As a practical conclusion, it is obvious that on ship-board great care ought to be taken to keep the chronometers out of the immediate vicinity of any considerable mass, or surface of iron; on which account they ought not to be kept in the cabins of the gun-room officers, which are on the sides of the vessel; and probably a strong iron knee, or even a gun, will be found at a very inconsiderable distance from the spot where the watch is most likely, in this case, to be deposited.'

A method is then proposed, and explained by reference to a delineation, for preventing the consequences to which the time-pieces on ship-board are liable from the effects of partial attraction; and the chapter concludes with a table in which the difference between the mean land and sea rates of the chronometer used by the Leven, during a voyage to the Cape de Verd islands in 1819, are shewn to have been very considerable, and calling for the attention of those who command ships.

The relative magnetic power of different descriptions of iron and steel, at different degrees of temperature,' is the subject investigated in the next section; and the experiments, from the accession of new and curious facts with which they have enriched the science of magnetism, will be considered as forming an important step towards a more perfect theory of this part of the philosophy of nature than any which has yet appeared.

The first course of experiments includes eight substances, viz. malleable iron; blistered steel, soft; blistered steel, hard; shear steel, soft; shear steel, hard; cast iron; cast steel, soft; cast steel, hard. Bars of each of these were provided, and each applied in such a manner to the compass, in their cold state, as to enable a determination to be made respecting the proportion of their several magnetic powers, compared with one another; which, in their nearest value to whole numbers, are thus expressed :

A variety of experiments follow, to try the relative magnetic power of iron and steel at different degrees of heat, all of which at a white heat lose their power on the needle: but, when the iron or steel has acquired the degree of heat termed blood-red, its influence in some instances was found much greater on the needle than when cold; and, in all cases, the blood-red is the degree of heat which produced the greatest magnetic effect.

D 4

A simi

A similar deduction follows some experiments on the comparative powers of malleable iron and shear steel at different degrees of heat; after which Mr. B. says,

I should, therefore, now have concluded my experiments on this subject, but for a circumstance which had been noticed, and which strongly attracted our attention. It had been observed both by Mr. Bonny castle and myself, in some preceding experiments, and others of which I have not given the results, that between the white heat of iron, when its power was actually zero, and the blood-red heat, at which its action manifested itself so highly, there was an intermediate state of the bar, during which it attracted the needle the contrary way to what it did when cold, viz. if the north end of the needle was attracted in the latter state, the south end was attracted while the heated iron passed through the shades of colour denoted by the workmen the bright red, and red heat. Our object was, therefore, now to examine this circumstance a little more minutely than we had hitherto done.' (P. 141.)

A great many efforts were made to arrive at a satisfactory reason for the peculiar effect mentioned in the foregoing citation, and a description is given of various other phænomena that appeared in the course of the experiments adopted on this occasion. The author then concludes thus:

The only explanation which seems to present itself of the cause of this anomalous action, is, that the bar cooling faster at its extremities than in its centre, one part of it becomes magnetic before the other, and hence gives rise to the irregular action above indicated. It must be acknowleged, however, that this explanation does not meet entirely all the phænomena recorded in the preceding table.' (P. 149.)

It ought, perhaps, to have been before signified that Mr. Barlow's Essay on Magnetic Attractions' now consists of three parts; the first of which, containing a detail of experiments, has been already explained. The second presents

a Theoretical Investigation of the Laws of induced and terrestrial Magnetism.' It appears that Mr. C. Bonnycastle, of Woolwich, (son of the late Professor of Mathematics) instituted an inquiry into the laws of the several phænomena which had exhibited themselves in the course of Mr. Barlow's experiments; founding it on the presumed similarity of action between electrified and magnetized bodies; and, employing the principles for establishing the laws of action in the former class of these bodies, as laid down in the volume of the French Institute for 1811, he had succeeded in constituting a partial theory of magnetism, from which the one here proposed differs but in some particulars that tend to make it somewhat

more

more general, and which do not confine it to Poisson's eledtrical principles.

The hypothesis upon which I shall proceed,' says Mr. Barlow, may be thus enunciated :

1. Magnetic phænomena are due to the existence of two fluids in a greater or less degree of combination, and such, that the particles of the same fluid repel, and those of an opposite nature attract, each other.

2. These fluids in iron bodies exist naturally in a state of combination and equilibrium, till that state is disturbed by some exciting cause.

، 3. But if a body, already magnetic, i. e. one in which these fluids are held in a state of separation, be brought within the vicinity of a mass of iron, such as is supposed above, the concentrated action of each fluid in the magnetized body will act upon the latent fluids in the quiescent body, by repelling those of the same, and attracting those of the contrary kind, and thus impress upon the latter a temporary state of magnetic action, which will remain only while the two bodies maintain their respective situ

ations.

4. The quantity of action thus impressed upon the iron body will depend, first, upon the intensity of the exciting magnet; secondly, upon the capacity of the quiescent body for magnetism, or the quantity of those fluids contained in it; and, thirdly, upon the cohesive power of the iron; which latter quality determines the depth to which the exciting magnet is able to disengage the two fluids.

، The above embraces every case ; viz. of any magnet, natural or artificial, developing the magnetism in any given iron body; but in that to which our attention will be principally directed, namely, the displacement occasioned by the magnetic action of the earth on spheres of iron, we shall find it more limited in its results, and more susceptible of correct mathematical investigation.

، 5. In this case, for instance, we may suppose the action to take place on every particle of the mass in lines parallel to each other, and corresponding with the direction of the dipping needle; also that every particle is at the same distance from the centre of the disturbing force, and consequently that the displacement in each particle is equal also; conditions which throw great facilities into the analytical investigation of the laws of action.'

We shall not, in the present place, offer any opinion on the merits of the theory: but we intend to say a few words respecting it when we have examined the whole that is adduced in its favor by the author, and have also gone through the subsequent part of the work, which treats of electromagnetic laws and experiments.

After a succession of experiments on spherical masses of iron, and on bars, in order to confirm the principles of the

theory,

theory, and the analytical deductions instituted to determine the value of the forces acting according to the terms of the hypothesis, this part of the investigation is closed in the following manner :

Upon the whole, therefore, I conceive that we may be allowed to cite these results as a further proof of the accuracy of the principles upon which our hypothesis is founded, and of the deduction we have made from it; viz. that the action of plain unmagnetized iron on a compass may be referred to two poles indefinitely near to each other in the common centre of attraction of the surface of the body, and, consequently, as a proof of the accuracy of the method proposed for correcting the local attraction of a vessel in all parts of the world.' (P. 188.)

Mr. Barlow now comes to apply the formulæ resulting from the investigation of the hypothesis to estimate the magnetic properties of the earth; and a number of authorities are collected relative to the quantity of the dip and variation of the needle in different parts of the earth, for the purpose of ascertaining by their means (provided that they have been taken correctly) the true place at the present time of the terrestrial magnetic pole, and its periodical motion. The analogy between the earth and an iron ball at this point vanishes; yet, although the inquirer fails in arriving at any positive conclusion, his opinions are of that stamp which commands interest; and we shall, therefore, close our examination of the second part of this volume with such extracts, as include the most popular points in this very peculiar feature of terrestrial magnetism.

Although in determinations of the dip and variation of the needle we cannot expect the utmost accuracy, yet it is very obvious,' says Mr. Barlow, from the preceding results, that the aberrations in the latitude and longitude of the magnetic pole are much greater than can be attributed to errors in observation. It will be seen that the place assigned to it differs in longitude as much as 55° between one set of observations and another, and as much as 10° in latitude. It will also be observed that the more we approach the north, and west, the more westerly we find the place of the pole, and the more easterly the place of observation, the greater is the latitude of the pole. In short, it is evident from the few examples we have taken, that every place has its particular polarizing axis, which probably in all cases falls within the arctic circle, and that this is the narrowest limit we are able to assign.

Instead, therefore, of the magnetism of the earth possessing that degree of uniformity which appertains to a perfectly formed iron ball, it may be rather said to resemble that species of action which we might expect to find in an irregularly formed mass of iron, approximating in its general character to that of a globe, but

not perfectly such; and if the magnetism of the earth be due to the distribution of iron in its interior, we ought in fact rather to expect à priori such a kind of action (as that which is experienced) than that which belongs to a perfectly formed iron sphere.

• It is true that the observations we have used were not made simultaneously, and that a change is perpetually going on in the direction of the axis of polarization, which circumstance alone would give rise to some discrepancies; but not to the amount shown in the preceding table.

Every place, therefore, appears to have its proper poles; and the only limit we are enabled to assign to their situations is, that as far as observations have yet been carried they appear to fall somewhere within the two frigid zones, but varying through all possible degrees of longitude and latitude within these limits.

• These aberrations being, however, attributed to local inequalities in the distribution of the ferruginous parts of the terrestrial sphere, we ought still to expect a certain degree of uniformity in the annual changes which take place in the situations of the poles of any particular place; supposing these changes to arise from some general cause acting equally on all. Let us then examine the circumstances attending the annual variation of the needle, and ascertain how far this phenomenon is reducible to determinate laws.' (P. 208.)

This inquiry forms the subject of the succeeding pages of this. part of the Essay; for the results of which we must necessarily refer to the volume.

Part the third of Mr. Barlow's Essay is divided into three sections, the first of which contains a memoir of the discovery of electro-magnetism by Professor Oersted, of Copenhagen: as also of the progressive expansion of the science in France under the skilful management of MM. Ampère, Biot, and Arago; and in England, aided by the philosophy of Davy, Cummings, and Faraday. The second section is occupied with investigations of the mathematical laws of electromagnetism; and the third presents a course of electro-magnetic experiments.

A few introductory remarks relate to certain approximations towards the discovery of this science, in the beginning of the present century: which are followed by an account of the several experimentál contributions furnished in succession by those who had taken an active interest in the promotion of the new-born science, until the author came forwards to throw into the already rich scale of its tributaries the accession of his talents. His particular object on this occasion will be best given in his own relation:

Such was the state of this science, when I undertook the experiments reported in the following section, and by which, if I have not deceived myself, the whole of the apparently anomalous

actions