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1915] AEOLIAN TONES 323
the latter can be in error. So far as I know, Strouhal's work has not been repeated; but the error most to be feared, that arising from the circulation of the air, acts in the wrong direction. In the hope of further light I have remounted my apparatus of 1879. The draught is obtained from a chimney. A structure of wood and paper is fitted to the fire-place, which may prevent all access of air to the chimney except through an elongated horizontal aperture in the front (vertical) wall. The length of the aperture is 26 inches (66 cm.), and the width 4 inches (10'2 cm.); and along its middle a gut string is stretched over bridges.
The draught "is regulated mainly by the amount of fire. It is well to have a margin, as it is easy to shunt a part through an aperture at the top of the enclosure, which can be closed partially or almost wholly by a superposed card. An adjustment'can sometimes be got by opening a door or window. A piece of paper thrown on the fire increases the draught considerably for about half a minute.
The string employed had a diameter of *95 mm.,, and it could readily be made to vibrate (in 3 segments) in unison with a fork of pitch 256. The 'octave, not difficult to'mistake, was verified by a resonator brought up close to the string. That the vibration is transverse to the wind is confirmed by the behaviour of the resonator, which goes out of action when held symmetrically. The sound, as heard in the open without assistance, was usually feeble, but became loud when the ear was held close to the wooden frame. The difficulty of the experiment is to determine the velocity of the wind, where it acts upon the string. I have attempted to do .this by a pendulum arrangement designed to determine the wind by its action upon an elongated piece of mirror (10'i cm. x 1'6 cm.) held perpendicularly and just in front of the string. The pendulum is supported on two points—in this respect like the one used for the • water experiments; the mirror is above, and there is a counter-weight below. An arm projects horizontally forward on which a rider can be placed. In commencing observations the wind is cut off by a large card inserted across the aperture and just behind the string. The pendulum then assumes a sighted position, determined in the usual way by reflexion. When the wind operates the mirror is carried with it, but is brought back to the sighted position by use of a rider of mass equal to '485 gm.
Observations have been taken on several occasions, but it will suffice to record one set whose result is about equal to the average. The (horizontal) distance of the rider from the axis of rotation was 62 mm., and the vertical distance of the centre line of the mirror from the same axis is 77 mm. The force of the wind upon the mirror was thus 62 x 485 4-77 gms. weight. The mean pressure P is
62 x -485 x 981 __ » dynes 77x16-2 cm.2 '
21—2 men what are perhaps misprints would need correction.on with soda ap to require the presence of oxygen (Mitcherlich, Smithells).