found, that with equal dips, north and south, he had equal local attractions, but reversed in direction: and the whole of the foregoing table indicates the same change. The north end of the needle being drawn forward, while the dip is north; and the south when the dip is south, at least the exceptions are only in places near the magnetic equator, and the amount of difference in these cases never exceeds a few minutes of a degree. The general decrease of effect from England to the equator, the increase again from the equator to Cape Horn, and the decrease thence as the southern latitudes diminish, are striking instances of the accuracy of the method of correction proposed. To which I may also add, as a still stronger case, the variations found with and without the plate, in experiments (31), (32), (33), in which the greatest difference Without the plate is With the plate, only 2° 53' It is thus rendered obvious, that the plate, as fixed in Portsmouth harbour, in lat. 50' 47' north, will correct the local attraction of a vessel in lat 60° 56′ south; the dip in the former case being 70° north, and in the latter about the same south. In short, it is rendered evident from the experiments made in the Conway, that the method of correction proposed is applicable through all navigable latitudes, from 50° north to the highest approachable southern regions.' (P. 25, 26.) The observations in the foregoing extract will be recognized as appertaining to the important question, relative to this method of correction,' that still remained to be decided,' as mentioned in the paragraph which has been already cited from this Report. As the whole of the experiments and certificates of approbation, which we have hitherto been considering, have resulted from observations made in latitudes south of England, it is a farther source of gratification to find 34 pages of the new Report filled with the particulars of numerous trials, to which the expedient has been submitted during a voyage from England to Spitzbergen. From the whole tenor of these it is conclusively shewn that the mariner's compass, an instrument which nature itself seems to have supplied as an essential auxiliary for promoting the intercourse of distant nations, has been effectually relieved from an imperfection to which it had been always before subject; and which rendered it not only a very doubtful and uncertain dependence, but caused it to become, in high latitudes, rather a 'dulce malum pelagi than the cynosura petatur sidoniis. 6 The experiments on board his Majesty's ship Griper, Captain D. C. Clavering, for correcting the local attraction, in a voyage from England to Spitzbergen, in 1823,' are thus introduced: As As the experiments which had hitherto been made were principally in regions where the local attraction is least considerable, it was desirable that they should be repeated in high northern latitudes, where it had been already ascertained by Captains Ross and Parry, that the disturbance from this cause was very great. An opportunity of making this trial occurred in the recent voyage of his Majesty's ship Griper to Spitzbergen; and the results, I trust, will be found highly important, and fully confirmatory of the general applicability of the method of correction in question. It may be proper to observe, however, that it had occurred to me before the return of the Conway, that the method proposed might be simplified, particularly in high northern latitudes (where it is of most importance), by placing the plate aft of the compass, thereby neutralizing instead of doubling the original effect of the vessel. The success of this severe trial of the application of the correcting plate will fully appear from the following letter from Captain Clavering to Mr. Barrow of the Admiralty. "Having been directed by their Lordships to make trial of Mr. Barlow's plate, under Mr. Foster's direction, I forward that gentleman's report, which it will be unnecessary for me to comment upon further than to acknowledge the extreme practical utility of it, as found during the whole of the voyage; as when once fixed abaft the compass (thereby neutralizing the effect of the iron on board), nothing further was necessary than to allow the variation of the place. "The very great local attraction in this ship is also something remarkable, and as it is now considerably greater than in the former voyage when with Captain Parry, we can only account for it by the addition of the patent capstan, and chain-cables, which can be proved before paying off by trial of the compasses when it is hoisted out. Should this be the case, it will be well for ships to be aware of the liability of this error. Our binnacle-compass has not been of the smallest use, and at present it differs with the ship's head at east and west points' (14° minus at east, and 14° plus at west)' besides traversing extremely sluggish." The ensuing important remarks occur also in a letter sent by Captain Clavering to Mr. Barlow: "You have seen by my report to the Admiralty, that the local attraction of the Griper before we left the Nore was 14° plus with the ship's head at west, and 14° minus at east, making a difference of 28° before we left England, and which soon after increased to 20° at each of those points, or more, viz. (ultimately to 37°) making in the latter case an extreme difference of about six points. Under such circumstances it is obvious that the compass would have been altogether useless, (as indeed it has always been admitted to be in these high latitudes,) but for your valuable correcting plate, with which, as I have already stated in my report, we found the compass to which the apparatus was attached as serviceable in these latitudes as in any other; for having once neutralized the local effect of the vessel at the Nore, we had only during the D 2 remainder remainder of the voyage to allow for the variation of the place, and were quite unembarrassed with any effect from local attrac tion. "I should also state, that, independently of the latter disturbance, we found all our other compasses so extremely sluggish that they would stand in any direction whatever."' (P. 28.) Mr. (now Lieut.) Foster's report, which follows Captain C.'s letters, is minute, copious, and altogether a valuable piece of instruction on the subject of local attraction; and on the practical operations required to be performed previously to departure from port and during a voyage, in order to properly insure, in all cases, the benefits which the correctingplate is capable of affording. Our object, on this occasion, has been to furnish such a sufficiency of extracts relative to the origin and progress of this valuable invention as may assist in promoting the general adoption of it, and in causing it to be every where duly appreciated; and, though many scientific points are recorded by Mr. Foster concerning its utility, yet we conceive it unnecessary to the purpose before expressed to separate them. We therefore recommend, to those who wish for the fullest information on the subject, an attentive examination of Mr. Barlow's Essay, and of the documents relative to the efficiency of the plan collected in the Report since published. The theory of Mr. B., on this particular species of magnetic influence, was founded on an hypothesis which resulted from his having discovered the existence of a magnetic equator, to which the dipping-needle was ascertained to be perpendicular in our latitude; and on a supposition that the same reciprocal posture of the dip and magnetic equator was invariably preserved through all other latitudes, he conceived the practicability of furnishing a simple movable apparatus for the purpose which we have been describing, that should possess the faculty of counterbalancing or obviating the whole influence of a cargo of iron on a ship's compass, under all circumstances that might operate in every part of the globe. This great object he has, in a short space of time, succeeded in perfectly achieving; and, from the mathematical vouchers brought forwards to corroborate the guarantee furnished by the several stages of his experimental process, this is the result which we had every reason to anticipate, after our examination of his former publication on the subject: as will be seen by referring to the opinions which we then expressed on the occasion. (See vol. xcii. pp. 18-29.) The conclusion of the Report to the Lords of the Admiralty, in which the practical proofs of the competency of the magnetic 14 magnetic plates are specified, cannot in justice be here with holden.' this case. The nature of the observations, and the judicious arrangement which Lieut. Foster has given to the results obtained in the Griper, render it quite unnecessary for me,' says Mr. Barlow, to offer any remarks to show the success and utility of the experiments in It is only requisite to state, that the local attraction of this vessel having been so much greater than I had contemplated, (viz. 14° at east and west,) the plate which I sent was not so powerful as it ought to have been; it was therefore necessary to bring it so near the compass as to produce some irregularities with the ship's head near the north and south points, in which position of the vessel there was but little more than four inches between the needle and the plate. This is a circumstance I have mentioned, at page 56. of the first edition of my " Essay on Magnetic Attrac tions," where it is stated, that when the needle and iron approach near to each other, the general laws of action fail; and to this circumstance, more than to the greatly diminished power of the needle, is (in my opinion) to be attributed the anomalies noticed in the experiments at Spitzbergen, and at the points in question. But, after all, I am convinced that, in the present infant state of the science, the experiments will be deemed as satisfactory as there could be any reason to expect. It appears then, that from latitude 80° N. to 60° 56' S. viz. through the entire range of all the navigable latitudes on the globe, the experiments have (even in the first three trials that have been made) been attended with the most favourable results, and there can be no doubt that further practice would lead to greater accuracy and give a value to the mariner's compass which it never yet possessed, and a degree of accuracy to our magnetic charts which would probably lead to the most interesting deductions relative to the laws of terrestrial magnetism. The importance of this principle of correction, even for the purpose of keeping the reckoning at sea, is sufficiently demonstrated in the two cases given by Lieutenants Mudge and Foster (pp. 11. and 39.) where, in the former case, the error by the common compass course was 19 miles in latitude, and 28 miles in longitude; while by the corrected compass course the error was reduced to two miles in latitude, and four miles in longitude; and in the instance furnished by Lieut. Foster, the error in latitude alone was 35 miles, which almost wholly disappeared on the corrected course. 'I am aware that seamen depend very little upon the reckoning by compass, while they can make the requisite astronomical observations, but as it frequently happens that many days may pass without their obtaining such observations, it cannot but be of considerable importance to them, in such cases, to possess the means of approximating the nearest possible to their true place. It is not however at sea that this method is of greatest use, it is in narrow channels, in piloting ships by means of charts and bearings, and in marine surveying, that it finds its most valuable application; in these instances nothing can supply the place of the compass, and it cannot but be important in such cases that its 'directive power should be freed from all irregularity. Every reader, whether a nautical man or not, must be aware of the great amount of error, and the fatal consequences which might arise in a few hours to a vessel in the Channel, in a dark and blowing night, having for its only guide a compass subject to an error of 14 degrees in opposite directions at east and west, the very course on which she would be endeavouring to steer; and who can say how many of the mysterious wrecks which have taken place in the Channel are to be attributed to this source of error: of which the most recent, that of the Thames, Indiaman, is a serious example. This vessel, besides the usual materials, guns, &c. had a cargo of more than 400 tons of iron and steel, and it may easily be imagined, that such a cargo would produce an effect on the compass at least equal to that of the Griper and Barracouta; and this alone would be quite sufficient to account for the otherwise unaccountable circumstance, that after having Beachyhead in sight at six o'clock in the evening, the vessel should have been wrecked on the same spot at one or two o'clock in the morning, without the least apprehension of being at all near shore.' Having now followed the author through the subject of local attraction, as far as it regards his plan for nullifying its power to alter the natural direction of the compass, we have next to examine the effects of local influence on another important instrument; which, in its application to maritime purposes particularly, and to the sciences of astronomy and geography generally, is of inestimable utility while its uniformity of action is not deranged: but, when the purity of its mechanism becomes by any cause vitiated, it is liable to lead to serious errors. The public are, therefore, much indebted to Mr. Barlow for investigating such a subject; and he has bestowed much pains and attention in endeavoring to ascertain the effects produced in the rates of chronometers by the proximity of masses of iron."' After having related a series of experiments, the author observes: The first general conclusion which may be drawn from these experiments, is, that the rate of a chronometer is undoubtedly altered by its proximity to iron bodies. Secondly, it appears that it is by no means a general case, that iron necessarily accelerates the rate of a chronometer, as would appear from Mr. Fisher's observations; for five out of the six chronometers which I have made use of were obviously retarded in every situation in which they were placed. In one instance only, viz. chronometer No. II., there is an indication of acceleration in one situation; but it is more doubtful than the retardation in all the other five.' 'As |