Wing compasses, or those with arc and set-screw, are easy to adjust accurately and will not slip, but, whatever kind you get, be sure that the points stay where you put them and do not spring away or wobble around.

The chief uses of this tool are to strike circles, to lay off angles and arcs, to take off measurements from a rule or some object, to lay off measurements, and to " scribe " in places where a gauge can not be used (see Scribing). In using compasses, particularly those which are not set by a screw, hold them and swing them around by the top at the hinged joint, rather than grasp them near the points, which may cause them to move or slip.

Circles or circular arcs can be struck roughly, as you doubtless know, with a string and a nail at the centre, the string being loose around the nail. This method is not very accurate, for obvious reasons, and is only suitable for rough work. A more accurate way is to drive two nails through a strip of wood at a distance apart just equal to the radius of the required circle, one nail being driven into the wood to act as the centre, the other doing the marking (Fig. 495). Instead of the marking nail a hole can be bored for a pencil. You can use a stick of this sort repeatedly by changing the position of the centre nail, or of the marking point. The same can be done with a brad or stout pin and a pencil, using stiff paper, card-board, or zinc instead of a stick. By such expedients you can do a great deal of work without buying compasses.

Fig. 495.