ANEMOMETER. A measurer of the ve locity or force of wind. To ascertain this force is sometimes of considerable mo ment to mechanical science. Cronne, in the 17th century, and Woinns and others in later times, have invented instruments for this purpose with butpartial success; in them the velocity of the wind was measured by its mechanical force, as by the compression of a spiral spring, or by the elevation of a weight round a centre, acting at the arm of a variable lever. Leslie's anemometer depended on the principle that a stream of air had a cool ing power in proportion to its velocity. Lind's anemometer raises a column of fluid above the general level of its sur face. It consists of two tubes, 9 inches long and inch wide, connected together at the bottom by a tube of very narrow bore; a thin metal cap, bent at right an gles, is fitted upon one extremity, so that it may receive the current of air in a hori zontal direction. The tubes are half filled with water, and a scale on which inches and tenths can be read off placed be tween the tubes. When the wind blows into the cap it depresses the water in the first tube and raise's it in the second, so that the distance of the surfaces of the fluid is the length of a column of water equal to the base of the column of fluid. The absolute velocity of the wind is cal culated from the height of the column of water, or it may be ascertained from tables made for the purpose.
Dr. Whewel's form of anemometer is one in much use. By means of a vane a windmill-fly is presented to the wind, the fly revolving with more or less velo city, according to the rapidity of the wind. By means of an endless screw and wheel-work attached, the motion of the fly brings a pencil down over a fixed cy linder, tracing a certain path, which may be longer or shorter, as the wind is rapid or slow. The pencil descends only one twentieth of an inch with 10,000 revolu tions of the fly. The surface of the cylin der is whited, and divided into thirty-two equal parts by vertical lines, the spaces corresponding to the points of the com pass, so that a mark left by the pencil in these spaces indicates the direction of the wind. The pencil moves in two ways: downwards to indicate the velocity, and laterally to indicate its direction. The cylinder is fixed, the vane and wheel work being on a turning-table to which the pencil is connected, and these are obedient to the wind. The friction in this machine is very great, arising from the wheel-work 'and the pencil; in the former it chiefly resides, because a rapid Motion has to be converted into a descend ing slow one.
Oslor's anemometer traces the direction of the wind and its pressure a givens pace, with the fall of rain, on a re gister divided into twenty-four spaces, corresponding to the hours of the day. A clean paper register is placed on the board every day, which is carried on by clock-work behind three pencils or in dices. The board moves on friction rol lers, and is kept constantly and hourly moving, so that a continued record or trace of the direction and pressure of the wind, together with the amount of rain, is left on the paper and it indicates the direction, the duration, and the force of the wind.
Mr. Philipps, in a paper read to the British Association, has reproduced Les lie's principle in his " Anemoscope," in which be proposes to measure the velo city of air by the rapidity of evaporation, and the cold produced thereby. When the bulb of a thermometer„covered with cotton wool, is immersed in water and exposed to the air, the evaporation is known to produce a given amount of diminution of temperature, and when the thermometer is moved through the air, the rapidity of evaporation is in creased. By repeated trials in tranquil air, and when the thermometer was in motion, he was enabled to ascertain the increased rates of cooling by various de grees of speed, and on the other hand to tell the amount of speed by the rapidity of cooling. He tested the instrument on the South-Western Railway (England), and when the carriages were at the velo city of thirty-six miles an hour, his new anemometer indicated correctly the amount of velocity in its being held two feet from'the carriage.
Dr. Robinson, of Armagh, Ireland, in 1846 constructed and worked an anemo the connection of the motion of which with the velocity is subject to lit tle variation, and is of easy determination. It consists of two or three arms attached to a spindle, carrying at their extremities hollow hemispheres 'of tin and copper, with the hollows of the hemispheres all turned in the one direction. The force of the wind exerted on the concave sur face being four times than on the convex, the spindle is made to turn in the same whatever waythe wind blows. Attached to the spindle are the count-wheels of a gasometer, and the ye locity thus determined is exactly the one third of that of the wind. So trivial is the friction in this machine compared with its power, that its motion was quite perceptible in breezes which were too gentle to disturb the leaves on neighbor ing poplar trees. It is the most correct anemometer as yet invented.