# Page:Popular Science Monthly Volume 91.djvu/639

Popular Science Monthly

Range-Finder to Locate Landmarks and Signal Fires

��WE NATURALLY think of a range- finder as that part of a gunner's equipment which tells him the distance to his target, thus enabling him to give the proper elevation to his gun. But the range-finder off the battlefield is a part of the equipment of a tramping kit which will tell the width of valleys and rivers and the distances to landmarks and signal fires.

��until the two inside edges of the images are parallel to the thin edge of the wedge. When the one image-edge is in line with the wedge's thin edge, the image-edge in the other mirror should also be in line with it. The image positions should appear as

���A range-finder suitable for the woodsman or camper to aid in locating landmarks and computing their distances or the width of rivers and valleys. It is made of odds and ends of scrap

��Any boy can make his own range finder from materials that might otherwise go to the scrap-heap.

The essential feature of the device is shown at A. This is cut from a wedge- shaped piece of wood 2 in. long, 1*4 in. wide, and 3/32 in. thick. From the thick end it is tapered to a very thin edge at the other end. Cut away a piece }^-in. wide extending from the thin end to J^ the way toward the thick end.

Two strips 2> l A in. long and J^ in. wide are cut from a good quality of thin mirror, and they are fastened to the prepared wedge of wood with bits of beeswax. Both mirrors face toward the front as shown.

Lay the mounted mirrors face-up on the floor, slipping something under one side of the wedge so that the lower mirror will be level. Cut from stiff white paper a piece exactly 2 in. square, then hold it in a horizontal position just 20 in. above the thin edge of the wood wedge, peering down at the mirrors through a hole punched in the center of the paper square.

Two images of the card will be seen, one in each mirror. Slip the card along, keeping it 20 in. from the wedge-edge, rotating it

��indicated by the shaded portions in the mirrors. If, however, the images overlap, then the mirror strips are too nearly parallel; if they do not touch, then the angle between the mirrors is too great. In either case the angle between the mirrors must be corrected by pressing the mirror strips firmly against their beeswax pellets

���A wedge shaped piece of wood with thin mir- rors attached is fastened to a baseboard

until they fulfill the alinement test at the 20-in. distance. A little patience is needed. When successful take a stick of red sealing wax and a hot nail and wax the edges of the

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