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Pencil Point D. Fig. 10 In selecting the foregoing instruments, care should be taken that they are all sector- jointed with double-leaves and well made ; there should be no shake or slackness in any of them, and they should be equally stiff in the joints at any point from being full open to closing. The test for a pair of compasses is to open out their legs well apart and then to fold each lower half-leg together if the points meet each other truly, they are correct in the joints ; if they cross one another, the joints are not properly made. Drawing or Ruling-pens are of two kinds viz., those made with a jointed nib, as in Fig. 11, and those without a joint, as in Fig. HA. The former, though more expensive, is to be preferred, on account of the facility in cleaning and sharpening ; but the latter is a very serviceable MECHANICAL AND ENGINEERING DRAWING 11 pen, if well made and finished. It will be observed in the sketch of the first, that the under or fixed nib is much straighter and thicker than the hinged one ; this is so made to resist the pressure of the hand upon it when drawn along the edge of the tee- or set-squares. In all ordinary pens the nibs are of equal thickness, and the hand-pressure tends to close them and prevent the flow of the ink ; but by providing a stout springless inner nib this tendency is overcome. The stem or handle of this pen, it will be noticed, is squared, to indicate how it should be held by the fingers when in use. Fig. 11 X>