Page:Encyclopædia Britannica, Ninth Edition, v. 2.djvu/35

Rh hurricane. Rivalling this in simplicity is a fourth class, that may be added to Dr Robinson's, viz., pendulum anemometers, where the pressure-plate is swung as a pendulum, and indicates the wind-force by its deviation from the vertical. The first of this form was invented by the Marquis Poleni, and gained the prize of the French Academy in 1733. They are still employed as simple rough-and-ready indicators to the eye of the strength of the wind. Several other anemometers have been contrived, which, from being essentially non-recording, may be mentioned as more curious than valuable. Among these are (1.) A musical anemometer, suggested by Dr Hooke, and constructed by Delamanon, in which the wind was caused to sound automatically, in a set of pipes, a particular note corresponding to its velocity; (2.) Leslie's anemometer, or rather anemometric principle, which deduces the velocity of the wind from its effect in cooling a thermometer, it being sup posed that the rate of cooling is proportional to the velocity of the wind; (3.) Brewster's evaporation anemometer, by which the velocity may be deduced from the amount of water evaporated in a given time from a rough open surface, such as sponge or flannel, the two being in proportion.

Osler's anemometer, the last of the pressure-gauges we shall mention, being one of the most trustworthy and most common registering anemometers in this country, merits a more particular description. It traces with pencils on a sheet of paper (which is moved along, either in a vertical or a horizontal position, at a constant rate by means of clock-work) lines which indicate changes of the wind, both in direction and in pressure. Its general principle will be readily understood without a diagram. Changes of direction are recorded in this way. The axle that carries the vane, and turns with it, has keyed on its lower end a pinion which, working in a horizontal rack, moves it backwards or forwards as the axle turns one way or the other; and a pencil fixed to the rack traces its movements on the register-sheet. Changes of pressure, or wind-force, are measured by means of springs. A square foot of a light metal plate, placed vertically and turning with the vane-rod, receives the impulse of the wind, and is forced against three springs, so arranged that one, two, or all of them may be pressed on according to the violence of the wind. The extent of compression against the springs is transmitted through the hollow vane-axle, by means of chains and pulleys, to a light spring, which presses a pencil gently against the paper. Variations of motion of the upper plate thus correspond to definite lateral deviations of this second spring pencil, and a wavy line is traced on the paper as it moves along. The register-sheet is ruled across its length into twenty-four equal parts, one of which passes the pencil points each hour; and lengthways it is ruled—first, with lines corresponding to the cardinal points, under the direction-pencil; and secondly, on the field of the other pencil, with lines spaced by actual experiment so as to correspond to gradations of 5, 10, 15, 20, &c. lb of wind-pressure on the square foot. Thus the pressure-curve, in the same way as on the indicator of a steam-engine, includes between it and the horizontal line of zero-pressure a space proportional to the amount of work done by the wind on the upper pressure-plate. Of course, so long as the wind blows in one direction, the direction-pencil traces a horizontal line; and if there be no wind at all, both pencils trace the zero horizontal lines. We have not space to enter particularly into the history of the direct velocity-measuring anemometers. As long ago as 1783, one was contrived on this principle by Mr Edgeworth for the measurement of air-currents; but to Dr Whewell belongs the credit of first appreciating at its true value the velocity as opposed to the pressure measure. In the ''Cam. Trans.'', vol. vi., he describes an instrument of his own devising, which was afterwards extensively used. Sir W. Snow Harris recommended it strongly to the British Association in 1844, exhibiting results of his own observations with it during three previous years. Owing, however, to some practical defects, such as the great frictional resistance of toothed wheels and endless screws, which have more effect on the indications of a gentle wind than of a high one, and to the want of an arrangement for indicating the times of the variations, Dr Whewell's instrument has now fallen into disuse. A description of it may be found in Drew's Practical Meteorology. One of the most valuable contributions to this science was made by Dr Robinson, of Armagh, who between the years 1843 and 1846 conducted a number of experiments and calculations on the proper form to be given to the revolving vanes. He found radical defects in the principle of all the vanes previously employed; either their moving power was not sufficiently great to make the frictional correction inconsiderable, or their velocity, in place of being less, was often three or four times greater than that of the wind, requiring greater complicacy of machinery to reduce the speed of the tracing point; or, lastly, their results were not identical, though constructed after the same type. The form adopted by Robinson to remedy these defects is that shown in the figure. Though it had been suggested to him by Edgeworth, it deservedly bears the name of Robinson's Cups (fig. 1), as he was the first to show experimentally and analytically its advantages. The arrangement consisted of four light metal hemispheres C, of as large a diameter as convenient, extended at the ends of light and strong metal radii, so that they shall be as far from

. 1.—Cups and Vane of Anemometer.

the axis of motion as possible, and so reduce the effect of friction to a minimum. They require no vane to keep them facing the wind, that represented by F, F, in the figure having a movement quite independent of the cups, as will be explained presently. He found that the cups move at almost exactly one-third of the rate of the wind a happy simplicity of ratio and this for all instruments made after one type. Experiments were made with sets of two, three, four, and six cups; but the velocity ratio—between the cups and the wind was simplest and most constant with four, the number now universally employed. In the paper already quoted (Trans. Roy. Irish Acad., vol. xxii.), Dr Robinson describes a self -registering apparatus, to which he had fitted the horizontal cups. But we shall describe instead one of the more modern and simple registers, which seems to leave little further to be desired in the way of compactness and accuracy. It is the invention of Mr Beckley, of the Kew Observatory, and is described in full in the Report of the British Association, 1858, p. 306. The outside, or wind-receiving parts are represented in the above figure, and consist of Robinson's cups C, for the wind-velocity, and a double fan F, or windmill governor, for wind-direction. The fans serve as a vane, turning so as to keep their axes constantly at right angles to the wind, and are more steady than the ordinary vane. They are connected with the outer brass tube B, and carry it round with them on any change of wind-direction. The second figure shows the manner in which the motions of the fans and cups are recorded. R is a cylinder, which carries the register-sheet, on which the 