ANEMOSCOPE.
Dr. Hales had various contrivances for this purpose. He found, that the air rushed out of a smith's bellows at the rate of 684 feet in a second of time, when compressed with a force of half a pound upon every square inch lying on the whole upper surface of the bellows. The velocity of the air, as it passed out of the trunk of his ventilators, was found to be at the rate of 3,000 feet in a minute, which is at the rate of 34 miles an hour. The same author says, that the velocity with which impelled air passes out at any orifice, may be determined by hanging a light Wire over the nose of a bellows, by pliant leather hinges, which will he much agitated and lifted up from a perpendicu lar, to a more than horizontal position, by the force of the rushing air.
M. Bouguer contrived a simple instru ment, by which may be immediately dis covered the force which the wind exerts on a given surface. This is a hollow tube, A A, B B, (Plate XVI. Miscel. fig. 13.) in which a spiral spring, C D, is fix ed, that may be more or less compressed by a rod, F•S D, passing through a hole within the tube at A A. Then having ob served to what degree different forces or given weights are capable of depressing the spiral, mark divisions on the rod in such a manner, that the mark at S may indicate the weight requisite to force the spring into the situation, C D: afterwards join at right angles to this rod at F, a plane surface, C F E, of any given area at pleasure ; then let this instrument be opposed to the wind, so that it may strike the surface perpendicularly, or parallel to the rod ; then will the mark at S show the weight to which the force of the wind is equivalent.
The following Table will give the dif ferent velocities and forces of the wind, according to their common appellations.
The force of the wind is nearly as the square of the velocity, or but little above it in these velocities. But the force is much more than in the simple ratio of the surfaces, with the same ve locity, and this increase of the ratio is the more, as the velocity is the more. By accurate experiments with two planes, the one of In square inches, the other if 32, which are nearly in the ratio of 5 to 9. Dr. Hutton found their resistances, with a velocity of 20 feet per second, to be the one, 1.196 ounces, and the other, 2.542 ounces ; which are in the ratio of 8 to 17, being an increase of between one fifth and one-sixth parts more than the ratio of the surfaces.