Anemoscope, a mechanical instrument for determining the course and velocity of the wind. That part which exhibits the former, or shews from what point of the compass the wind blows, consists of an index, moving round an upright circular plate, like the dial of a clock; on which, instead of the hours, the thirty-two points of the compass are represented. The index which points to the divisions on the dial, is turned by a horizontal axis, having a trundle-head at its outward extremity. This trundle-head is moved by a cog-wheel, on a perpendicular axis; at the top of which is fixed a vane, moving with the course of the wind, and imparting motion to the whole machine. The contrivance is extremely simple, and requires in its construction only, that the number of cogs in the wheel, and rounds in the trundle-head, be equal; because, when the vane moves entirely round, the index of the dial should also make a complete revolution. An anemoscope of this construction is placed in one of the turrets of Buckingham-house, the residence of Her present Majesty.
The anemoscope invented by Mr. Pickering, and published in die Philosophical Transactions, No. 4/3, is a machine four feet and a quarter high, consisting of a broad and weighty pedestal, a pillar attached to it, and an iron axis, about half an inch In diameter, fastened into the pillar. Upon this axis turns a wooden tube; at the top of which is placed a vane, of the same materials, twenty-one inches long, consisting of a quadrant, graduated, and shod with an iron ring, notched to each degree j and a counterpoise of wood on the other, as represented in the figure. Through the" centre, of the quadrant runs an iron pin ; upon which are fastened two small round pieces of wood, serving, as moveable radii to describe the degrees upon the quadrant, and as handles to a velum or sail; the pane of which is one foot square, made of canvas stretched on four battens, and painted. On the upper batten, next to the shod rim of the quadrant, is a small spring, which catches at every notch, corresponding to each degree, as the sail may be raised on die pressure of the wind, and thus its falling-back prevented, when the force of the wind decreases. At the bottom of the wooden tube is an iron index, which moves round a circular piece of wood fastened to the top of the pillar, on the pedestal, where the thirty-two points of the compass are described. We have annexed a representation of this machine: a is the pedestal; b, the pillar on which the iron axis is fitted; c, the circle of wood representing the points of the compass; e, the wooden tube upon its axis; f, the velum; g, the graduated quadrant; h, the counterpoise of the vane. The subjoined figure represents the velum", "which may be taken oft": a is the plane of the velum ; b, the spring; c.c, the Wooden radii; d, d, the holes through which passes the pin, in the centre of the quadrant.
This instrument serves the following useful purposes:
1. Having a circular motion round the iron axis, and being furnished with a vane at the top, and an index at the bottom, as soon as the artificial cardinal points described on the round piece of wood on the pillar are fixed to the corresponding quarters of the heavens, it faithfully points out the quarter from which the wind blows.
2. Being furnished with a velum, or sail, elevated by the wind, along the arch of the quadrant, to an height proportionate to the power of the column of wind pressing against it, its relative force and its comparative power, at any two times of examination, may be accurately taken.
3. By means of a spring fitted to the notches of the iron, with which the quadrant is shod, the velum is prevented from returning upon the fall of the wind ; and the instrument, without the trouble of watching it, ascertains the force of the highest blast, since the last lime of examination.
This machine may be confidently depended upon, as the velum is hung so nicely, that it is susceptible of the most gentle breeze, and will also describe the force of the wind in a violent storm. There is, however, reason to apprehend, that by" 'exposing the anemoscope to all winds, especially to irregular blasts and squalls, for a length of time, it may become inaccurate. The observer ought, therefore, to take the lube with its vane and velum, in his hand, with a view to learn the force of the wind; and, after having made his observation, he should return with the machine into the house, till the violence of the storm subside.