ANEI\IOSCOPE, from oeve•-05, the wind, and cs.oz-Eto, observe, is properly an instrument for observing the direction of the wind, or the point of the compass from which it blows ; though it has been confounded with the word anemometer, and sometimes with hygrometer. It is synonymous with the words plagoscope and plogo graph, employed by Lcupold.
The common anemoscope, an instrument known to the ancients, may be seen on the top of every spire, and therefore does not require a figure for its illustration. A vane of thin metal is fixed at the top of a vertical rod, so that the greater part of the vane may be on one side of the rod. This rod moves upon a pivot, and always points to the quarter from which the wind blows. When great accuracy is required, the direction of the wind may be indicated, either upon a horizontal, or a vertical dial-plate. When the dial-plate is required to be hori zontal, we have only to fix it upon the base in which the pivot of the rod moves, and place an index upon the rod itself. When the dial-plate is vertical, a horizontal bevel-wheel must be fixed on the vertical rod, so as to drive a vertical wheel, with the same number of teeth, fixed upon a horizontal axis, at the extremity of which is fastened the index. When the vane and the vertical rod perform one complete revolution, or part of a revolu tion, the horizontal wheel will make the vertical wheel, and consequently the horizontal axis, with its index, per form the same revolution, or part of a revolution. When, from particular circumstances, the dial-plate can neither be horizontal nor vertical, but forms an oblique angle with the vertical rod, the index of the dial-plate may he put in motion by Hooke's Universal Joint. This simple contrivance we owe to the ingenuity of Mr Miller, optician in Edinburgh.
As most of the anemometers which have been de scribed under that article, have likewise the properties of anemoscopes, it is unnecessary to repeat the descrip tion of them in this place. We shall content ourselves
with describing two anemoscopes which do not seem to be generally known. The one is the invention of Dr Wren, and the other of P. Beaudoux.
The anemoscope of Dr Wren, consists of a vane fixed to a vertical rod, on whose lower extremity is fastened horizontally a circular plate. An arm, moveable by a clock, in the direction of the radius of this circular plate, carries a pencil which, in consequence of its own mo tion, and the motion of the plate by means of the vane, describes upon its surface irregular lines, from which the changes of the wind may be deduced.
The anemoscope of Beaudoux is not unlike that which forms a part of Lomonosow's anemometer. Plate XXIX. Fig. 8.
A circular box, with 16 or 32 cavities, answering to the points of the compass, is fixed at the bottom of a ver tical rod, moved by a common vane. About a foot above this box, the vertical rod carries a cross arm, at each of whose extremities is fixed a vessel filled with sand. When the vane moves this cross arm, the sand will evidently run into the cavities, and indicate, by the quan tity of sand in each compartment, the time during which the wind has blown from the corresponding quarter. One of the vessels of sand is placed nearer the centre of the rod than the other, and throws its contents into a set of cavities nearer the centre of the box than those which answer to the other vessel. The bottoms of each cavity are inclined planes, to facilitate the motion of the sand into a drawer below, from which it is returned into the vessel for future observation. See Birch's History of the Royal Society, vol. i. p. 341. Leupold's Theat. ?gerostat. Emerson's Mechanics. p. 253. Cotte's Me moires de illeteorcdogie. (A)