A later type of Sir Wm Thomson’s 10” compass and binnacle.
The card is an improvement on that of No. 46, but the principle of construction is the same and remain practically the same at this day (1930).In this specimen the binnacle conforms generally to the patent No. 679 of 1879 in which the horizontal magnets are carried in holes bored in the woodwork. Two rows of holes are in the fore-and-aft direction, one row at each side of the binnacle, and there is one athwartships row at the aft side.Instead of the bowl being carried in gimbal journals in the flange of the binnacle, an elastic grommet of twisted wire is suspended by two cups on the grommet resting on two balls secured to the flange of the binnacle in the fore-and-aft direction. From the grommet, in the athwartship direction, two sockets are suspended by short chains and these engage in knife-edged gimbals on the gimbal ring. The bowl is fitted with knife-edged gimbal pins close up to it supper edge – instead of the usual position co-incident with the horizontal plane of the point of suspension of the card. These are in the fore-and-aft direction and engage in suitable journals in the gimbal ring.The result of the arrangement of gimballing is to make the movement of the bowl pendulous instead of oscillating as with normal gimballing. The combination of the high point of gimballing of the bowl and the oil chamber below it appears to have been intended to reduce as far as possible any tendency of the bowl to move about its fore-and-aft gimbal pins.The inventor altered the type of gimballing because it was found that vibration due to the screw, and the firing of heavy guns, caused the light card to be unsteady when mounted in a bowl gimballed in the normal fashion as in No. 46, and thus it was necessary to evolve a new method of gimballing (see last paragraph of No. 46).
One result of the pendulous suspension of the bowl was that, after correcting the heeling error by vertical magnets, the latter had to be lowered two inches to allow for the effect produced by them on the compass card by the translation of the bowl. Captain Creak’s Elementary Manual for the Deviations of Compasses, p 118, clause (e), states that “this empiric rule of lowering the vertical magnets two inches was obtained from a series of experiments with magnets of varied strength and length etc”.
The bowl of this exhibit conforms to a patent of Lord Kelvin’s No. 22032 or 1902, by which the bowl is adapted for lighting from below by having a hemispherical glass oil chamber fitted below the bowl proper.
The azimuth mirror conforms to a further patent, No. 9514 of 1905, in which it rotates on a raised inner edge of the verge ring of the compass bowl instead of being centrally pivoted as had been the case previously. Readings can be taken with this instrument in two ways: (i) by looking direct at the object over the top of the prism. The degrees of the card reflected in the prism are then seen close below the object; (ii) by looking through the lens at the degree divisions of the compass card, and at the image of the object in the prism which is seen on the proper degree of the card. The first method is applicable to objects on the horizon, and is more particularly useful for taking bearings of distant landmarks which are too indistinct to be seen when reflected in the prism. The second method may be used either or taking bearings of objects on the horizon or for taking bearings of the sun, moon or stars.
The binnacle is fitted with sphere brackets, a tube for Flinders’ bar mounted in brackets, also a bucket for heeling error correctors. The hood is adapted to permit of bearings being taken by the azimuth mirror with the form shipped – a great advantage in wet weather.