equator, will be 5' or 6'. In London, the magnetic colatitude being 35° 12', it ought to be about 13′ or 15'; at Port Bowen, 1° 52', or 2° 15', all which quantities are very consistent with observations at those places.

Again, it follows from the hypothesis, that the daily variation ought to be much greater, and the change of daily intensity also greater in our hemisphere, while the sun has northern declination, than when his declination is south, or than when he is in the equator; because he will then approach so much nearer the magnetic pole than in the latter cases; and, in whatever way the influence takes place, we may expect it to be greatest when its action is most direct: this circumstance is also fully confirmed by observation.

There is, however, one point, and only one that I am aware of, that has the appearance of being opposed to the theory we are examining, and this in candour ought to be stated. It is this, that although we ought to find, as we really do, a greater change in the daily intensities, as the sun advances to the north, yet the mean daily intensity ought to be nearly the same; whereas, by referring to our tables, it will be found to be constantly decreasing, from the 1st of January, when the experiments began, to the end of April, when the needle was magnetized, without any such change of temperature, as is sufficient to account for the circumstance. This anomaly, as it is the only one we have met with, leads us to suspect some other cause, and I think it by no means an improbable one, that the daily and hourly use of this needle for four months, might lead to a deterioration of its own magnetic power; and that some such effect was noticed, seems probable, by the needle being re-magnetised on the 1st of May, particularly as its intensity was so much increased by this operation, which could not have happened, had the needle maintained itself in a state of saturation.

=

If this was the case, it necessarily prevents us from comparing the intensity of one month with that of another, although the hourly changes will be too small to be affected by this cause. I have not at present made any reference to the experiments performed at the Whale Fish Islands, because their number is inconsiderable, and they cannot, therefore, be supposed to have the same weight as the preceding, in a case of this kind. It is, however, satisfactory to find, that they still agree with the hypothesis which has been advanced. If we go through the same calculation here as in the other examples, we find the magnetic polar distance ML 14°; the one of the equator Q = 68° 43′, answering to 4h 32m, or 7h 28m A. M., the time when the sun was on the magnetic meridian of Whale Fish Island; and 1h 32m P. M., for the time when the sun was at right angles to the same, and when, as we have seen, the daily variation ought to be the greatest westerly. The time of maximum is registered from 1h 10m to 1h 30m. Again, the amount here (the magnetic polar distance being 14°) ought to be 32' or 38', according to our preceding determination, and the quantity actually observed on one side of the meridian only as 23'; which, if the night easterly variation had been taken, would certainly have brought the total somewhere about these limits.

After such close accordances as those we have shewn, between theory and observation, in so many, and in such distant and peculiar situations on the globe, no one can, I think, hesitate in pronouncing, that that theory, in its general character, must be correct, although there may be variations due to dif ferent causes, which modify the results, as the wind and other circumstances influence the tides, without, however, in any way throwing a doubt upon the theory, which makes the moon the great primary agent, productive of those daily changes. These influencing or disturbing causes may furnish the subject of another communication. This I shall conclude by examining what is the present state of our knowledge, and what are our future hopes of arriving at a more perfect knowledge of the laws, which govern the phenomena of the magnetic needle in different parts of the earth. With reference to the former, I think we may venture to say, 1st, We know, that, from whatever source the earth derives its magnetism, it is by some inductive principle, the phenomena which it exhibit being wholly inconsistent with a state of determinate magnetic polarization,"Essay on Magnetic Attraction," p. 208.; 2dly, I have shewn, by an incontestible experiment, that such a power may be induced in a globe of any matter, and all the phenomena of terrestrial magnetism exhibited by the agency of electricity or galvanism, independently of any magnetic body whatever; 3dly, Professor Leibech has shewn, that such an electric motion may be induced in a body composed of different metals, by merely destroying the equilibrium of caloric within them; and, 4thly, It is shewn, by the highly valuable experiments of Captain Parry and Lieutenant Foster, that a magnetic disturbance is actually produced on the needle, and on the general magnetism of the earth, through the medium of the solar influence.

We have thus at least a glimpse of the probable cause of that hitherto mysterious, but invaluable property, of a magnetized needle, which disposes it to take up a determinate position; and, as we owe this conclusion, in a great measure, to the happy thought of Lieutenant Foster, which led him to make a series of simultaneous observations on the intensity of the horizontal and dipping needles, we do sincerely hope he may be enabled

to complete a comparison he has so happily begun, by being allowed to pursue his experiments in the southern hemisphere.

If, as has been said, all knowledge is valuable, that must be more especially so which contributes to the comforts, necessities, and preservation of human existence; and that a correct knowledge of the theory of terrestrial magnetism is entitled to rank in this class, cannot be doubted, when we reflect, that it would enable us to add to the facilities of commerce, to the security of navigation, and tend materially to the preservation from shipwreck and death, of those brave men to whom England is so much indebted for her pre-eminence amongst nations.

It is a subject, however, which cannot be pursued with advantage in the closet; at least the data must be drawn from accurate observations made in various and remote situations on the globe, which can only be obtained through the assistance and support of governments,-and no government can be so much interested in the inquiry, as that which directs the efforts of a nation, whose pride and boast is to be the first maritime power on the globe. The British Admiralty, then, is the rock on which we build our hope for a more complete knowledge of the laws of terrestrial magnetism; and, after the liberal support it has already given to these inquiries, we feel confident that our hope is not ill founded.

At some future time I propose to examine the magnetic experiments of Captain Sabine, and endeavour to show, that the theory in question will also serve to explain the anomalies he found in the deep intensity of the needle in the torid and temperate zones, and which led him to look for a pole of intensity distinct from that of direction,-not reflecting that the formula he employed was rendered inapplicable the moment he assumed the separation.

On the Use of a Simple Syphon as a Hydrometer. By Mr HENRY MEIKLE. Communicated by the Author.

AMONG

MONG the numerous instruments for comparing or ascertaining the specific gravities of liquids, the " pump areometer” has been for a considerable time known. This consists of a syphon, having its extremities immersed in two different liquids, whose specific gravities are to be compared, and having a pump or syringe communicating with the upper or bent part; so that on exhausting a portion of the included air, the atmospheric pressure raises the liquids through heights, which are inversely as their specific gravities. The reason of this is obvious, and was long ago particularly noticed by Boyle. In the last volume of the Philosophical Magazine, is a description of a different instrument, consisting of a double syphon, with four parallel legs; into each pair of which, a different liquid being put, with a portion of air between, the effective columns compressing that air will be inversely as their specific gravities. This instrument possesses the remarkable property, that if the bore of the tube, however narrow, be uniform, its indications will be entirely free from capillary action; because both extremities of the same liquid being equally affected by capillary attraction, the difference of their heights, or the effective column, is not altered thereby.

It appears, however, that, when the liquids are transparent, the syphon may be applied in a still more convenient form than either of those just mentioned, though, to avoid capillary action, the tube in this, as well as in the pump areometer, must not be narrow. Thus, if the legs of a simple glass syphon be immersed in different liquids, the lengths of the columns, depressed by the included air, will be inversely as their specific gravities. The vessels containing the liquids only require to be transparent, such as glass bottles or jars. Any scale of small equal parts may be attached to the tubes; but it will be still simpler, and more convenient, for corrosive liquors to graduate the tubes themselves; for, in proper hands, a glass tube is as easily divided into equal parts as any thing else.

But with the assistance of a little calculation, the simple syphon may be used for comparing the specific gravity of an

opaque liquid with that of a transparent one, as for instance water.*

Let a be the volume which the air included in the syphon had under the external barometric pressure b, and c the increase of pressure occasioned by the immersion which will be proportional to c', the volume of air below the surface of the water; also let e be the volume of water in the bottom of the tube. Then the ab reduced volume of air is and the volume of the other b+c'

ab

a c

liquid within the tube, is a

e, which

b+c

b+c

subtracted from g, the whole contents of the tube under the

level of the opaque liquid, gives g+e

ac

b+c

for the column

of air below the surface of the opaque liquid, and if by this we divide c', the quotient is the specific gravity sought.

In this case, it is supposed that the bore of the tube is uniform, and that both legs enter their respective liquids at the same instant. But when the two specific gravities are very different, unless care be taken not to immerse the syphon too far, some air may escape from the end which descends into the lighter fluid. This may also be avoided, by gradually lowering the vessel containing the heavier liquor, or raising the other whilst the syphon is descending; but that will seldom be necessary, and the escape of a little air will not affect the simpler method, to be used when both liquids are transparent.

On the Live Marine Cockles, said to have been found at a great distance from the Sea in Yorkshire. In a letter to Professor JAMESON. By W. C. TREVELYAN, Esq. M. W. S., &c.

[Nearly a year ago, my intelligent friend Mr Witham of Lartington, had sent to him in Edinburgh several specimens of live marine cockles, said to have been found in a bog, considerably above the level of the sea, and fully forty miles distant

* In some opaque liquids, as for example mercury, the top of the column within the tube may be rendered visible by bringing the leg of the syphon close to the side of the jar; in such cases, no additional calculation is required.