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APPARATUS AND METHODS] within which barometric pressure is less than in the neighbouring free air. The draught up the chimney is due to the pressure of the air at the lower end or fireplace pushing up the flue into this region of low pressure, quite as much as it is due to the buoyancy of the heated air within the flue. From such experiments as these there has been developed the vertical suction-tube anemometer, as devised by Fletcher in 1867, re-invented by Hagemann in 1876, and introduced into England by Dines.

In his Meteorological Apparatus and Methods (Washington, 1887) Abbe gives the theory of this class of anemometers and develops the following additional forms: Two vertical tubes, whose apertures are respectively directed to the windward and the leeward, and within which are two independent barometers, give the means of determining the barometric pressure plus the wind pressure and minus the wind pressure respectively, so that both the velocity of the wind and the true barometric pressure can be determined. If instead of a simple opening at the top of the tube we place there horizontally the contracted Venturi’s tube, we obtain a maximum wind effect, which gives an accurate measure of the wind velocity, and is the form recommended by Bourdon as an improvement on that of Arson. In all anemometers of this class the inertia of the moving parts is reduced to a minimum, and the measurement of rapid changes in velocity and of the maximum intensity of gusts becomes feasible. On the other hand, these researches have shown how to expose a barometer so that it shall be free from the dynamic or wind effect even in a gale. It has only to be placed within a room or box that is connected with the free air by a tube that ends in a pair of parallel plane plates. When the wind blows past the end of this tube it flows between these plates in steady linear motion, and can produce no disturbance of pressure at the mouth of the tube if the plates are at a suitable distance apart. This condition of stable flow, as contrasted with permanent flow, was first defined by Sir William Thomson (Lord Kelvin) (see Phil. Mag., Sept. 1887). Such a pair of small circular plates can easily be applied to a tube screwed into the air-hole at the back of any aneroid barometer, and thus render it independent of the influence of the wind. As to the exposure of the anemometer, no uniform rules have as yet been adopted. Since the wind is subject to exceedingly great disturbances by the obstacles near the ground, an observer who estimates the force of the wind by noticing all that goes on over a large region about him has some advantage over an instrument that can only record the wind prevailing at one spot. The practice of the U.S. Weather Bureau has been to insist upon the perfectly free exposure of all anemometers as high as can possibly be attained above buildings, trees and hills; but, of course, in such cases they give records for an elevated point and not for the ground. These are therefore not precisely appropriate for use in local climatological studies, but are those needed for general dynamic meteorology, and proper for comparison with the isobars and the movements of the clouds shown on the daily weather map.

Hygrometer.—Moisture floats in the atmosphere either as invisible vapour or as visible haze, mist and cloud. The presence of the latter generally assures us that the air is fully saturated. The total amount of both visible and invisible vapour contained in a unit volume of cloud or mist is directly determined by the Schwackhofer or Svenson hygrometer, or it may be ascertained by warming a definite portion of the air and fog and measuring the tension of the vapour by Edelmann’s apparatus. Both these methods, however, are in practice open to many sources of error. If only invisible aqueous vapour is present we may determine its amount by several methods: (a) the chemical method, by absorbing and weighing it; (b) the dewpoint method, by cooling the air down to the temperature where condensation begins; (c) Edelmann’s method, by absorbing the moisture chemically and measuring the change in vapour tension; (d) by adding vapour until the air is saturated, and measuring either the increased tension or the quantity of evaporation; (e) the psychometric method, by determining the temperature of evaporation.

Rain and Snow Gauge.—The simple instrument for catching and measuring the quantity of rain, snow or hail that falls upon a definite horizontal area consists essentially of a vertical cylinder and the measuring apparatus. The receiving mouth of the cylinder is usually terminated by a cone or funnel, so that the water running down through the funnel and stored in the cylinder is protected from evaporation or other loss. The cylinder is firmly attached to the ground or building, so that the mouth is held permanently at a definite altitude.

The sources of error in its use are the spattering into it from the ground or neighbouring objects, and the loss due to the fact that when the wind blows against the side of the cylinder it produces eddies and currents that carry away drops that would otherwise fall into the mouth, and even carries out of the cylinder drops that have fallen into it. As a consequence all the ordinary rain-gauges catch and measure too little rainfall. The deficit increases with the strength of the wind and the smallness or lightness of the raindrops and snowflakes. If we assume that the correct rainfall is given by a gauge whose mouth is flush with the level of the ground and is surrounded by a trench wide enough to prevent any spatter, then, on the average of many years and numerous observations with ordinary rain-gauges in western Europe, and for the average character of the rain in that region and the average strength of the attending winds, the deficit of rain caught by a rain-gauge whose mouth is 1 metre above the ground is 6% of the proper amount; if its elevation is 1 ft. above ground, the deficit will be 3%. This deficit increases as the gauges are higher above the ground in proportion approximately to the square root of the altitude, provided that they are fully exposed to the increase of wind that prevails at those altitudes. It is evident that even for