Wind GAGE, an instrument for measuring the force of the wind upon any given surface. It was invented by Dr Lind, who gives the following description of it, Phil. Trans. vol. lxv.

This instrument consists of two glass tubes AB, CD, (fig. 5.) of five or six inches in length. Their bores, which are so much the better for being equal, are about four-tenths of an inch in diameter. They are connected together like a siphon, by a small bent glass tube a b, the bore of which is about one-tenth of an inch in diameter. On the upper part of the leg AB there is a tube of latten brass, which is kneed, or bent perpendicularly outwards, and has its mouth open towards F. On the other leg CD, is a cover with a round hole G in the upper part of it two-tenths of an inch in diameter. This cover and the kneed tube are connected together by a slip of brass c d, which not only gives strength to the whole instrument, but also serves to hold the scale HI. The kneed tube and cover are fixed on with hard cement or sealing wax. To the same tube is soldered a piece of brass e, with a round hole in it to receive the steel spindle KL; and at f there is just such another piece of brass soldered to the brass hoop g h, which surrounds both legs of the instrument. There is a small shoulder on the spindle at f, upon which the instrument rests, and a small nut at i, to prevent it from being blown off the spindle by the wind. The whole instrument is easily turned round upon the spindle by the wind, so as always to present the mouth of the kneed tube towards it. The end of the spindle has a screw on it; by which it may be screwed into the top of a post or a stand made on purpose. It has also a hole at L, to admit a small lever for screwing it into wood with more readiness and facility. A thin plate of brass k is soldered to the kneed tube about half an inch above the round hole G, so as to prevent rain from falling into it. There is likewise a crooked tube AB (fig. 6.) to be put occasionally upon the mouth of the kneed tube F, in order to prevent rain from being blown into the mouth of the wind gage when it is left out all night, or exposed in the time of rain.

The force or momentum of the wind may be ascertained by the assistance of this instrument, by filling

the tubes half full of water, and pushing the scale a little up or down till the 0 of the scale, when the instrument is held up perpendicularly, be on the line with the surface of the water in both legs of the wind-gage. The instrument being thus adjusted, hold it up perpendicularly, and turning the mouth of the kneed tube towards the wind, observe how much the water is depressed by it in the one leg, and raised in the other. The sum of the two is the height of a column of water which the wind is capable of sustaining at that time; and every body that is opposed to that wind will be pressed upon by a force equal to the weight of a column of water, having its base equal to the altitude of the column of water sustained by the wind in the wind gage. Hence the force of the wind upon any body where the surface opposed to it is known may be easily found; and a ready comparison may be made betwixt the strength of one gale of wind and that of another.

The force of the wind may be likewise measured with this instrument, by filling it until the water runs out of the hole G. For if we then hold it up to the wind as before, a quantity of water will be blown out; and if both legs of the instrument are of the same bore, the height of the column sustained will be equal to double the column of water in either leg, or the sum of what is wanting in both legs. But if the legs are of unequal bores, neither of these will give the true height of the column of water which the wind sustained. But the true height may be obtained by the following formulae.

Suppose that after a gale of wind which had blown the water from A to B (fig. 7.) forcing it at the same time through the other tube out at E, the surface of the water should be found standing at some level DG, and it were required to know what was the height of the column EF or AB, which the wind sustained. In order to obtain this, it is only necessary to find the height of the columns DB or GF, which are constantly equal to one another; for either of these added to one of the equal columns AD, EG, will give the true height of the column of water which the wind sustained.

1. Let the diameters, AC, EH, of the tubes be respectively represented by c d; and let a = AD, or EG, and x = DB, or GF: Then it is evident, that the column DB is to the column EG as c^2 x to d^2 a. But these columns are equal. Therefore c^2 x = d^2 a; and consequently x = \frac{d^2 a}{c^2}.

2. But if at any instant of time whilst the wind was blowing, it was observed, that, when the water stood at E, the top of the tube out of which it is forced, it was depressed in the other to some given level BF, the altitude at which it would have stood in each, had it immediately subsided, may be found in the following manner: Let b = AB or EF.—Then it is evident that the column DB is equal to the difference of columns EF, FG. But the difference of these columns

is as d^2 b = d^2 x; and consequently x = \frac{d^2 b}{c^2 + d^2}.

For the cases when the wind blows in at the narrow leg of the instrument: Let AB = EF = b, EG or AD = a, GF = DB = x, and the diameters EH, GA, respectively

respectively =d, c, as before. Then it is evident, that the column AD is to the column GF as ac^2 to d^2x. But these columns are equal; therefore d^2x = ac^2; and consequently x = \frac{ac^2}{d^2}. It is also evident that the column AD is equal to the difference of the columns AB, DB; but the difference of these columns is as b c^2 - c^2 x. Therefore d^2x = b c^2 - c^2 x. Whence we get x = \frac{b c^2}{d^2 + c^2}.

The use of the small tube of communication a b (fig. 5.) is to check the undulation of the water, so that the height of it may be read off from the scale with ease and certainty. But it is particularly designed to prevent the water from being thrown up to a much greater or less altitude than the true height of the column which the wind is able at that time to sustain, from its receiving a sudden impulse whilst it is vibrating either in its ascent or descent. As in some cases the water in this instrument might be liable to freeze, and thus break the tubes, Dr Lind recommends a saturated solution of sea salt to be used instead of it, which does not freeze till Fahrenheit's thermometer falls to 0.