COMPASS, THE MARINERS. This in valuable instrument is a cylindrical box, gene rally of brass, in the centre of which is fixed vertically a steel pin terminating in a fine co nical point, and on this is accurately balanced a magnetised needle, or a circular card to the underside of which such a needle is attached in the direction of a diameter. In the centre of the needle is usually inserted a piece of agate in the lower part of which is sunk a co nical hole to receive the point of the pin ; and the needle with the card is supported on that point so as to traverse freely in a horizontal position. The rim of the compass-box, or the circumference of the card, is divided into thirty-two equal parts called points, which have symmetrical designations on opposite sides of each of the four cardinal points as they are called, namely, the north, the east, the south, and the west. In some compasses the points are subdivided into quarters of 111 degrees each, and sometimes into single degrees.
When the needle is attached to a card, the north point on the card is directly over the northern extremity or pole of the needle. To insure the horizontality of the compass-card the cylindrical box in which it is enclosed is supported in a hoop at opposite points by pins projecting from it, so as to allow the box to revolve inside the hoop. This hoop is sup ported in the same manner on pivots, the line of which is at right angles to the first pivots ; so that, between the rotation of the compass. box in the hoop and the hoop itself, the former can always find its position of equilibrium, which is the horizontal position. The small oscillations of the apparatus are immediately destroyed by the friction. The apparatus is then said to be supported on gimbles, or gim bals.
The Mariner's Compass still remains, in principle, what it has been for ages ; but there have not been wanting attempts to improve it in minor details. Mr. David Napier included
in his patent of 1848 a kind of compass-box having an analogous movement to that of his barometer [BAROMETER]. There is ordinary clock-work machinery placed at the bottom of the compass-box. Connected with this are a lever and a pricker, which rise and fall at cer tain regular intervals of time. Within the compass-box is placed a ruled circular piece of paper containing 21 concentric circles, and a number of radiating lines corresponding to the points of the compass in a compass card. The vertical pricker is made to travel over the surface of the paper from the inner concen tric circle to the outer one, and in a line pa rallel with the keel of the vessel, once in 21 hours. The pricker punctures the paper at certain regular intervals of time ; while the paper follows the movements of the magnet and compass card ; the consequence of which is that the direction of the ship's course will be indicated by the punctures on the radiating lines, while the hours of the day which corre spond to these directions will be marked by the punctures on the concentric circles. A new paper has to be used every day: each paper being a record of one day's proceedings.
The Admiralty have recently bestowed a good deal of attention on the storm compasses of Mr. Walker, which are becoming extensively used in the navy. During a storm the ordi nary compasses are liable to great derange ment by the violent commotion ; and Mr. Walker has sought to devise such an adjust ment of the apparatus as will bring this evil to a minimum.