Science Siftings

New Zealand Tablet, 14 September 1911, Page 1827

Science Siftings

By Volt.'

Waterproofing Fabrics. The French army is interested in a new method for waterproofing fabrics which is claimed to be superior to usual processes. English manufacturers have been in the lead in bringing out waterproof fabrics for many years past. Besides rubber for fabrics, we have the use of insoluble gelatine, boiled linseed oil, shellac, metallic soaps, and others, which have had more or less success. It appears that the best product for waterproofing is acetate of aluminium, and when applied to the fabric it can be made to form alumina, in a gelatinous state, and volatile acetic acid. When such fabric is dried it is waterproof, but has not the objectionable feature of rubber and similar material, as it allows the air to circulate through it. The fabric is also quite supple, and this is especially necessary, so that clothing can be made of it. As the French army department is looking for the best method of waterproofing which can be used for military cloaks as well as for tents, some experiments were made, and it was found that the above process is the best, according to the opinion of the best authorities." Conductors of Sound. If one takes a strong piece of thread, about as long as one's two arms, loops the centre around the handle of a silver spoon so that it will strike the sharp edge of a table or shelf, there will be a pleasant singing sound, that is,softer or louder as the spoon is brought nearer to or held away from the ear. If the ends of the thread are twisted around the index finger several times, and the finger thrust into the ears while the spoon is struck, the result will be altogether astonishing. Not only can the sound be heard with much greater distinctness, but the waves of air caused by the sound and the swinging of the spoon can be felt in the ear. In this instance it is clear that the thread carries sound better than the air, though air is the medium through which sound usually reaches our ears. A little telephone can be made on this plan, with a silk thread and a tin roll or cylinder, perhaps a tin can of the size of a lamp chimney as a receiver. One can easily talk with a person twenty or twenty-five yards distant in this way. Water, too, is a good conductor of sound. On moist days or across a body of water, for instance, church bells can be heard much further and more plainly than in dry weather. If two stones are struck together under water, the sound can be heard very ■plainly, and fish will swim away at the faintest surface sound. Solid bodies carry sounds with particular directness. If one lays a watch on a wooden board or an iron bar or pipe, and the ear is then held to the board or the pipe, the ticking can be heard much further than through the air. But if the watch is put upon a porous body or wrapped in cotton, the sound will be weakened. It is this property of solid bodies to carry sound that led to the invention of the stethoscope, the instrument which physicians use to listen to the sounds of the heart and lungs. The thunder of cannons, the hoof-beats of horses, the march of soldiers, the rush of a train can all be heard at longer distances by placing the ear upon the earth. All these things show that solid and liquid bodies carry sound i better than do the air and porous bodies.