accurately as possible, one inch apart, and centered in the longitudinal groove, but are made larger than the holes in the blocks, so as to provide for slight adjustment. Large Beam Caliper [Illustration: Fig. 18. Large Beam Caliper] Fig. 18 shows a large beam caliper designed for machinists and patternmakers. It consists of a beam _MN_ and the legs _R_ and _S_, made of cherry wood to the dimensions indicated. The legs are secured in position on the beam by means of the thumb screws _A_, which jam against the gibs _C_ at the points of the screws. The gibs have holes countersunk for the screws to enter, to hold them approximately in place, and the nuts _B_ are of brass, fitted into the filling pieces _P_ that keep them from turning. The filling pieces are riveted to the legs by means of cherry dowels _D_. One leg _S_ is provided with a fine adjustment consisting of flexible steel spring _H_, ending in a point which is adjusted by the thumb screw _E_. This screw is locked in adjustment by the check nut _G_ bearing against the brass nut _F_, which is inserted in the leg as shown.[8] * * * * * CHAPTER III MICROMETER MEASURING INSTRUMENTS Of all measuring instruments used in the shop intended for accurate measurements, those working on the principle of the ordinary micrometer calipers are the most common. In the present chapter we shall describe and illustrate a number of different designs of these tools, intended to be used for various purposes. The instruments shown in Figs. 19 to 23 were built, in leisure hours, by Mr. A. L. Monrad, of East Hartford, Conn. Micrometer for Snap Gages [Illustration: Fig. 19. Micrometer for Snap Gages] Fig. 19 shows a form of micrometer that has proved very handy for measuring snap gages, and thicknesses, and can also be used as a small height gage to measure the distance from a shoulder to the base, as shown in Fig. 20. In measuring snap gages or thicknesses, the outside and inside of the measuring disks are used, respectively. This instrument may also come in very handy when setting tools on the planer or shaper. As will be seen in the engraving, there are two sets of graduations on the sleeve _A_, thus enabling the operator to tell at a glance what measurement is obtained from the outside or the inside of the measuring disks. Each of the disks is 0.100 inch thick, so that the range of the micrometer is 0.800 and 1.000 inch for the outside and inside, respectively. The details of the instrument are as follows: