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Rh altitudes of 5 or 10 ft. the records become appreciably discrepant from those obtained at the surface of the ground. The following table shows in the last column the observed ratio between the catches of gauges at various altitudes and those of the respective standards at the level of the ground. Unfortunately, there are no records of the force of the wind to go with these measurements; but we know that in general, and on the average of many years, corresponding with those here tabulated, the velocity of the wind increases very nearly as the square root of the altitude. Although this deficit with increasing altitude has been fully recognized for a century, yet no effort has been made until recent years to make a proper correction or to eliminate this influence of the wind at the mouth of the gauge. Professor Joseph Henry, about 1850, recommended to the observers of the Smithsonian Institution the use of the “pit-gauge.” About 1858 he recommended a so-called shielded gauge, namely—a simple cylindrical gauge 2 in. in diameter, having a wide horizontal sheet of metal like the rim of an inverted hat soldered to it. This would undoubtedly diminish the obnoxious currents of air around the mouth of the gauge, but the suggestion seems to have been overlooked by meteorologists In 1878 Prof. F. E. Nipher of St Louis, Missouri, constructed a much more efficient shield, consisting of an umbelliform screen of wire-cloth having about sixty-four meshes to the square inch. This shield seems to have completely annulled the splashing, and to have broken up the eddies and currents of wind. With Nipher’s shielded gauges at different altitudes, or in different situations at the same altitude, the rain catch becomes very nearly uniform; but the shield is not especially good for snow, which piles up on the wire screen. Since 1885 numerous comparative observations have been made in Europe with the Nipher gauge, and with the “protected gauge” devised by Boernstein, who sought to prevent injurious eddies about the mouth of the gauge by erecting around it at a distance of 2 or 3 ft. an open board fence with its top a little higher than the mouth of the gauge. The boards or slats are not close together, but apparently afford as good a protection as the shield of Professor Nipher, and give good results with both snow and rain.

In general it is now conceded by several high authorities that the measured rainfall must be corrected for the influence of the wind at the gauge, if the latter is not annulled by Nipher’s or Boernstein’s methods. A practicable method of measuring and allowing for the influence of the wind, without introducing any very hazardous hypothesis, was explained by Abbe in 1888 (see Symons’s Meteorological Magazine for 1889, or the U.S Monthly Weather Review for 1899). This method consists simply in establishing near each other several similar gauges at different heights above the ground. but in otherwise similar circumstances. On the assumption that for small elevations the diminution of the wind, like that of the rainfall, is very nearly in proportion to the square root of the altitude, the difference between the records for two different altitudes may be made the basis of a calculation which gives the correction to be applied to the record of the lower gauge, in order to obtain the rainfall that would have been caught if there were no wind. It is only when the catch of the gauge has been properly corrected for the effect of the wind on the gauge that we obtain numbers that are proper to serve for the purpose of determining the variation of the rainfall with altitude and locality, the influence of forests and the periodical changes of climate. Methods of measuring dew, frost, hail, sleet, glatteis and other forms of precipitation still remain to be devised; each of these has its thermodynamic importance and must eventually enter into our calculations.

It has been common to consider that the rain-gauge cannot be properly used on ships at sea, owing to the rolling and pitching of the vessel and the interference of masts and rigging; but if gauges are mounted on gimbals, so as to be as steady as the ordinary mariner’s compass, their records will be of great importance. Experimental work of this sort was done by Mohn, and afterwards in 1882 by Professor Frank Waldo; but the most extensive inquiry has been that of Mr W. G. Black (see Journal Manchester Geographical Society, 1898, vol. xiv.), which satisfactorily demonstrates the practicability and importance of the marine rain-gauge.

Evaporometer.—The moisture in the atmosphere comes from the surface of the earth or ocean by evaporation, a process which goes on continually, replacing the moisture that is precipitated as rain, hail, snow and dew, and maintaining the total quantity of the moisture in the atmosphere at a very uniform figure. The rate of evaporation depends on the temperature, the dryness, and the velocity of the wind. It is not so important to meteorologists to know where the moisture comes from as to know its amount in the atmosphere, and in fact no method has yet been devised for determining how much moisture is given up by any specific portion of the earth, or ocean, or forest. Our evaporometers measure the quantity of moisture given off by a specific surface of water, but it is so difficult to maintain this water under conditions the same as obtain in nature that no conclusions can be safely deduced as to the actual evaporation from natural surfaces. The proper meteorological use of these evaporometers is, as integrating hygrometers, to give the average humidity of the air, the psychrometer giving the conditions prevailing at any moment.

Among the many forms of evaporometer the most convenient is that devised by Piche, which may be so constructed as to be exceedingly accurate. The Piche evaporometer consists essentially of a glass tube, whose upper end is closed hermetically, whereas the lower end is covered by a horizontal disk of bibulous paper, which is kept wet by absorption from the water in the tube. As the water evaporates its descent in the tube is observed, whence the volume evaporated in a unit of time becomes known. So long as the paper remains clean, and the water is pure, the records of the instrument depend entirely upon the evaporating surface, the dryness of the air, and the velocity of the wind. Careful comparisons between the Piche and the various forms of absolute evaporometers were made by Professor Thomas Russell, and the results were published in the U.S. Monthly Weather Review for September 1888, pp. 235–239. By placing the Piche apparatus upon a large whirling machine he was able to show the effect of the wind upon the amount of evaporation. This important datum enabled him to explain the great differences recorded by the apparatus established at eighteen Weather Bureau stations; based upon these results, he prepared a table of relative evaporation within thermometer shelters at all stations. The actual evaporation’s from ground and water in the sunshine may run parallel to these, but cannot be accurately computed. It is probable that Professor Russell’s computations are smaller than the evaporation’s from shallow bodies of water in the sunshine, but larger than for deep bodies, like the great lakes, and for running rivers. Recent elaborate studies of evaporation have been undertaken in Egypt and in South Africa—but perhaps the most interesting case occurs in southern California. Here the Colorado river, having broken through its bounds, emptied itself into a great natural depression and formed the so-called “Salton Sea,” about 80 m. long, 20 wide and 100 ft. deep, before it could be brought under control. This sea is now isolated, and will, it is hoped, dry up in eight or ten years. Meanwhile the U.S. Weather Bureau has established a large number of evaporation stations in and around it, and has begun the study not only of the relation between evaporation, wind and temperature, but of the eventual disposition of this evaporation throughout the atmosphere in the neighbourhood of the sea (see the Reports of Professor F. H. Bigelow in U.S. Monthly Weather Review, 1907–1909, as also the elaborate bibliography of evaporation in the same volumes). Although the influence of the evaporation on local climate is scarcely appreciable to our hygrometric apparatus, yet it is said to be so in the development and ripening and drying of the dates raised on the U.S. government experimental "date farm” a few miles north-east of the Salton Sea.