IV. COMPASSES. 647 



IV. COMPASSES. 



SPECIMENS OF THREE DIFFERENT SIZES (4-iNCH, G-INCTT, 8 -INCH, 

 IO-INCH) OF MARINER'S COMPASS, WITH SUN AND STAR 

 AZIMUTH MIRROR, AND BINNACLE CONTAINING CORRECTORS 

 FOR QUADRANTAL SEMICIRCULAR AND HEELING ERRORS. 



Sir William Thomson. 



3145b. I. Binnacle, with correctors for quadrantal semi- 

 circular and heeling errors, with mirror azimuth instrument on 

 bowl, for taking bearings of sun, stars, or terrestrial objects. Small 

 size compass (4-inch card), suitable for armour-clads or other ships, 

 with quadrantal error exceeding 1 1. 



3145c. II. 6-inch Compass Card, suitable when quad- 

 rantal error is from 7 to 11. 



3145d. III. Medium size (8-inch card) suitable for 

 standard or steering compass in any ship, iron or wooden, steam or 

 sailing, having quadrantal error less than 7. 



3145e. IV. Large size (lO-inch card) suitable for stan- 

 dard compass when quadrantal error does not exceed 5. 



In the improvements here illustrated the object primarily aimed at was to 

 obtain a compass to which the Astronomer Royal's correctors could be 

 applied with safety and convenience. The quadrantal correctors must not be 

 so near the needles of the compass as to sensibly affect its direction through 

 magnetization of the soft iron by the influence of the needles, otherwise the 

 quadrantal error will, if truly corrected in middle latitudes, be over corrected 

 in high magnetic latitudes, and under corrected in low magnetic latitudes. 

 Thus when the 12-inch iron cylinders of the Liverpool Compass Committee 

 are applied with their ends at a distance of 7 inches from the centre of an 

 Admiralty compass card in this country, they correct a quadrantal error of 

 12, but of this 7 ^ is due to magnetization of the iron by the compass 

 needles, and only the remainder or 5 is genuine, that is to say, dependant on 

 the magnetization of the correctors by the terrestrial magnetic force. Blocks 

 of iron weighing many tons would be necessary to safely correct for all lati- 

 tudes a quadrantal error of even so moderate an amount as 5 or 6, when the 

 compass needles are of so great magnetic moment as those of the Admiralty 

 standard compass. But if, as in the several sizes of compass now exhibited, 

 the needles are of thin steel wire from an inch and three quarters to three 

 inches long, a quadrantal error of any amount not exceeding 21 may be cor- 

 rected perfectly in all latitudes by a couple of globes of iron of not more than 

 6 inches diameter fixed on two sides of the compass. 



To correct a quadrantal error of 21^ a couple of globes, each 6 inches in 

 diameter, fixed on the two sides of the binnacle at a distance of 6 inches 

 asunder may be used, with the bearing point of the compass midway between 

 them. Hence, to allow room for the case containing the compass, and for 

 the gimbals supporting it, the diameter of the compass card must not be more 

 than 4 inches (I.). This is the smallest of the four sizes now exhibited. 



When the quadrantal error is 11 the globes, if of 6 inches diameter, must 

 be placed 9 inches asunder and the 6 inches diameter compass card (II.) may 

 be used. If with the same size of globes the distance asunder is 12 inches, 



