[CONCLUDED FROM PAGE 232.] The report of Capt. Pendergrast and Mr. Dunnington, master of the II. S. steamer Merrimac, also settles the fact that the corrections of Morris were perfectly satisfactory during about fifteen months' experience, between the latitutes of 50 and 13 N. The reports say :— U. S. STEAM-FRIG ATE MERRIMAC, ) BOSTON, April 15, 1857. ) SIR—I have the honor to enclose herewith the report of Lieut. Dunnington, in regard to the standard-compass of this ship, as corrected for local attraction by Capt. Morris, and I fully concur in opinion with Mr. Dunnington as to the invariable correctness of our compass. As applied to this ship, it has proved all that could be desired, and I have no hesitation in recommending his plan for general use in all our public ships. I am very respectfully your obedient servant, G. J. PENDERGRAST, Captain. U. S. STEAM-FRIGATE MERRIMAC, ) BOSTON, April 14, 1857. ) SIR—In obedience to your orders, I herewith enclose a number of azimuths taken for the purpose of testing the standard-compass of this ship, which was corrected by Capt. Morris in January, 185G, for local deviation. The azimuths I have put in the form prescribed by the Bureau of Ordnance and Hydrography. You will see, by the observations, that the standard-compass has invariably given within two degrees the variation as shown by the chart ; and this slight difference, I think, is owing to my not always finding a place where the azimuth would give the same heading as the standard-compass. It is also very difficult to read the headings of the ship by both compasses to a degree, especially the one in the binnacle, when taking observations. Running from New York to Southampton, we changed the variation gradually from 9 W. to 30 W. The sights taken show that the standard-compass was always correct. In running from Southampton to Barba-does, we changed the latitude from 50 N. to 13 N., changing the variation from 30 W. to 1 30' E. The observations show the compass to have been correct during the entire run. As I have always found the standard-compass correct by my observations and the different bearings, I take great pleasure in saying that I am fully convinced that Capt. Morris has succeeded in correcting the standard-compass of this ship for local deviation. Very respectfully your obedient servant, JNO. W. DUNNINGTON, Lieutenant, U. S. N. Great doubt was expressed, even after the experience of the Merrimac and the Mahlon Betts, as to whether the corrections applied in north latitude would be of any value in the southern hemisphere, and their were many substantial reasons for this doubt in the minds of scientific and practical men. Almost every vessel of iron corrected in England had been obliged to resort to a table of errors, at least, and many of these, in spite of the corrections and the tables, were found to be quite out of the way in moderately high southern latitudes. There is still some reason to doubt the entire success of Capt. Morris's plan, in south latitude, in iron ships, because we have no positive proof of it, but I trust this doubt will be solved when we hear from the Argentina. But in steamships of wood the plan has been tried in two or three cases in the other hemisphere. Capt. Dearborn, of the Yang-Tze paddle steamer, reports from Bombay that " nothing could work better than my compasses." In this case the error was considerable, on some courses several points before leaving New York. The corrections were also applied to the II. S. steam-frigate Minnesota, Capt. S. F. Dupont, who writes to me from China that his report to the Bureau will be " altogether favorable," and that he fonnd no deviation on getting into south latitude, and that all his landfalls were correct by compass. These are satisfactory evidences of the correctness of Morris's principle, as far as they go, but the savans say there is still room for doubt as to whether he has found a way of correcting all cases of deviationarising from local attraction in all places. Let us grant that this is true, and let us solve the doubt, Jby giving him every steamship and every iron ship in the country, to correct and especially those which are going into south latitude. The U. S. steam-frigate Merrimac has gone to the Pacific and has been heard from as far as Rio Janeiro, but I have seen no official or other report of her compasses since she went into the southern hemisphere. That they will have proved to be correct, like the Minnesota's, I have no doubt. The limits of a letter will scarcely warrant me in going more at length into the importance of correcting all ships for local attraction. It is a subject which merits the serious consideration of shipowners and underwriters, for it is a prolific source of loss of life and property ; it should commend itself to the notice of Boards of Trade, Chambers of Commerce, and of Congress, just as much as the lighting of the highways of the sea, just as much as any other means for the security of navigation. Happening to be on board of an English iron vessel recently, the Witch of the Seas, I inquired of the captain if he had any trouble with his compasses ? He answered :—" Nothing but trouble ; I have had them corrected several times in England, by the most approved methods ; they do tolerably well in the Channel, but on going south, they gradually get out, until in high south latitude, I can depend on them no longer. I am obliged to correct them daily by azimuths, and on near-ing the land, at the Cape and in Australia, I might as well be without a compass." It must be remembered that Capt. Morris professes to entirely neutralize the effect of local attraction within a certain distance of the standard compasses ; he makes no table of corrections ; does not necessarily swing the ship to ascertain the amount and the character of the error ; uses no "mass of unmagnet-ized iron," as recommended by Airy, the Astronomer Royal, and never resorts to placing his compass aloft, excepting in making experiments. He uses magnets of greater or less intensity, prepared and packed in a peculiar manner. Trusting that these remarks may prove interesting, I leave the subject with the single suggestion that however imperfectly I have treated it in a scientific view, you may depend on my disinterestedness, and my single desire is to do Capt. Morris and navigation a service. R. B. FORBES