Air put in motion by some physical cause, so as to become a current or stream. Winds are denominated according to the points from which they blow; see Compass. A variety of machines have been invented at different times for ascertaining the strength or velocity of the wind; the annexed cut represents one which possesses the advantages of simplicity of construction, and of being unerrm in its indications. It is thus formed: - A square open frame of wood or iron, a be, is supported by the shaft d; two cross pieces are fixed at ef, carrying an horizontal axis, which is moved by the action of the wind upon four sails, i i i i, fixed to one end of the axis, and disposed to be influenced by the wind in the usual manner. Upon this axis is also fixed a conical barrel of wood, on the smaller end of which, n. is attached a line with a weight, l, appended to it The wind now acting upon the sails, causes the barrel to revolve, and the line to be wound round its superficies. To prevent any retrograde motion, a ratchet wheel o, is fixed to the base or larger end of the cone m, having a clicker falling into its notches as it revolves.

It is evident that the force of the weight will continually increase as the line advances towards the base of the cone, by the power being applied at a greater distance from the axis or fulcrum; consequently the variable force of the wind may be readily ascertained, by fixing the line at the smallest end, andmarking the barrel with spiral lines, as taken up by the coiling of the rope round its superficies; placing, also, between the line so drawn, numerical signs to denote the force of the wind; which might be calculated with tolerable precision, according to the known principles of the lever. The diameter of the base of the cone should.be such, in comparison with the smaller end, that the very strongest wind should have scarcely sufficient power to bring it on to the end of it.

The different velocities, forces, and corresponding popular appellations of winds are given in the following table, derived from the experiments of the late celebrated engineer, John Smeaton; and detailed in the Philosophical Transactions.

 Velocity. Perp. Force on one square foot in lbs.avoirdupois. Appellations. Per hour Miles. Per Second Feet. 1 1.47 .005 Scarcely perceptible. 2 2.93 .020 Perceptible. 3 4.40 .044 4 5.87 .079 Gentle breeze. 5 7.33 .123 10 14.67 .492 Pleasant brisk gale. 15 22.00 1.107 20 29.34 1.968 Very brisk gale. 25 36.67 3.075 30 44.01 4.429 High winds. 35 51.34 6.027 40 58.68 7.873 Very high winds. 45 66.01 9.963 50 73.35 12.300 A storm or tempest. 60 88.02 17.715 A great storm. 80 117.36 31.490 A hurricane. 100 146.70 49.200 A dreadful hurricane that overturns buildings, trees, etc.

To which may be added a still more remarkable instance of the impetuosity of a hurricane, as related by M. Rochou. The velocity of the wind, a* observed by him, was no less than 109 miles an hour, or 159.88 feet per second; and its force against a perpendicular plane of a foot square, was estimated at 58.45 pounds avoirdupois. Of the causes and theory of winds, many very able philosophers have treated largely; as Des Cartes Rohault, Bacon, De Luc Halley Prevost, Derham, Eles, Muschenbroeck, Dalton, and others. We have not room to introduce even a short abstract of their several theories, but must refer the curious reader to their writings, and the various parts of the Philosophical Transactions.