��mirror strips securely to the wood wedge, being very careful not to disturb the adjust- ment of the strips. Make the sealing wax very hot with the nail so that it will not break loose.
The other part of the work is easy. Cut from a board a piece measuring 8 in. long, 2 x /l in. wide and Yi m - thick, and nail a wood strip of ^-in. square cross-section across one end at a 45 deg. slant. Glue the wood wedge against this strip so that the mirrors stand as in B, facing away from the supporting strip.
To complete the range-finder cut a piece of tin 1% in. square. Cut into it a slit 1 in. long and 1/64 in. wide and tack the tin against the end of the baseboard so that the slit and the thin edge of the wood wedge are parallel and the same distance from the side edge of the board.
The distance of an object, C, may be determined as illustrated. Turn the left cheek toward the distant object and, look- ing straight ahead, hold the slit of the range-finder close up to the left eye and look through the slit into the mirrors. Point the instrument in such a direction that the image of the distant object, as seen in the upper mirror, lies exactly above and directly in line with the thin edge of the wedge. At the same instant, glance past the wedge-edge and note some feature in the landscape which is in line with this edge, a tree, D, for instance. Now with the eye at the slit walk directly toward this object, D, counting the number of paces taken, still keeping the wedge-edge in line with the tree. Meanwhile the image of C in the upper mirror moves on out to the right and at the same time its image as seen in the lower mirror moves up from the left until it in its turn comes into line with the wedge-edge. At the instant this happens note how many paces, E, have been taken from the first observing station. Multiply this number by 10 and you have the number of paces, F, from the first point of observa- tion out to C. If you have learned to make a stride of 36 in., then the distances are known at once in yards, otherwise you must know the length of the natural stride before you can obtain distances in yards. Of course the paced distance can be measured with a tape-line. If the paced distance is 88 yards then C is 880 yards or half a mile away.
It is evident that to measure with this instrument the distance to an object 5 miles away one must pace off half a mile. This
��Popular Science Monthly
��is sometimes inconvenient. It is therefore desirable, for objects at this distance, or farther away, to use a range-finder whose wedge is of only half the angle described above; the 2-in. paper square must then be held at a 40-in. distance in adjusting the mirror strips. The paced distance is then to be multiplied by 20.
This type of range-finder yields astonish- ingly accurate results if it is equipped with good mirrors. It is a scientific instrument based upon the laws of reflection of light and surveyors' triangulation methods and is worthy of any boy's serious consideration.
��A Convenient Pivoted Card File for the Desk
FILING systems are necessary in all lines of business, and for convenience of small accounts or for data a single tray is often applicable. The desk tray illus- trated is especially designed and it fits into a slide made for it, so that it is never
���A desk card file tray pulls out the same as a drawer and turns on a pivot when drawn
in the way. Another feature of the tray is that it turns on a pivot and the cards face the person sitting at the desk.
Flanged bottoms hold the guide cards in place. There is a follower-block, simple in construction and operation, that holds the tabs in a readable position. Side tabs, as shown at A, permit more distinctive subdivisions. With each closing of the drawer an automatic angle-block tilts the cards back parallel with the follower-block. To further facilitate reference the index drawer is pivoted to its carrier-shelf.
The frame of the drawer is made up of two thick ends to which thin bottom and side pieces are nailed, the exact dimensions of which depend upon the size of the cards as well as upon the materials at hand. To the front end block, planed to serve