Chap. VIII.

GEOMETRY.

829

This is usually made of brass or wood indiameter, mounted in a

Of the Compass and the Magnetic Needle .the shape of a small circular box, about 4 or 5 inchessquare mahogany case, as shown at GHKI. Each side isdivided, and a line drawn across it at right angles; at C, thecentre, several circles are described, the outer one dividedinto 360 degrees ; in the centre is suspended the needle ; on thetop are two sights, through which the observations are to betaken; the box is covered with glass set in a brass rim, themargin of which is divided into 360 degrees. The compasswhen used is attached by a joint, which enables it to be movedin any direction on the stand provided for it; a ball and socketis usually found to be the most convenient for the purpose.

The mariner’s compass has a circular card attached to its needle,which turns with it, on the circumference of which are markedthe degrees, and also the thirty-two points or rhumbs likewisedivided into half and quarter points. The pivot rises from thecentre of the bottom of a circular box, called the compass-box,which contains the needle and its card, and is covered with aglass top to prevent the needle from being disturbed by theagitation of the air. The notation is marked out by dividingthe circumference into four quadrants by two diameters at right Fig. 1035.angles, the extremities of which are the four cardinal points marked N, S, E, W. Bisectingeach of the quadrants, the several points of bisection are denoted by placing the two lettersat the extremity of the quadrant in juxtaposition ; thus NE denotes the point which ishalf way between north and east, and so with N\V, SE, SW. Let the octants next bebisected, the points of division are denoted by prefixing to each of the above combinations,first the one and then the other of the two cardinal points of which it is formed. ThusNE gives NNE and ENE, and so with the others: sixteen points having thus been

Fig. 1034.

Fig. 1036. Fig. 1037.

fixed, their distances are to be bisected, and each of the points so found is expressed by thatone of the preceding points already named to which it is nearest, followed by the name ofthe cardinal point towards which its departure from the nearest points leads it, the twobeing separated by the little letter 6; thus the point half-way between N and N N E isNbE, that which is half-way between NNE and N E is NEbN: the whole of thethirty-two points are thus established.

The variation of the needle is its deviationfrom the north; and to find the true meridian ofa place by the sun, we may fix a piquet upon ahorizontal plane, either perpendicular or inclined,as that at A B, with a plumb attached, as BC:from the point C, which is found by the plumb,and with the radius at which the point of theshadow D finishes before noon, describe the arcI) F; then in the afternoon observe where theshadow touches the arc a second time, as at F,and draw the line D F; bisect this line, and raisethe perpendicular C G, and this line will be themeridian. By placing the compass at I and K,its variation may then be observed from the truemeridian.

Fig. 1038.