the compass, or at right angles to the magnetic meridian, I proceeded to ascertain experimentally the deviations produced, either by the same pole constantly, or by the mean of the action of each pole alternately, first at the distance of one length, and then successively at other distances to the extent of ten lengths of the bar.

Preparatory, however, to a general application of the results thus obtained, it will be useful to ascertain by calculation the actual force exerted by any magnet on a compass at different distances, according to the above law of attractions.

VOL. XIII. NO. XXV.-JULY 1832.

G

When the bar is placed in the prescribed position, with the north pole at the distance of one length from the compass, then the action of the south pole, tending to counteract that of the north, will be in the inverse relation with the nearest pole of 22 to 12. That is, if the force of the nearest pole be called 1, then the force of the remote pole will be inversely as 4 or 4th, which being in the contrary direction to that of the nearest pole, reduces its action to ths. In like manner, at two lengths of the bar, the force of the nearest pole being now inversely as the square of 2 or 4th is reduced by the remote pole at three lengths distance, the action of which is inversely as the square of 3 or th; hencerepresenting the actual influence or

1

3

4

= 9

5 369

general resultant of the whole of the magnetic forces in the magnet acting upon the compass.

But we may obtain a general expression for all distances, either in lengths or fractions of lengths of the bar.

Let F be the influence of the nearer pole at the distance a.

Fa2
x2

Then will 22 represent the influence at the distance x, and

the counteracting influence of the remote pole at the dis

As the force F, however, being the separate action of a pole not practicably separable, is not a quantity that can be immediately ascertained by experiment, this expression requires to be extended so as to connect it with the value of the assumed force :

Let R be the resultant influence of a magnet on the compass at distance a, and, in the first instance, let a be equal to x.

The nature of the forces acting upon the needle, at various distances, with the general results, and a reduction of these results to other ratios, are exhibited in the following Table.

G 2