ART. V. An Essay on Magnetic Attractions, and on the Laws of Terrestrial and Electro-Magnetism; comprising a popular Course of curious and interesting Experiments on the latter Subject, and an easy experimental Method of correcting the local Attraction of Vessels on the Compass in all Parts of the World. By Peter Barlow, Associate in the Society of Civil Engineers, and of the Royal Military Academy. Second Edition, much enlarged and improved. Illustrated with Plates by Lowry. 8vo. pp. 303. 12s. Boards. Mawman. 1823.

D ISCOVERIES by which light is thrown on those operations of nature that seem instrumental to the mechanism of the universe, and particularly if such discoveries appear at the same time calculated to improve the means of carrying forwards the intercourse of the moral world, are among the most interesting subjects that can be brought under our observation. From the many useful and valuable facts which have remained concealed through so many ages to the present, it may be remarked that physiology, notwithstanding it has been emulously cultivated by every enlightened nation, has advanced but tardily in times past. It may be useful also to observe that facts in the science of natural philosophy may be very satisfactorily established, and may remain acknowleged and admired from generation to generation, without any doctrines being deduced from them. Those who have been recently turning their attention with such encouraging success to the subject of magnetic influences will, we hope, be induced to enlarge their field of speculation; and to double their scrutiny, in order to detect, as far as possible, the agency of magnetic, galvanic, or electric effluvia in producing the atmospheric and almost all other natural phænomena that we daily experience, and to ascertain the laws according to which they are exerted. - After Copernicus had succeeded in restoring the Pythagorean system, and it had become authenticated by the common consent of all contemporary astronomers of any note, nearly a century elapsed before a Kepler rose to explain according to what laws the several members of the system were actuated to exhibit their peculiar phænomena; and after these facts and laws were both thus established, another century expired before the world heard a Newton declare the reason of the visible motions of the heavenly bodies, and demonstrate the mathematical principles on which the Pythagorean theory was founded.

These are lessons that urge us, when we have been chasing philosophy successfully through many windings and doublings, not to stand still and leave to those in our rear the honors which we, by a little farther exertion, might probably

achieve.

achieve. We would not be supposed to insinuate that Mr. Barlow has not followed up his pursuit with energy; on the contrary, it is not only the ability but the zeal that he has manifested in his scrutiny which lead us to wish and hope for the performance of more, relative to the same subject. It is now sufficiently ascertained that electricity and magnetism are closely allied; and magnetism, we know, has a very near relationship to that universal effect in nature which is understood by the term gravitation. Whether the electric and magnetic fluids, which seem to pervade all substances in a greater or less degree, have any part in producing the phænomena ascribed to gravitation, remains yet to be discovered: but it is, at any rate, desirable to become farther acquainted with that which, the more we know of it, the more it seems an essential agent in the various modifications of all material substances, and in the physiology of the atmosphere. Thus will it be seen how far these suggestions bear on the subject treated in the volume before us; and when we shall have examined the new facts and doctrines concerning magnetic and electro-magnetic attractions, that are afforded by Mr. Barlow, and shall have advanced such occasional remarks as appear due to the more important points of information which we may find in his Essay, a few farther observations relative to the intimacy between electricity, magnetism, and gravitation may probably be elicited.

Four years ago, we had the opportunity of expressing our approbation of the contents of a volume written by Mr. Barlow on the same subject as the present; (see Monthly Review, vol. xcii. pp. 18-29.) and the substance of that work, with some additional particulars, and a continuation of experiments relative to the effects on the mariner's compass-needle produced by the guns and other iron matter distributed about a ship, or used in its construction, form the first part of the publication before us. In this portion, however, we have merely the proposition for correcting the local attraction of vessels: after which we advance to the proof produced of its practical efficacy. In April, 1820, H. M. S. Leven being in readiness to proceed to the western coast of Africa, under the command of Captain Bartholomew, Mr. Barlow attended at Northfleet to superintend the operations necessary for determining the quantity of local attraction by which the compass of the vessel was influenced, and to give the officers directions for applying and using his apparatus for its correction. The detail of these operations, in the author's report to the Admiralty, will be read with interest by those whose attention is turned to the science of navigation.

On

On the return of the Leven from this service, that ship as well as the Barracouta, was fitted for a survey of the eastern coast of Africa, under the command of Captain Owen; and in January, 1822, Mr. B. was again called to see to the due preparations being adopted on board those ships, for the farther proof of the efficiency of his invention. For the particulars of the method of ascertaining how far the needle was drawn from its natural point, by the attraction of the metal contained in each of the ships, for a description of the correcting plate employed on this occasion, and the mode of adjusting it, and for the directions given to the officers for using the plate at sea, the reader must necessarily be referred to the work itself.

Concerning the test to which this expedient for neutralizing the forces, which prevented the compass from adequately performing the part on ship-board for which it is expressly consulted, had been submitted during the voyage of the Leven to the western coast of Africa, the author says:

To the officers of this vessel I am under the greatest obligation for the pains they took to give the method I had proposed the most impartial trial, and for the results they furnished me with on their return. From Captain Baldey (who had succeeded to the command of the vessel during the voyage, on the death of Captain Bartholomew,) I received a copy of the mean results of nearly 100 series of observations, with and without the plate; the local attraction at each, the variation, the latitude, the longitude, &c.; and about the same number from Lieutenant Vidal, Lieutenant Mudge, and from the Master, Mr. Higgs. It would be irksome to the reader to give him the series of observations: it will be sufficient to say, that they are not only highly satisfactory to me, but also to the officers themselves, as will appear from the following letter addressed to me by Captain Baldey on the return of the vessel.

"I have left for you, in the care of Lieutenant Mudge, a copy of the results of a series of observations, made by me on board his Majesty's ship Leven, with your correcting plate, attached to Gilbert's patent azimuth compass, the original having been already transmitted to my Lords Commissioners of the Admiralty, and I beg to congratulate you on the success which has attended the experiments.

"You will perceive that in several instances our binnacle compasses differed from each other a half to three-quarters of a point: which, however, we were always able to correct by your plate, and in all cases our place by reckoning, when thus corrected, agreed as closely with the observations as we could have reason to expect. Indeed little need be said to shew how very erroneous a place by reckoning must be found, after a run of several hours five, six, or seven degrees out of the supposed

course.

course. At sea such an error, although very considerable, is not perhaps of much importance, but in making land, in entering a channel, and in narrow seas, it might be, and doubtless has been, frequently attended with the most fatal consequences."

An instance is then adduced in a letter from Lieut. Mudge, in which it is stated that, after a run of only 24 hours, the latitude by common reckoning differed 19 geographical miles from the latitude by observation; and the longitude by the same reckoning differed 28 miles from the longitude by observation which great disparity, in so short a space of time, was ascribed to a current, until Lieut. Mudge, consulting the compass, with the correcting plate affixed, discovered that the starboard or steering compass varied seven degrees from the one which had been freed from the effect of local attraction; and that the reckoning by the corrected compass agreed with observation as nearly as, under any circumstances, reckoning and observation can be expected to concur.

The Essay, as far as it refers to the subject of local attraction on the compass-needle, finishes with the testimonies in favor of the practical utility of the invention received from the officers of the Leven, and then proceeds to detail an examination of the effects produced on the rate of chronometers by the attracting matter on board of vessels: but, before we enter on our account of this new subject, it will be proper to notice the additional experiments relative to local attraction made in long voyages by his Majesty's ships Conway and Griper, in high latitudes, both north and south; and to consider the farther evidences which they have each furnished as to the great services derived from the application of the correcting plate. The certificates of these ships on this subject are contained in a printed Report* to the Lords Commissioners of the Admiralty, by Mr. Barlow; which report is dated February 14th, 1824, and consequently furnishes a continuation, up to the 'present time, of the practical results that have been officially obtained relative to this important discovery.

After having enumerated the particulars already mentioned, as derived from the Leven, the Report thus continues:

An important question, however, relative to this method of correction, still remained to be decided. Captain Flinders had observed that with an equal north and south dip he found an equal quantity of local deviation, but in a contrary direction, the north end of the needle in one instance, and the south end in the other,

*This Report forms an Appendix to the Essay, and is published by the same booksellers.

being drawn forward by the action of the iron in the vessel; and it was of course of the highest importance to ascertain how far the power of the plate was competent to correct this strongly marked difference in the action of the ship. This point has, however, been completely answered by the observations made on board his Majesty's ship Conway, by Captain Basil Hall, in a voyage from England round Cape Horn to different parts on the western side of America, in the years 1820, 1821, and 1822.'

When the ship returned to Woolwich, Mr. Barlow went on board to superintend the experiments then to be performed.

'As the object was to ascertain the amount of deviation in the direction of the magnetic needle, by the combined action of all the attracting matter on board the ship; and as it was required to determine this quantity at various positions of the ship's head, she was warped to one of the transporting buoys in the middle of the harbour, round which, as a fixed centre, she was successively drawn, by means of hawsers, and held steadily at the required points while the observations were made.'

The particulars of these preliminary operations, and the amount of local attraction observed at each point, are then detailed; after which Captain Hall thus proceeds to relate the process subsequently followed at sea:

A set or several sets of azimuths were taken without the plate, then another set or sets with the plate affixed, the ship's head and all other circumstances remaining the same: the variation of the compass was computed from these observations; that variation resulting from the first azimuths, taken without the plate, is affected simply by the local attraction of the ship, and may be termed the deviated variation: that resulting from the azimuths when the plate was affixed, by an action twice as great; first by the ship and next by the plate, and may be termed the double-deviated variation. The difference between these variations is the amount of the local attraction or the deviation, and this applied to the deviated variation gives the correct magnetic variation.

It is easy to see how this correction is to be applied by merely observing whether the north end of the needle has been drawn tọ the west or to the east, by the application of the plate, and considering that the ship's attraction must have had a similar effect on the needle.'

The next eight pages of the Report contain the series of observations made at sea, in order to determine the amount of local attraction in different latitudes (from 51° N. to 60° S.) by means of Mr. Barlow's plate, the results of each experiment throughout the voyage being arranged in a table: after having given which, Mr. B. adds,

'On examining the numbers given in the above tabulated results, their general agreement with the deductions of Captain Flinders will be immediately obvious. That distinguished officer REV. MAY, 1824.

D

found,