sufficiently so to produce a vibration of the needle, when applied outside the glass of 14 or 15 degrees, and to retain the same 1o out of its natural direction; and the lighter needles belonging to my other compasses were drawn and retained by the same piece of brass four, five, or six degrees from their true magnetic bearing, although applied outside the glass, and therefore at nearly a quarter of an inch from the extremity of the needle. This piece of brass was by far the most powerful in its effects; but still every screw and attached piece in the instrument had acquired the same quality to a certain degree, so that no dependence could be placed on the needle except when these were all removed, which rendered its application inconvenient.' (P. 16, 17.)

Considerable utility, we conceive, will result from such information as we have just cited.

Although the following hypothesis is advanced with much diffidence, it would be culpable in us to pass over a conjecture which not only bespeaks ingenuity and deep knowlege of the subject, but which there is every reasonable probability to suppose will ultimately be corroborated by experiments, and become a valuable rule in subsequent investigations. After having furnished a formula for ascertaining the variation of the directive force in different latitudes, in which the index of the cosine of the dip denoted by m is indeterminate, Mr. B. proceeds to observe;

'If we might be allowed to draw any inference as to the probable value of m, in the above expression, it seems reasonable to assume, that if the power of the attracting body were increased or diminished in the same ratio as the directive force, that is, according to our hypothesis, as the cosine of the dip, the deviation would remain constant. Now we have seen that the square of the tangent is as the cube of the force, or the tangent as the power of the force; whenee it follows, that if the above increase or dimi nution of force do not take place, but that it remains constant, then the tangents ought to be found inversely as the power of the directive force; that is, as cos. d'; or, m = 4. It would, however, be best to deduce this index from actual observation, though, if the foregoing hypothetical reasoning prove correct, it will follow that at the equator the effect of iron on a ship's compass is less than in our latitude in the ratio of 1 to 5 nearly; while at the pole, where cosine d'o, it gives the angle of the magnetic north 90°. But at the pole the horizontal needle has no natural magnetic direction; the formula, therefore, may still be considered as applicable even in this extreme case.' (P. 53.)

I

The confirmation of this hypothetical inference will be expected with no small degree of interest, by all who are either speculatively or practically concerned in the subject of magnetic attractions.

The

The law of action, says Mr. B., as it obtains between a magnetic needle and regular masses of iron, being established, either exactly or approximatively, by the foregoing experiments, my next object was to ascertain whether the same law was applicable to irregular masses.' We have then a detail of experiments made on an iron 24-pounder cannon, nine feet six inches long, weight 58 cwt., with formulæ, and examples of computation; after which the section is thus concluded:

These experiments will be, I trust, quite sufficient to satisfy every one, that the same laws which I first obtained from observation on regular masses of iron, are equally applicable to irregular masses; and that they furnish us with the means of computing the local attraction of a ship's guns upon her compass, under all circumstances, and in all parts of the world: at least, if (as there is the strongest reason to believe) the plane of no attraction varies its position in different latitudes, so as to be every where inclined to the horizon at an angle equal to the complement of the dip.' (P.65.)

From the deficiency of observations accurately collected in different parts of the world, the author has scarcely yet had a fair opportunity of proving the efficacy of his rules, with regard to the effect of local attraction on ship-board, where the compass is particularly situated with respect to the masses of iron distributed about the vessel. He has, however, availed himself of the observations furnished by Capt. Sabine, during the late expedition to the north-west; and the greatest deviation, according to Mr. Barlow's rule, applied to Capt. Sabine's observations, is 27° 26'; which Capt. Sabine found by actual observation to be at least 25°, "if it did not exceed that amount." The agreement is therefore as great as can be expected.'

Attempts to produce and prescribe formulæ are necessarily futile, unless our principles are self-evident, or equally irrefragable, for they can only accidentally hit the truth: but rules resulting from principles properly attested, and deduced by a proficient in mathematical reasoning, are infallible. Now we have no hesitation in pronouncing the rules furnished by the present author, on the subject of magnetic attraction, to be of the latter character, and therefore intitled to implicit confidence: for, though we have at present but a paucity of evidence as to their efficacy on ship-board, yet they have produced such a series of approximations to the several tests, as cannot fail to astonish those who are best acquainted with the nature of the task which has been (as it were) at once so effectually performed.

Mr. Bar

6

Mr. Barlow, however, has extended the advantage of his observations and experiments farther than the mere rules, and has proposed a method of determining the local attraction of a vessel by experiment.' The experiments relative to the practical apparatus intended to be used on board of ships having been detailed, he says,

'These results, although they exhibit some small aberrations, are sufficient to shew that the principle itself is correct, and that with greater precision, and a more accurate mode of suspending the shell, greater accuracy might have been attained. These experiments had been performed before I had made the singular discovery that the power of an attracting body of iron resided in its surface; and I therefore at that time foresaw an impediment to the practice of this method on board a ship, in consequence of the mass of iron which I thought would be necessary to produce the desired effect: but having since found that surface is the principal thing to be attended to, this difficulty is avoided, as a mere globular iron shell, or a simple circular plate of the same metal, is amply sufficient for the purpose. I therefore ordered a double circular plate of iron to be made, fifteen inches in diameter, weighing only 4lb. 13 oz., with which I repeated my former experiments, and made several others, the whole of which gave the most satisfactory results; and by afterwards attaching the same plate to a ship's binnacle, I found that its power was far greater than would be requisite for doubling the effect of the guns of any vessel of the navy, although applied exterior of the binnacle, and nearly fifteen inches distant from the pivot of the needle.

I cannot resist adding a note in this place which may be thought of some importance. Upon my examining the compasses in store in Woolwich dock-yard, for the purpose of selecting one for my experiment, I could scarcely bring myself to believe that the instruments exhibited to me were those actually employed in his Majesty's vessels: the cards, bowls, needles, &c. seem all worthy of each other; equally clumsy and imperfect; —and it does appear to me very unaccountable that vessels of such immense value, and the safety of so many valuable lives, should be endangered by the employment of instruments that would have disgraced the arts as they stood in the beginning of the eighteenth century.'

Our readers will recollect that we have already adverted to this subject, in terms and with information similar to those of Mr. Barlow.

We have now given an outline of that part of the work which appertains to local attraction; and we have no doubt that the practical navigator, as well as the inquirer into the phænomena of nature, will receive the many new and important communications, which the author has made, with applause and satisfaction. In the last two sections are considered the

phænomena of the daily variation of the compass, and the general nature of magnetic action, which are both discussed with much skill; and many suggestions are contributed, that are well intitled to the serious attention of future experimentalists, and lovers of this science.

In concluding these remarks, and awarding to Mr. B. the merit so justly due to him, we may be allowed to mention that several little inadvertences in diction and phraseology occur, of which the expression as respects' in the title is a sample; and we the more regret such incuria because his communication is not merely to England but to the world. From the perspicuous manner in which he usually writes, we judge him to be very capable of avoiding these inaccurate expressions.

The essay which has also been supplied by Mr. Barlow, concerning the strength and stress of timber, a matter of essential import to every practical engineer and architect, contains many well digested truths; which, if some of them were not previously unknown, appear in too many instances to have been neglected. We believe that no English author has professedly treated on this subject until it came under the investigation of Mr. B.: but the French government have employed, particularly in late years, some very eminent men to examine into the texture and quality of the several kinds of building timber, and to furnish an estimate of the relative strength of each. In the early part of his volume, Mr. Barlow has given an historical sketch and dissertation, in which the nature and merits of the several theories are discussed; and from which it appears that the experiments were, in some cases, very inefficiently made on which the principles of computation were founded: while in others assumptions had supplied the place of fact, so that the conclusions which had been drawn were calculated more to mislead than teach. In consequence of these fallacious computations, and considering the mischief which might ensue from adopting them, the important subject was brought under Mr. B.'s scrutiny.

The way in which the author proceeds in his inquiries is such as must, in all cases, if conducted in a critical manner, insure success. It will be evident that the object of such a work as the one that we now describe must be to take into `consideration the force of any pressure, or strain, on any piece of timber about to be used; and the substance necessary to sustain or endure that force according to the texture, &c. of the timber. First, then, a course of experiments is necessary to determine the relative strength of every kind of buildingtimber;

timber; and from these data are to be deduced the rules to govern the judgment of the artificer in selecting and employing, in all cases, materials proportionate to the stress to which they are respectively to be afterward subjected.

It cannot fail to appear, to any one who considers the nature of the subject on which Mr. Barlow has here been employed, that only a comparative estimate of the strength of each sort of timber can be obtained; as it is well known that situation affects the growth of trees, and that a variety of accidental circumstances will render even those of the same nature very different in quality. After the elements of calculation, therefore, have been determined as carefully as the nature of the case will admit, and the various rules resulting from them have been deduced with all possible accuracy, much must depend on the arbitrary judgment of the artificer in selecting his material; that is, he must be what is termed "a good judge of timber;"-otherwise, he may possibly fail in his expectations, though in his work he may have adopted every piece according to rule.

We can only say that Mr. Barlow seems to have bestowed the utmost care in the pursuit of his object throughout; - that, as far as the advantage of calculation can be applied, he has applied it; and that, if the rules which he has furnished and the practical examples to illustrate them be duly observed, much improvement will be derived in the particular part of those mechanical and architectural professions to which they expressly relate.

ART. IV. Memoirs of Mr. John Tobin, Author of the "Honeymoon. With a Selection from his unpublished Writings. By Miss Benger, Author of "Memoirs of Mrs. Elizabeth Hamilton." Crown 8vo. pp. 450. 12s. Boards. Longman and

Co. 1820.

A

MONG the few candidates for legitimate fame in the province of the drama within the last half century, the name of Tobin stands so high that, with the exception of one or two of his theatrical rivals, he may justly lay claim to the very first rank of stage-writers of our own times. In one respect, indeed, he is alone and unrivalled; viz. in the happy adaptation of a poetic style to dramatic dialogue. Formed in the cast and character of his plays on the model of Shakspeare and his contemporaries, he is yet no servile imitator; and while in the tone and tenor of his language he leaves the laboured copyism of Fazio far behind, in the opinion of his warm admirers he will be considered as excel

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