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Anemometer

wind, force, water, instrument, tube and line

ANEMOMETER. An in strument for measuring the strength or velocity of the wind. Among various machines which have been constructed for this purpose, the following one has been found to answer very well. It consists of an open frame a b c, supported by a shaft d, upon which it turns by the action of the wind upon the vane e. f f are sails, fixed to one end of the axis g, and disposed to be influ enced by the wind in the usual manner. Upon this axis is also fixed a conical barrel of wood Ak, on the smaller end of which k is attached a line I, with a weight appended to it. The wind acting upon the sails, causes the barrel to revolve, and the line to be wound up on its superfices. To prevent any retrograde motion, a ratchet wheel o is fixed to the base or larger end of the cone, having a clicker falling into the notches as it revolves. It is evident that the power of the weight will continually increase as the line advances towards the base of the cone, as the weight acts at a greater distance from the axis or fulcrum ; consequently, the variable force of the wind may be readily ascertained by fixing the line at the smallest end, and marking the barrel with spiral lines, as taken up by the coiling of the rope round its superficies, placing also between the lines numerals to denote the force of the wind, which may be calculated with tolerable pre cision upon the principles of the lever. The diameter of the cone should be such in comparison with its smallest end, that the force of the strongest wind should have scarcely sufficient force to bring ,$he line on to the base of the cone.

Although the instrument above described gives an accurate idea of the com parative force of the wind at different times, it does not point out the actual force exerted on a given surface, nor can observations made with one instrument in a particular place, be compared with observations made by another instru ment elsewhere. It is also cumbersome, and not portable. In the Philosophical Transactions for 1775, Dr. Lind gives a description of a very ingenious portable wind gauge, which indicates the actual force of the wind by the column of water which it will support. This instrument consists

of two glass tubes a b c d, which should not be less than 8 or 9 inches long, the bore of each being about of an inch diameter, and conilited together by a small bent glass tube e of about' only of an inch bore, to check the undulations of the water caused by a sudden gust of wind. On the upper end of the tube a 6 is fitted a thin metal tube f which is bent at right angles, and has its mouth open to receive the wind blowing into it horizontally. The two branches of the tube are at liberty to turn round a steer spindle g, which passes through two slips of brass h i near the top and bottom of the instrument. The spindle is fixed into a block of wood by a screw in Its bot tom. When the instrument is used, a quantity of water is poured in until the tubes are about half full, and the instrument being then held perpendicularly, with its mouth exposed to the wind, the water will be depressed in the tube a b, and proportionably elevated in the tube c d; and the distance between the surfaces in the two tubes measured by a sliding scale of inches, and parts k attached to the instrument, will be the height of a column of water which the wind is capable of sustaining at that time; and as a cubic foot of water weighs 1000 oz. or 62i lbs. nearly, the twelfth part of which is 5h lbs., therefore every inch the surface of the water is raised, the force of the wind will be equal to so many times 5h lbs. on the square foot. This instrument shows the force, but not the velocity, of the wind ; but as the force is as the square of the velocity, if the velocity due to a given force be ascertained, a table of the velocities corresponding to each inch the water is elevated, may be calculated and engraved upon the scale of equal parts. The following table, showing the corresponding height of water, velocity of the wind, and the force exerted upon a square foot of surface, has been calculated from some experiments made by Dr. Hutton.