COMPASS C011RECTIOX. 



COMPASS CORRECTION. 



odeoce of thought Mid rigid iui|urtiality which ought to charao- 

 thk work, we may remark that it U Mid by the Astronomer- 



1, Cylinder containing vertical magnet ; 2. Kej- to raise and lower the 

 mapiu-t< ; S, Frame in which the m;i|tnr-tR arc placed ; 4, Magnets ; 

 9, Vertical screws for raising and lowering the magnet frame ; 6, Screws 

 to prevent the vertical screws altering their position ; 7, Guides to the 

 magnet frame. 



Royal publicly to be " perfectly correct," as to the principles under 

 which it is constructed, and " entirely satisfactory" to him. The one 

 property which (while there might seem to be some complexity in the 

 mode of suspension) distinguishes this apparatus from others, is the 

 facility with which the adjusting magnet can, under very simple 

 printed instructions from the maker, be altered in position, so as to 

 compensate the changes produced from alteration in the magnetic con- 

 dition of the ship. 



Nothing is intended herein to disparage the beautifully constructed 

 and excellent compasses made by those whose names have been men- 

 tioned as gome of the principal makers of this country. A very 

 elegant form of campus, in which the magnet is attached to a vertical 

 metallic zone, and the whole inclosed in a glass globe, has been invented 

 recently by a Mr. Gowland, of Liverpool. It is for practical men to 

 satisfy themselves thoroughly if the three grand desiderata of the 

 compass be fulfilled in any one instrument ; then are : 



1. Steadiness, without sluggishness. 



2. Activity and sensibility, without oscillation. 



3. Facility for correction under local attraction, without complication. 

 COMPASS CORRECTION. Under the word BEARING, some notice 



has been taken of the most recent improvements in the method uf 

 using the compass for nautical purposes. 



It U ordinarily supposed by landsmen, that with a well-made com- 

 pass, and a knowledge of the variation thereof, as depending on ter- 

 restrial changes [VARIATION], and this to be applied on either hand of 

 any given point, according as the magnetic needle deflects towards the 

 east or west from the true pole of the world, or the meridian, a 

 navigator might find his way to distant port* of the earth ; but the 

 need of correction from other causes, daily or hourly in some ships, 

 gave rise to and constitutes what has been so long called and agitated 

 an the compass question. 



The navigator's desire at sea is simply to know on irltat nni/le. from 

 a meridian, or true north and south line, Aw tkip ! tailing. But before 

 he can arrive at a satisfactory conclusion as to his correct course, the 

 various channels through which local attraction affects his compass 

 have to be estimated; in other words, ho must free his compam 

 bearing from all casual local magnetic influences ; and this constitute* 

 the work of cum/iott cvrrrrtinn. If such were easily performed, on 

 merely knowing the deviation [DEVIATION], an ordinary additive or 

 Militractivc operation would suffice ; but it unfortunately happens that 

 the quantity of iron used about a ship, and especially hi a steamer, 

 presents a combination of disturbing forces so intricate in their rein- 



that no ma* of cjyrriciux Aould rely on any men tyOem / 

 uutntmntcU correction, independent o/ actual ubter<<atio of kiatmlf/ 

 body, much leu will be rely mldy on a correcting card, or correcting 

 magnet*. It way here be well to explain so clearly the present state 

 of things, that the difficulties which have hitherto clouded the brow of 

 the master mariner, plunged him into uncertainties and unnecessary 

 labour, depriving many a weather-beaten and stgrm-toesed mariner of 

 his nightly rest, may, in those whose intelligence really befits them for 

 their duties, be totally tet atide ; and that ships may henceforth be 

 navigated with a confidence and security profitable alike to their owners, 

 their crews, and the public. 



The system of correction at present adopted in Her Majesty's navy 

 U the following : A line-of-battle ship, having on board nearly 600 tons 

 weight of iron (runs, shot, and shell, being moreover a screw steamer, 

 with all her massive machinery, an immense iron funiirl, iron water 

 tanks, iron anchors, iron cables, Ac., is fitting for sea, off one of Her 

 Majesty's dockyards, say for example, Sheerness, either before or after 

 the usual trial trip (generally before), and it is thought necessary to 

 swing her for compass correction. Where no distant and conspi- 

 cuous object is in sight, this is done by sending a trusty officer on shore 

 at on assumed station, say at the Isle of Grain, with an azimuth com- 

 pass. The ship is then turned about, so as to bring her head to every 

 point of the compass, and by comparing simultaneous observations 

 (made during the time of swinging) between the ship and the .- 

 and the station and the ship, some notion can be formed of the local 

 attraction on board : thus, if the officer on board at a certain moment 

 sees (for example) his assistant at the station on shore bearing due west 

 by ship's compass, and the assistant on shore at the same time sees the 

 ollicer on board bearing, not east, the opposite point, but E.S.E., there 

 must be, in one or both instruments, a total deviation of two 

 or 22 30' ; but as the compass on shore is carefully placed, so as to be 

 beyond the influence of any local attraction, the deviation is always 

 attributed to some magnetic disturbance on board. These deviations, 

 as found on each point, are carefully noted and collected, and form 

 what is generally called a correcting card. Various methods have 

 been proposed for the registering of these deviations ; perhaps uotliing 

 more simple and ingenious has been offered than the one by Sir Archi- 

 bald Smith, in the form issued by the Admiralty to each of Her 

 Majesty's ships. The swinging of a ship, then, is nothing more than a 

 mode of finding the error of her compass at a certain moment ; but as 

 there are other methods of ascertaining this so important a fact, it is 

 necessary to consider which of the many it the mutt accurate, simple, and 

 convenient in application and practice. 



The swinging of a line-of-battle ship U a work of labour, time, and 

 expense ; it often occupies three or four days, the employment of about 

 300 men, the wear and tear of two, three, or four hawaers, the letting 

 go and weighing two or three heavy kedge anchors ; all this is neces- 

 sary in swinging a ship of the line. But the advantage of ascertaining 

 precisely the state of such ship is paramount, and we should spare no 

 time, labour, or money, if a permanent result could thua be obtained, 

 It unfortunately happens, however, that any change which may occur 

 in the magnetic condition, or even position of the iron on board, such 

 as the training of a gun, the firing of a broadside, lighting the fires of 

 the engine, moving the ponderous funnel up or down, and various other 

 Mii:i'."i>l,ii>lc o| ^-rations on shipboard, may so affect the compass as to 

 render a eorrecting-card, obtained at one time with so much labour, 

 totally useless at another. It is true that some ships have been found 

 to undergo very little changes during long periods; but, on the other 

 hand, it is equally well known that steamers which have preserved a 

 remarkably equable state of magnetic condition for years, have sii< - 

 shown disastrous changes. To mention only one case out of lup- 

 in support of this assertion an iron steamer, belonging to one' ot" the 

 principal companies of Liverpool, which had shown great uniformity 

 as to local attraction for sevi-i. 1.1, in 1857, proceeding down 



Channel, under the command of a distinguished officer, who was on his 

 73rd voyage post that coast. It was a pleasant, calm, hazy evening, and 

 the commander supposed, from her standard compass, that the ship 

 was heading fairly tor the Longship's light. But just after sunset a 

 cry suddenly arose from the look-on t man, of " Rocks ahead '." and to his 

 amazement the experienced commander found himself running stem-on 

 to the Stones in St. Ive's Bay. It was afterwards found by the writer 

 of this, that the local attraction on board must have varied 16 degrees 

 after leaving Liverpool. The ship and crew were barely saved I mm 

 destruction; an instance of the high importance of the subject now 

 under our consideration; for we have no meant of ascertaining the 

 amount 'in any particular Map to vary her condition, hence 



''i fid value nf any currecting-tardi whatever. The subtle dis- 

 turbing clement may at any moment, from unforeseen causes, accumu- 

 late in this gun, or that range of shot, or in some particular iron knee, 

 or chain, or iron stanchion ; so that the wary master or commander 

 may suddenly, ami at any time, be deprived of that in his so-called 

 correcting-card, the loss of -which would necessarily entail 

 anxiety, until opportunity presented itself of checking the state of the 

 compass by a celestial observation. 



As an important public question is deeply involved in this scrutiny 

 of the practice of swinging a ship, an operation shown to be not only 

 expensive and troublesome, but attol*ttly / loo much rr/ianrc 



be plated n^oa it, it may reasonably be asked why we permit fallacies to 



