Anemometer (Gr. wind, and measure), an instrument for measuring the force of the wind. Attention was first given to this subject by Dr. Croune in 1667, and instruments were contrived by him and by Wol-fius and others in the last century. These have all given place, however, to recent inventions of more perfect construction. The first attempts were to measure the force of the wind by its pressure upon a vertical plane, kept in position by a spring or by a weight suspended by a cord wound around a conical spiral axis, which weight the wind would raise more or less according to the degree of pressure on the vertical plane. A bag of air opening into a glass tube which was shaped like the letter U, and contained a fluid which by compression of the bag was forced down one leg and up the other, was another contrivance for the same purpose. Another form of it was to dispense with the bag and turn one extremity of the tube against the wind, expanding it to a funnel shape, so that the wind might blow directly into it and press upon the surface of the fluid, The tube was drawn out to a small diameter in the curve at the bottom, so as to check the sudden fluctuations caused by irregular blasts of wind.

By means of this simple instrument, Dr. Lind, who invented it, ascertained the force of the wind at different velocities by the height of the column of water raised by it. A gentle breeze, moving at the rate of nearly four miles an hour, raises a column of water 1/40 of an inch, which is equivalent to a pressure of 2 1/3 ounces upon a square foot. A high wind moving 32 1/2 miles per hour raises the column 1 inch, with a pressure of nearly 5 1/4 pounds on the square foot. A column of 3 inches indicates a pressure of 15 1/2 pounds, and a velocity exceeding 56 1/4 miles an hour. At 9 inches the wind is a violent hurricane moving 97 1/2 miles an hour, and exerting a pressure on the square foot of 46 7/8 pounds. The atmospheric pressure being a little over 2,000 pounds on the square foot, or equal to a column of water 33 feet high, the greatest force exerted by the wind is feeble in comparison with this. - A more complicated apparatus was invented by Dr. Whew-ell, and another by Mr. Osier, both of which have been used in England at the meteorological observatories and government institutions.

Both are self-registering, and determine the force of the wind by the number of revolutions of a windmill fly, the axis of which by perpetual screws and toothed wheels is connected with the registering pencil. In Whewell's instrument the windmill with its wheels and vane is on a horizontal plate, which revolves on the top of a vertical cylinder. The pencil is attached to a little block of wood or nut, through which passes a screw from the horizontal plate above to a circular rim below the cylinder, all which revolves around the cylinder as the wind changes. A straight rod also goes through the pencil block or nut, up and down which it slides as the screw turns. According as the wind blows gently or strongly, this screw turns slowly or fast, and carries the pencil down the cylinder at a proportional rate. Its point reaches the surface of the cylinder and marks upon it its position, and as the frame turns with the change of direction of the wind, the course of the wind is registered upon the face of the cylinder. For this purpose it is divided by vertical lines into 16 or 32 equal parts corresponding to the points of the compass. This instrument is deficient in not recording the time during which each wind blows, nor the times of its changes, nor its force at any particular moment.

It merely gives the order of the changes, and the entire quantity that blows from each point. This is known by the vertical length of the pencil mark in each division of the cylinders corresponding to the courses. It is defective also by the friction of its machinery. Osier's instrument, constructed on similar principles, is more complicated than Whewell's. Its register is divided by lines into spaces, which represent the 24 hours of the the day, and in these spaces pencils inscribe lines, one of which indicates the direction, another the pressure, of the wind, and a third, connected with a rain gauge, the quantity of rain which has fallen at every hour. The register moves along by clockwork under the pencils, and at the meteorological observatory at Greenwich a new one is employed every day. In the royal exchange in London one of these instruments is in use with a register made to last a week. By the lines inscribed on the register the integral or quantity of the wind can he calculated that has blown to each point of the compass during the periods of the observations; and thence the resultant, or average effect of all the winds. - The instrument now in use in the United States office for weather reports is Robinson's anemometer, which consists of four horizontal arms (see figure) radiating from a central point, at which is a vertical axis of revolution.

A hollow hemispherical cup is attached to each arm in such manner that when the wind is pressing upon the concave side of a cup on one arm, the cup on the opposite arm presents its convex side toward the wind. The wind exerts more pressure on the concave side than on the convex, and hence causes the arms to revolve. The rate of revolution per minute gives the velocity of the wind. Each instrument has to be tested by placing it upon a moving body on a calm day. In this way it is easily found what the number of revolutions is which the instrument will give for any velocity; it is then placed upon a high building, and its axis attached to a recording apparatus similar to that described above. - Biram's anemometer is an instrument for measuring and registering the quantities of air which circulate through the passages of mines. It was invented in consequence of the recommendation of a committee appointed by the British house of commons, that the use of such an instrument should be adopted as a precaution against the explosions in coal mines. It is a disk of a foot diameter, made to revolve when placed in a current of air, and furnished with registering wheels like those upon a gas meter.

Any want of attention on the part of those having charge of supplying the required current of fresh air is thus readily detected.