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AEROPLANE PROPELLERS. 116

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AEROPLANE PROPELLERS. 116 The earliest aeroplanes had propellers cut "ut of solid timber. These have long ago • boon given up, and the modern propeller or tractor is constructed of a number of different layers of wood (says a contem- N porary). Of these there are usually seven, although in the very newest types twenty or more may be used. In all the range of con- structional work no job demands greater care than the construction of an aeroplane- screw. The lives of the pilot and observer depend upon its perfection, and each sepa- raw layer or section is separately examined and stamped before being handed over, to the workmen. The woods used for 'the tractors of the fast scouts are walnut, beech, or elm; for slower machines, mahogany is considered' most satisfactory. The Ö various layers are glued together, and after much experimenting it has been found that there is nothing like casein or cheese glue for, this purpose. Cheese glue is no new invention. It was discovered by a monk some 800 years ago. New, soft cheese is first crushed in a mortar and washed in warm water; then hardened again in cold water. It is then mixed with quicklime and water to the thickness of cream, and the result is a glue which resists damp better than any other similar compound. For aeroplane propeller construction the actual cheese is not used, but "casein," which is the principle of cheese. After the sections have been glued together they are subjected to enormous pssnre, Then the tips are sheathed with copper. After that nothing remains but to ivaferTjroof and varnish the wood. and it is readv for use.

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