NA TURE 



{April 22, 1 88(3 



One of the first of these instructive models was that 

 constructed for Sir George Airy, and used during his 

 lectures to illustrate his method of correction of the 

 deviations of the compass. It consisted of a model of the 

 wooden hull of a vessel. In the centre of the deck a 

 compass was mounted, disturbing magnets and pieces of 

 soft iron being concealed underneath it, producing semi- 

 circular and quadrantal deviations as in an iron ship. 

 These deviations were then corrected by placing the 

 model ship with its bow alternately on the north and 

 south magnetic points, when the compass was made to 

 indicate the same directions by means of transverse 

 magnets on the deck ; and then on the east and 

 west points magnetic, the correction of the semicir- 

 cular deviation being completed by longitudinal mag- 

 nets on the deck. Lastly, with the model placed in 

 a north-east and south-west direction magnetic, scrolls 

 of soft iron were placed on either side of the compass — 



an imaginary line transverse to the model passing hori- 

 zontally through the centre of the scrolls and compass- 

 card — until the compass pointed correctly. 



Of more recent models there are three which are of an 

 instructive character: one designed by Dr. Neumayer 

 of the German Naval Observatory at Hamburg ; the 

 second in England by an official of the Board of Trade ; 

 and the third, which is the most complete both for experi- 

 ments and purposes of instruction, by the United States 

 Navy Department. 



The accompanying woodcut on the scale of one- 

 twentieth of the original model is taken from Paper 

 No. 2 of the Archiv der Deiitsclicn Scewartc, VI. Jahr- 

 gang, 18S3, where an account of the experiments to be 

 made with it is given in full detail. The following is a 

 description of the several parts shown. 



S is a pillar fixed in the floor of the room, upon which 

 pivots the wooden board a b, with the line of its central 



axis marked. At the point T, a compass-card is fixed to 

 s, with its north and south points adjustable in the mag- 

 netic meridian. 



Supported by the two brass uprights «'//, is the 

 second board in the form of a ship's deck pivoting at 

 d d' , so that it can be inclined sideways, as when a ship 

 inclines under pressure of sail or when rolling, but kept 

 horizontal as required by the screws s s. An arc, o p^ 

 marked to degrees, shows the angle of inclination. A 

 gimballed compass, C, with sight vanes, is mounted on 

 the deck, and when the lubber's point and the pin i are 

 in line, as seen through the vanes, the compass sup- 

 port is secured by the clamping-screw, o P' is a gradu- 

 ated arm revolving round the base of the compass stand, 

 grooved to receive a bar-magnet, and with a pointer, ;■, 

 showing the number of degrees the arm has been turned 

 in azimuth. /; and /;' are brass bearers for carrying the 

 rods of soft iron used in disturbing or correcting the 



compass, with screws, s, for clamping the rods at any 

 required distance. 



The model, as described thus far, is entirely free from 

 any magnetic body external to the compass, and may, by 

 means of the latter, be placed with its marked axis in 

 the magnetic meridian, the compass card at T being fixed 

 in that direction for future reference. The means for 

 producing the disturbing forces on the compass similar 

 to those found in iron ships are these. M n is a magnet, 

 which may be so adjusted in the groove that, by moving 

 the arm O P' in azimuth, semicircular deviation of any 

 desired form may be produced. In the figure the mag- 

 net M N is placed to produce the semicircular deviation 

 of a ship built with her head north-north-west, and the 

 resulting south (or blue) pole is found in the point R. 

 The soft iron rod v v in its vertical position represents 

 the stern-post of a ship, producing that part of the semi- 

 circular deviation in compasses placed near it, which 



