Page:Popular Science Monthly Volume 92.djvu/617

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���FOR PRACTICAL WORKERS

��A Chemical Preparation to Preserve Cut Flowers

IF a little saltpeter, or carbonate of soda, is added to the water in which flowers are left standing, they can be kept comparatively fresh for more than two weeks. Another method is to add a small amount of ammonium chloride, or camphor, to the water. The presence of one of these substances stimulates the plant cells and acts in opposition to germ growth. Flowers that have wilted can be revived for a time if the stems are inserted in a solution of weak camphor water. — Herman Neuhaus.

��Simple Construction of a Useful Range Finder

ONE of the simplest range finders ever devised consists of a short tube, one end closed by a cap pierced with a pin-hole, the other end covered with a wire screen of square meshes.

The manipulation is as simple as the construction of the instrument. With the eye close to the pin-hole, look through the tube at some distant object of known

���The tube with its peep hole and screen cov- ered end and formula to compute distances

length (say a distant freight car). The ware mesh will stand out clearly in the field of view, and the number of spaces which are outlined against the object may be readily counted off. (In the accompanying illustration the tower is

��six spaces high.) It is evident that the object is farther from the peep-hole than the wire mesh, in the same ratio as the length of the object is to the aggregate width of screen openings which enclose it. (See preceding diagram). Suppose a 50-ft.

���In looking through the range finder the object is covered with the mesh screen

freight car is covered by four spaces of a 20-wire-to-the-inch screen. The distance to the car is obtained by multiplying the length of the range finder by the ratio of 50 ft. to 4/20 of an inch, i. e., by the ratio of 3000 to 1. If the range finder is 2 ft. in length, the car is two thousand yards distant.

It is plain that this type of range finder is restricted to use with objects of a knowTi dimension. The height of a distant man may be taken as 70-in., with a probable resulting error of less than 5 per cent. Objects of a fairly standard length should be chosen. Recording to fifths of a screen space is comparatively easy. Even a novice may determine ranges with an error not exceeding 10 per cent.

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