Since, in Pickering's anemometer, the force of the wind varies as the sine of the angle of elevation, we have only to determine the force which corresponds to any given angle, and, by means of a table of sines we may find the force which corresponds to any other angle; and consequently by the table at the end of this article, the velocity which is clue to that force. This theoretical division of the instrument, however, is not so accurate as if the limb had been divided experimentally. By means of a string attached to the centre of gravity of the sail, passing horizontally over a pulley, and carry ing different weights successively, we may easily find the force of the wind in pounds and ounces; or what is still better, we might fix the whole instrument upon Smeaton's apparatus, and graduate it in the same man ner as Zciher graduated the anemometer of Bougucr.
A very curious anemometer for measuring the sum of the velocities of any wind, and at the same time the variations of its direction, has been invented by Michael Lomonosow, and described in the Commentaries of Pe tersburgh for the year 1749. This instrument is repre sented in Plate XXVIII. Fig. 8. where A is a wheel composed of 16 sails, 24 inches long, 2 broad, and a line thick. These sails, which are kept together by iron wires, cc, gg, are fixed to the axis b, forming with it an angle of about 24 degrees. Half of the wheel A is included in the thin wooden box BCDE, and upon its axis is fixed a tooth d, which drives the wheel F, two feet in diameter, and furnished with 800 teeth. On the axis of F is fixed a pinion, h, which gives motion to a I 2 third wheel, 1\1, a foot in diameter, and carrying 400 teeth on its circumference, and also to its axis, fi, which A half a foot in diameter, and half an inch lung. At e is fixed a spring for pressing the wheel Al to the pinion b, to be removed from it at pleasure, by the string pqr. At in is another spring, which prevents toe wheel F from moving backwards. The wheels F and 111 are shut up in the box BCDE; and a wooden vane Q, five feet long, is fixed to the box at BD. The whole machine is fastened to the perforated rod, CK, which is cylindrical, excepting between L and K, where it has the form of a prismatic tetragon. In the roof TT, which supports the anemometer, a hole is made sufficiently large to admit the rod CK, which moves in the cone GG, which is protected from rain and snow by the hollow cone RR, fixed to CK, and moving along with it. A string, Jilt, covered with wax or pitch, is coiled round the axis of the wheel AI, and passing down the perforated rod CK, it winds round the circumference of the wheel H, divided into degrees, which are pointed out by the in dex n, fixed to the vertical rod. The wheel H moves in
the narrow box NN, and has a circular glass-tube, uvy, bent round its periphery. This tube has a bore about the 12th of an inch in diameter, and is filled with mer cury nearly to its extremities 11 y, which are in a hori zontal line. The box N, has a small hole in its bottom at k. The rod CK is placed in the centre of the circu lar receptacle X, divided into 32 equal cavities, Fig. 9, by thin wooden partitions, answering to the different points of the compass. The parts o of each cavity-box terminate in an acute angle, and are open above, like the remaining part of each cavity. The graduation of the scale on the wheel H, is performed experimentally as in other anemometers, by placing the instrument in a wind whose velocity is determined by the celerity of a light body floating in it, and by marking the space run over by the wheel H, and the number of turns performed by the sails.
When the instrument is exposed to the wind, and any of the points of the compass-box X, adjusted to the corresponding points of the horizon, the thin box NN, which is opposite to the vane Q, will shew the direc tion from which the wind blows. The wheel A being put in motion, will drive the wheels F, Al, and also the wheel H, by means of the string Jilt, so that the index 91 will point out the number of degrees upon the scale proportioned to the velocity of the wind. In the follow ing way may be shewn, during the absence of the ob server, the sum of the velocities of any wind. The spring e may be drawn back by the rope fir, so that the wheel AI; and its axis p, may be moved without coming in contact with the pinion h, and that the wheel H may be turned into that position where the index n will point to the beginning of the scale. By letting go the string rqh, the wheel AI will again come into contact with the pinion h, and may be put in motion by it. When the wind blows, the wheel H will be put in motion, and one of the apertures of the glass-tube being depressed, the mercury will flow out of it into the box NN, and pass through the hole k into the cavity of the box X, which answers to the quarter from which the wind blows. If the direction of the wind changes, the hole k being above another cavity, will convey the mercury into it, and thus determine, by the quantity of mercury in each, the quantity of wind that has blown from any quarter in a given time.