MESSRS. EDITORS—This is a subject of such vital importance to the safety of steamships, and to iron vessels whether sailing or steamships, that I call your attention to it in the hope of bringing Capt. Griffith Morris's plan for applying corrections into more extensive notice. I beg leave to ask your perusal of a small pamphlet published by me in the early part of the last year, wherein the subject of local attraction and magnetism is touched upon. It is there clearly seen how far the light of science has shone on the subj ect, and how far philosophers have been at fault in curing the disease called "local attraction." While Barlow, Airy, Gray, Scoresby, Harris, Walker, Faraday, and other scientific and practical men have studied tho subject on shore, and have written much regarding it, not one of them has found an infallible remedy ; few have studied it at sea on board of iron ships, and only one of the great names above given has, to my knowledge, made a voyage in an iron ship to the southern hemisphere, and he (Dr. Scoresby) reported on his return from Australia that the only safe remedy in iron ships was to elevate the compass above the reach of the magnetic current or local attraction, and I believe this is the practice on board of iron ships generally. The inconvenience of trusting to a compass elevated many feet abovo the deck can bo readily estimated by navigators; it must be subject to great vibrations in steamers, especially in propellers, and in sailing ships to many accidents. The English nautical magazines are full of discussions on local attraction, a reference to which would fill a volume, but it may not be out of place to quote a few lines from a letter published in June, 1856, in the Mercantile Marine Magazine, from W. W. Eundell, Secretary of the Liverpool Compass Committee: " But it will also be seen by any one who visits our shipping, that nine out of ten ships have long, and more or less massive horizontal iron spindles connected with the steering apparatus—in some cases at a higher and some at a lower level than the compass card; and I proceed to draw your attention to the action of such bars upon the compass. Now, these bars, and in fact, the fore and aft horizontal portion of all the iron in the ship, produce an effect which has been aptly termed ' quadrantal,' (for the reason that it changes its character in each quadrant of the compass,) and produces easterly deviation when the ship's head is between N. and E. ; westerly deviation when between E. and S.; easterly deviation again when the ship's head is between S. and W., and again westerly in the remaining quadrant. It is worthy of remark, that this deviation is independent of locality, and has the same effect at the equator as at the pole. "As before, I will not attempt to estimate the' amount of this kind of error, but only di-reet your attention to the fact, that in two quadrants of the compass it will tend to decrease, and in the other two to aggravate, those deviations which arise from iron situated aft and below the compass. "There is another cause of error which must be mentioned here, though it does not affect so large a number of ships as the preceding—I allude to the double binnacle. In many cases where these are used, the compasses are placed so close as to disturb each other, and the result is, a quadrantal error of precisely the opposite kind to that arising from fore and aft iron ; but, like it, alternately increasing and decreasing the errors due to that cause of deviation which was first discussed. " These remarks convey but a very imperfect idea of the eompass question; they should be taken in the restricted sense in which they are given, and as illustrating the possible accumulation of small errors ; but I would protest against their being received as merely theoretical, for they are the result of ample and practical experience. My desire is to briefly state as much as may enable any one who will give the subject a little attention to understand the nature of the deviations likely to arise from the iron fittings about the rudder and steering apparatus, on board wooden ships. When deviations occur in our merchant ships, which do not arise from iron cargo, the cause is almost invariably found in this direction, and the el rors are consequently of the opposite kind to those observed in men-of-war. [This should be borne in mind by persons reading different works which have been published on compass deviations, in which the observations have been mostly derived from officers of the royal navy.]" In the July number of the same magazine may be found a " summary of results" derived from G. B. Airey Esq., Astronomer Royal, among which he says: " On reviewing the results of the preceding examinations, I think I am justified in denouncing any system of navigating a ship by forming a table of compass deviations at the starting port, and using that table until means of correction can be obtained from observations, as dangerous, and I think it ought to be discontinued ;" and again he says, "I apprehend that the necessity of using a table at all (that is, of steering by one nominal course when another course is intended) is, especially in difficult channels a very serious evil from which the method of steering by a corrected compass is entirely free." There seems to be no doubt whatever that several expedients aro resorted to in England and elsowhere, on that side of the Atlantic, by which compass errors are obviated on certain courses; a table of variable errors being made for the other points, arid that by constant watching, by azimuths, and other means, ships go and come safely. But no settled plan exists by which local attraction, in all places, North or South, is entirely neutralized, excepting tho plan of Capt. Morris. If he can do this he has certainly attained what much more learned men have failed to do. The question then is, how far have we the proof that Capt. Morris has attained this most important object? In this locality, we know that ho has navigated an iron tow-boat for twelve or thirteen years, most of the time by virtue of corrections applied by himself, and which he says were permanent until he put in new boilers last summer, when he readjusted his compasses. Next in importance comes the iron schooner Mahlon Beds, corrected nearly three years ago for a very large error making her compasses quite valueless. This vessel has been to the West Indies, and Capt. Godfrey reports, on several occasion!, that his compasses are correct on all courses. The steamers Palmetto, Capt. 0. Baker; S. A. Stevens, Capt. Stow ; J. Whitney, Capt. S. Howes; Henry Morison, Capt. Loring ; Wm. Jenkins, Capt. Hallet; Nantasleel, Capt. Eouel, and others, who have tested Capt. Morris's method on our coast and in the Delaware and Chesapeake, have perfect confidence in their compasses. In my own experience I have tested his corrections in an iron yacht, the Edith, during one summer's cruising. I gave him but a few hours to do the work, and he considered it imperfectly done, but I found my compasses as correct as the most fastidious navigator could desire. I have also applied the corrections to the iron steamer Argentina, now bound to the La Plata ; on several trial trips her compasses were found quite correct, although, like the Edith's, they were very much out on E. and W. courses before the application of tho corrections. It will thus be generally conceded that Capt. Morris understands correcting compasses in this latitude in wooden and iron steamers and iron sailing vessels, in one case as far south as Trinidad. R. B. FORBES. [TO BE CONCLUDED IN OCR NBXT.]. SEED-SOWEB.—A correspondent writes us that as yet he has heard of no " light broad-east sower," constructed so as to be readily adapted to all kinds of grain and grass seed, 7 where the operator can ride in a comfortable V arm-chair.