On examining the numbers contained in the above tabulated results, their general agreement with the deductions of Captain Flinders will be immediately obvious. That distinguished officer 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 the 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 as found with and without the plate, in experiments 31, 32, 33, in which the greatest difference It is thus rendered obvious, that the plate, as fixed in Portsmouth Harbour, in Lat. 50° 47' N., will correct the local attraction of a vessel in Lat. 60° 56' S.; 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. Only one point could now be considered as doubtful, respecting the efficacy of this method of correction. It had been ascertained by the observations of Captains Ross and Parry, that the effect produced by the iron of the ship increased with immense rapidity, and amounted to the most fearful quantity in approaching towards the pole: Would the plate increase in power with equal pace? To ascertain this point, Lieutenant Foster, who had already received the thanks of the Board of Longitude for the experiments on this and other scientific subjects in the Conway, was now appointed to the Griper, which was about to leave England for Spitzbergen, under the command of Captain D. C. Clavering, with orders to continue his experiments on local attraction under the superintendence of the above officer at every opportunity. We are sorry we are not able to give the entire detail of these experiments, which are the more interesting as they were made in a part of the world where the dip is very great, and where hitherto the compass has been considered as an useless instrument. Moreover, the attraction of this vessel, before leaving England, was very much greater than usual, so that on all accounts the plate was in this instance submitted to the most severe trial. By a series of observations, made while the vessel was lying at the Nore, it was found that the bearing of a distant object differed 28°, with the ship's head at east and west; that is, the local attraction at each of these points was 14°, and proportionally great at all the other points, an excess of attraction which Captain Clavering attributed to the effect of the spindle of the patent capstan, a suggestion which was verified by experiment on the return of the vessel, as we have already stated. To counteract this strong power, it was necessary to bring the iron plate, which was 14 inches in diameter, to a distance (from the middle of the pedestal) of 7 inches; and the centre of it 7 inches below the pivot of the needle, in which situation abaft the compass, it counteracted the local attraction of the ship, and left the needle free to obey the natural directive power of the earth. This was proved by taking the variations of the needle with and without the plate, (as already explained in the Conway experiments,) from England to the North Cape; when the close agreement of the former, and the great discrepancy in the latter, were so marked, that from this time the vessel was navigated during the remainder of the voyage altogether by the corrected compass, and with the best possible success. She was moreover swung in three different ports during the voyage, viz. at Hammerfest, at Drontheim, and at Spitzbergen, and the local attraction ascertained at every point; first without and then with the plate: this was found to be at the east and west, or maximum points, without the plate, as follows, viz. Hammerfest, England, 24° 10' 34 42 21 23 14 00 Whereas, with the plate affixed, the deviations were reduced to quantities very little exceeding what might be attributed to errors of observation. It will, however, be more satisfactory to state some of these results at length. The following Table shews the Variation as observed with and without the Plate, at different times during the Voyage. Variation Variation Latitude. Longitude. Ship's Head. without the with the Time of obser A single glance at this Table is sufficient, to perceive the very irregular character of the variations, as determined without the plate, and their close approximation to uniformity when the plate was affixed. Extract from Journal of H. M. S. Griper. H. M. S. Griper at Sea, 25th May 1823. Lat. 69° 16′ N. Lon. by Chr. 7° 54′ E. P. M. REMARKS, &c. Fresh breezes and cloudy. P. G. 4. Fresh breezes with a ditto P. G. Midnight, moderate, and 6 3 E by SSE by NN ditto T. D. Variation 2 points west. 8. Squally weather. 10 3 2E by S ENE ditto 11 3 EAS ENE Lat. observed at Noon, 69° 12′ 10′′ N.; Long. by Chr. 10° 14′ 15′′ E. The preceding is a copy of an extract from the journal of the vessel, showing the courses, &c. as kept both by the corrected and uncorrected compass, and the amount of errors corrected by the plate. The course, according to both reckonings, is shewn in Fig. 5, and the numerical results will stand as below: Course and distance made good between the observations on the 25th and 26th of May, 1823: Course= S. 85° E., distance 50 miles. By the Plate compass course= E. distance 51 miles. the Plate, Latitude observed May 26. meter 10 14' E. Course N. 56° E., distance 58 miles. = : 69° 12′ 10′′ N.; Longitude by Chrono Latitude by the Plate Compass `= 69° 16′ 00′′ N.; Longitude 10° 17′ E. without the Plate, Making a difference in the latitude of 35 miles. The following table shews the local attraction of the Griper, as ascertained by swinging her at Hammerfest, and the second Table shews the same when the plate was fixed. Hammerfest, Latitude 70° 40′ N. Longitude 23° 45′ E. Variation 11o 26′ W. Note. The dip is supplied by Captain Sabine throughout. These results will be found fully to bear out the representation made of them by Captain Clavering to the Admiralty, who, in his letter to John Barrow, Esq. says, "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, farther than to acknowledge the extreme practical utility of it, as found during the whole voyage; as when once fixed abaft the compass (thereby neutralising the effect of the iron on board), nothing farther was necessary than to allow for the variation of the ptace." And, in his letter to the author, he says, after speaking of the great amount of the attraction in the Griper: "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 sta ted in my report,, we found the compass to which the apparatus was attached as serviceable in these latitudes as in any other." What is here alluded to is an advantage attending the plate beyond what the author had foreseen, viz. that it not only causes the compass to work correctly, but it also enables it to |