Feb. 28, 1878] 



NATURE 



353 



that with it it is not necessary to look through a small aperture 

 in an instrument moving with the compass-bowl, as in the 

 ordinary prism compass, or in the original nautical azimuth 

 compass (described 280 years ago by Gilbert, Physician in 

 Ordinary to Queen Elizabeth, in his great Latin book. " On the 

 Magnet and on the Earth a great Magnet "j, which is very much 

 the same as that still in use in many of the^best merchant steamers. 

 In using the azimuth mirror the eye may be placed at any 

 distance, of from an inch or two to two or three feet, from the 

 compass, according to convenience, and in any position, and may 

 be moved about freely through a considerable range on either side 

 of the line of direct vision through the lens, without at all 

 disturbing the accuracy of the observation. This last condition 

 is secured by the lens being fixed in such a position of the 

 instrument that the divided circle of the compass-card is in its 

 principal focus. Thus the virtual image of the divided circle is 

 at an infinite distance, and the images of distant objects seen coin- 

 cidently with it by reflection in the plane mirror show no shifting 

 on it, that is to say, no parallax, when the eye is moved from the 

 central line to either side. From the geometrical and optical 

 principles explained previously, it follows also that if the 

 azimuth instrument be used for taking the bearing of an object 

 whose altitude is less than 27°, then the effect of turning the frame 

 carrying the lens and mirror in either direction will seem to 

 carry the object in the same direction relatively to the degrees of 

 the card ; or in the contrary direction if the altitude exceeds 

 27°. But if the altitude of the object be just 27°, then the 

 azimuth instrument may be turned through many degrees on 

 either side of the compass-card, without sensibly altering the 

 apparent positions of the objects on the degree-divisions. 



II. — An Adjustable Deflector for completely dtterminin^ the 

 Compass Error when Sights of Heavenly Bodies or Compass 

 Marks on Shore are not available. 



About thirty years ago Sir [Edward Sabine gave a method 

 in which, by aid of deflecting magnets properly placed on pro- 

 jecting arms attached to the prism circle of the Admiralty 

 standard compass, a partial determination of the error of 

 the compass could be performed at any time, whether at sea 

 or in harbour, without the aid of sights of heavenly bodies 

 p, or compass marks on shore. The 



; adjustable magnetic deflector before 



you is designed for carrying out in 

 practice Sabine's method more rapidly 

 and more accurately, and for extend- 

 ing it, by aid of Archibald Smith's 

 theory, to the complete determination 

 of the compass error, with the excep- 

 tion of the constant term "A" of the 

 Admiralty notation, which in almost 

 every practical case is zero, and can 

 only have a sensible value in virtue of 

 some very marked want of symmetry 

 of the iron work in the neighbour- 

 hood of the compass.^ When it exists 

 it can easily be determined once for 



' I had a curious case lately of the effect of 

 unsymmetncal iron on a midship steering com- 

 pass, due to a steam-launch about thirty feet 

 long placed fore-and-aft on the port side of 

 the deck with its bow forward and its stern 

 five or six feet before the thwart-ship line 

 through the position of the compass. Adjust- 

 ment having been performed by means ol the 

 globes and magnetic correctors to correct the 

 quadrantal error (D), and the semicircular 

 error, it was found (as was expected) that the 

 compass was correct on the east and west 

 points, but showed equal westerly errors of 

 about 3J0 on the north and south points. There 

 were, therefore, approximately equal negative 

 values of "A" and " E " each i|°. The cap- 

 tain was, of course, warned of the change he 

 would find when he was relieved of the steam- 

 launch at Rangoon, the port of his destination. 

 The explanation of the westerly deviation 

 when the ship's head was north or south, by 

 the inductive magnetism of the steam-launch, 

 according to which its stern would be a true 

 north pole when the ship is on the north course, 

 and a true south pole when the ship is on the 

 south course, is obvious from the annexed diagram, in which the letters 

 n, s, denote true north pole and true south pole of induced magnetism in 

 he steam-launch when the ship's head is north magnetic. 



all and allowed for as if it were an index error of the compass 

 card, and it will, therefore, to avoid circumlocutions in the 

 statements which follow, be either supposed to be zero or allowed 

 for as index error. 



The new method is founded on the following four principles : — 



1. If the directive force on the compass needles be constant 

 on all courses of the ship, the compass is correct on all courses. 



2. If the directive force be equal on five different courses, it 

 will be equal on all courses. 



3. Supposirg the compass to be so nearly correct or to have 

 been so far approximately adjusted, that there is not more than 

 eight or ten degrees of error on any course, let the directive forces 

 be measured on two opposite courses. If these forces are equal 

 the compass is free from semicircular error on the two courses at 

 right angles to those on which the forces were measured ; if they 

 are unequal there is a semicircular error on the courses at right 

 angles to those on which the forces were measured, amounting 

 to the same fraction of the radian (57 "3°) that the difference ot 

 the measured forces is of their sum. 



4. The difference of the sums of the directive forces on oppo ,ite 

 courses in two lines at right angles to one another, divided by 

 the sum of the four forces, is equal to the proportion which the 

 quadrantal error, on the courses 45° from those on which the 

 observations were made, bears to 5 7 "3°. 



The deflector may be used either under way or in swinging the 

 ship at buoys. The whole proce?s of correcting the compass by 

 it is performed with the greatest ease and rapidity when under 

 way with sea room enough to steer steadily on each course for a 

 few minute?, and to turn rapidly from one course to another. 

 P'or each operation the ship must be kept on one course for three 

 or four minutes, if under way, by steering by aid of an auxiliary 

 compass, otherwise by hawsers in the usual manner if swinging 

 at buoys, or by means of steam-tugs, A variation of two or 

 three degrees in the course during the operation will not make a 

 third of a degree of error in the result as regards the final correc- 

 tion of the compass. The deflector reading is to be taken 

 according to the detailed directions in sections 14 and 15 of the 

 printed "Instructions." This reading maybe taken direct on 

 the small straight scale in the lower part of the instrument The 

 divided micrometer circle at the top is scarcely needed, as it is 

 easy to estimate the direct reading on the straight scale to a tenth 

 of a division, which is far more than accurate enough for all 

 practical purposes. This reading with a proper constant added 

 gives, in each case, the number measuring in arbitrary units the 

 magnitude of the direct force on the compass for the particular 

 course of the ship on which the observation is made. 



The adjustment by aid of the deflector is quite as accurate as 

 it can be by aid of compass marks or sights of sun or stars, though 

 on a clear day at any time when the sun's altitude is less than 40'', 

 or on any clear night, the adjuster will of course take advantage 

 of sights of sun or stars, whether he helps himself also with the 

 deflector or not. 



Ill, — N'ew Form oj Marine Dipping Needle for facilitating the 

 Correction of the Heeling Error. 



This instrument is designed as a substitute for the vibrating 

 vertical needle, hitherto in use for carrying out the observations 

 of vertical force, whether on board ship or on shore, required for 

 performing the operations described in Part iii. Section 4, and 

 ths last three pages of Part iv. of the Admiralty Manual. It 

 consists of a light bar-magnet or "needle" of hardened steel 

 wire, supported by means of a very small aluminium cradle on a 

 stretched platinum wire, of which the two straight parts on the 

 two sides of the needle are, as nearly as may be, in a line through 

 its centre of gravity. One flat end of the needle is painted white, 

 with a black line through its middle parallel to the platinum 

 wire. When the instrument is properly placed for use the 

 platinum wire is horizontal, and the needle is brought into a 

 horizontal position by turning one end of the platinum wire un'.il 

 the elastic force of the torsion balances the turning motive 

 (or " couple ") due to the vertical component of the magnetic 

 force of the locality. A divided circle is used (as the torsion 

 head of the original Coulomb's Torsion Balance) to measure the 

 degrees of torsion to which, according to Coulomb's original 

 discovery, the turning motive is proportional. Thus, the mag- 

 netic moment of the needle being constant, the vertical compor ent 

 of the magnetic force in the locality of the observation is measured 

 simply in degrees or divisions of the torsion head. A glass plate, 

 fixed in a vertical position parallel to the platinum wire and close to 

 the painted end of the needle, has a horizontal score across it on the 



