It is pleasing to turn from an instrument of this na ture, to one that is distinguished by simplicity and no velty of conception. Professor Leslie, to whom the sciences are under numerous obligations, has described, in his Essay on Heat, an instrument for measuring the velocity of the wind. He found, by experiment, that the cooling power of a stream of air is proportional to its velocity, and hence he deduced the following for mula, to express the relative degree of cooling, t V where T is the time in which a body loses an all quot part of its heat in the still air, t the time when it loses the same quantity when exposed to the wind, and V the velocity of the wind in miles per hour. In order to find V when T and t are given, the formula becomes , T , V= I •-•-•••--- X The instrument for ascertaining the quantities T and t, is nothing more than a common ther mometer, with its ball greater than hall an inch in dia meter, and filled with alcohol tinged with arehil. When the thermometer is held in the still air, its temperature is marked ; it is then heated by the application of the hand, till the alcohol rises a certain number of degrees, and the time which it takes to descend through half that number of degrees is carefully marked. This time Mr Leslie calls the fundamental measure of cooling. The same observation is made when the ball is exposed to the impression of the wind, and the time which the al cohol takes to descend through half the number of degrees that it rose, is called the occasional measure of cooling. Hence we have front the formula the follow ing rule. Divide the fundamental by the occasional mea sure of cooling, and the excess of the quotient above unit, being multiplied by 41, will express the velocity of the wind in miles per hour. In order to simplify the obser vation, a sliding scale of equal parts should be applied to the tube. After the ball has acquired the proper temperature, the zero of the slide is set opposite to the limit of the coloured liquor in the stem ; and if, after being heated, it stands at of the scale, the time which it thence takes to sink to 10°, is measured by a stop watch. When the common scale of the thermometer is used, let us suppose, that in the still air the tempera ture is 50°, that when warmed by the hand, the liquor rises to 70', and that it requires 100 seconds to cool down to 60. Suppose also, that when exposed to the wind, and heated with the hand, it takes only 10 seconds to fall through the same number of degrees ; then we have the velocity of the wind V=W-1 x miles per hour. This calculation may be avoided, by having a table engraved upon the scale, for the series of occa sional intervals of cooling.
The various anemometers that have been described, excepting those of Delamanon and Mr Leslie, measure the velocity of the wind by its mechanical effects. The compression of a spiral spring, the elevation of a weight round a centre, acting at the arm of a variable lever, are the means which have been employed to balance, and consequently to measure the force of the wind. The spring, however, which, from its simplicity, has been most commonly used, is far from being accurate. Every spring loses its elasticity after frequent compressions, and therefore the scale which ascertains its force most be perpetually varying. The following contrivances which have occurred to the writer of this article, seem to possess several important advantages for measuring the velocity of the wind.
The first of these instruments is represented in Plate XXIX. Fig. 12, where A is a plane surface, to he ex posed to the wind. It is fixed at right angles to the
toothed rod AB, which drives the pinion C. Round the axis D of this pinion, winds a string DE, which raises the long cylindrical weight F, out of a fluid contained in the glass vessel G. When the instrument is placed in the still air, and the weight F completely submerged in the fluid, the apparatus is in equilibrio by means of a counter-weight fixed on the axis D, on the other side of the pinion, and the index points to 0 on the scale.
When the anemometer is exposed to the wind, the toothed rod All drives the pinion C, and thus raises the weight E out of the fluid. But it is evident, that as the cylinder E rises, its weight gradually increases, till it is completely raised above the fluid, when it has received an increase of weight equal to the weight of a column of fluid of the same size as the cylinder. This variation of weight which the cylinder sustains, may be increased by augmenting its size, or by employing a fluid of greater density. The length of the scale is ob viously equal to the length of the cylinder, so that the circumference of the pulley or axis D should have the same length. The instrument may be accommodated to winds of any intensity, by varying the quantity of sur face which receives the impression of the wind; and is graduated in the same way as other anemometers.
The anemometer represented in Plate XXIX. Fig. 13, measures the force of the wind by the effect which it produces in compressing a columicor air, contained in the tube BC, and ball C. A short column of mercury, or any coloured fluid, is placed at B, the beginning of the scale, and the force of the wind, concentrated by the conical mouth AB, forces the liquor up the tube, upon which the scale BC is fixed for measuring the degree of compression, and consequently the force of the wind. An index made of iron, as in Six's thermo meter, may be made to float in the liquor, and remain in the part of the tube to which it was pushed by the wind. It is then drawn back to the fluid by a magnet, to be ready for another observation. If the greatest force of the wind which it is required to measure, should be able to compress a quantity of air into 5-6ths of its bulk, then the contents of the tube BC, should be a little more than 1-6th of the contents of the ball C, so that the greatest wind may just force the liquor within a little distance of the ball.
The instrument shewn in Plate XXIX. Fig. 14, de pends on the same principle as the preceding, but is perhaps more commodious and accurate. The metal cap AB, bent at a right angle, is fixed upon the top of the glass tube, B, C, which communicates at C, with another glass tube, DE, of a much smaller bore, with a bulb, E, at its extremity. A quantity of mercury or any other fluid is poured into the tube BC, and of course rises to the same level m n, in both tubes. When the mouth A, of the instrument, is exposed to the wind, the fluid at m descends in the tube, and by rising in the stem DE, it compresses the inclosed air, till there is an equilibrium between the elasticity of the air and the force of the wind. In order to prevent the oscillation of the fluid, a round and thin piece of wood floats upon its surface at nz. It is evident that the force which ba lances that of the wind, arises both from the elasticity of the air in the ball and the stem, and from the weight of the column of fluid, which is raised in the tube DE, above the fluid surface m, in the other tube BC : But as the scale may be formed by experiment, it is unne cessary to consider the effects of these separate resis tances.