Page:EB1911 - Volume 18.djvu/294

APPARATUS AND METHODS] The annual average velocity of hourly movement in metres per second without regard to direction may be summarized as follows:—

The movements of the upper clouds are more rapid in winter than in summer at these northern stations, but among the median and lower clouds a retardation takes place apparently due to the ascending currents that form rain and snow. Above 8000 metres at Upsala the average velocity in winter exceeds 30 metres per second, whereas in summer it is 20; at Toronto and Blue Hill the absolute velocities are larger but in the same ratio. In the United States the maximum velocities from the west attain 100 metres per second and over 80 or 70 metres per second are not rare, but in Europe the corresponding figures are 70, 60, 50. (See also .)

The observational basis of meteorology is the frequent and, if possible, continuous record of the temperature, moisture and barometric pressure at different altitudes in the free atmosphere, the direction and velocity of the wind, the rain and snow-fall, and the kind, amount and motion of the clouds. For Europe these data have been furnished with more or less accuracy and continuity by thousands of observers ever since 1653, when Ferdinand II., grand duke of Tuscany, organized a system of daily observations in Italy under the general supervision of Luigi Antinori. During the 19th century great efforts were made to obtain equally full records from all parts of the land and ocean, and thousands of navigators were added to the great corps of observers. Other matters have also been investigated, the most important being the intensity of radiation from the earth at night-time and from the sun by day-time, the optical phenomena of the sky, the amount of dust in the air, the electrical condition and the chemical constitution of the atmosphere. Although all the instruments used belong to the category of physical apparatus, yet certain points must be considered as peculiar to their use in connexion with meteorology.

Thermometer.—In using the thermometer to determine, the temperature of the free air it is necessary to consider not merely its intrinsic accuracy as compared with the standard gas thermometer of the, International Bureau of Weights and Measures at Paris, but especially its sluggishness, the influence of noxious radiations, the gradual change of its zero point with time, and the influence of atmospheric pressure.

Sensitiveness.—The thermometer indicates the temperature of the outside surface of its own bulb only when the whole mass of the instrument has a uniform temperature. Assuming that by appropriate convection we can keep the surface of the thermometer at the temperature of the air, we have still to remember that ordinarily this itself is perpetually changing both in rapid oscillations of several degrees and in diurnal periods of many degrees, while the thermometer, on account of its own mass or thermal inertia, always lags behind the changes in the temperature of its own surface. On the other hand, radiant heat passes easily through the air, strikes the thermometer, and raises its temperature quite independently of the influence of the air whose temperature we wish to measure. The internal sluggishness or the sensitiveness of the thermometer is usually different for rising and for falling temperatures, and is measured by a coefficient which must be determined experimentally for each instrument by observing the rate at which its indications change when it is plunged into a well-stirred bath of water whose temperature is either higher or lower than its own. This coefficient indicates the rate per minute at which the readings change when the temperature of the surface of the bulb is one degree warmer or colder than the temperature of the bath. Such coefficients usually vary between th of a degree centigrade for sluggish thermometers, and one or two degrees for very sensitive thermometers; Suppose, for instance, that the coefficient is one-half degree, then when the rate of change in the temperature of the air is one degree per minute this is exactly the same as the rate of change which the thermometer itself undergoes when its own temperature is two degrees different from that of the air; consequently, the thermometer will lag behind the air temperature to that extent and by the corresponding amount of time, assuming that the air itself flows fast enough to keep the surface of the bulb at the air temperature. When the air temperature ceases to rise or fall, and begins to change at the same rate in the opposite direction, the thermometer will fail to record the true maximum or minimum temperature by an appreciable error depending upon the rapidity of the change, and will follow the new temperature changes with the same lag. For example, in the case just quoted, if a rising temperature suddenly changes to a falling temperature, the error of the thermometer at the maximum temperature will be two degrees, and yet the thermometer may be absolutely correct as compared with the standard when it is allowed five or ten minutes time to overcome the sluggishness. It is very difficult to obtain the temperature of the free air at any moment within th of a degree Centigrade, owing to the sluggishness of all ordinary thermometers and the perpetual variations in the temperatures of the atmospheric currents.

Radiation.—When a thermometer bulb is immersed in a bath of liquid all radiant heat is cut off, but when hung in the open air it is subject to a perpetual interchange of radiations between itself and all its surroundings; consequently its own temperature has only an indirect connexion with that of the air adjacent to it. One of the most difficult problems of meteorology is so to expose a thermometer as to cut off noxious radiations and get the true temperature of the atmosphere at a specific place and time. The following are a few of the many methods that have been adopted to secure this end: Melloni put the naked glass bulbs within open sheltering caps of perforated silver paper. Flaugergues used a protection consisting of a simple vertical cylinder of two sheets of silver paper enclosing a thin layer of non-conducting substance, like cotton or wool. The influence of radiation upon a thermometer depends upon the radiating and absorbing powers of its own surface; a roughened surface of lamp-black radiates and absorbs perfectly; one of chalk powder does nearly as well; glass much more imperfectly; while a polished silver surface reflects with ease, but radiates and absorbs with the greatest difficulty. Fourier proposed to use two thermometers side by side, one of plain glass and the other of blackened glass; the difference of these would indicate the effect of radiation at any moment; but instead of plain glass he should have used polished silver. His method was quite independently devised and used by Abbe in 1865 and 1866 at Poulkova, where the thermometers were placed within a very light shelter of oiled paper. In order to use this method successfully, both the black and the silvered thermometers should be whirled side by side inside the thermometer shelters (see Bulletin of the Philosophical Society of Washington for 1883). Various forms of open lattice-work and louvre screens have been devised and used by Glaisher, Kupffer, Stevenson, Stowe, Dove, Renou, Joseph Henry and others, in all of which the wind is supposed to blow freely through the screens, while the latter cut off the greater part of the direct sunshine and other obnoxious radiations by day, and also prevent obnoxious radiation from the thermometer to the sky by night. The Italian physicist Belli first proposed a special artificial ventilation drawing the fresh air from the outside and making it flow rapidly over the thermometer. Even before his day de Saussure, Espy, Arago and Bravais whirled the thermometer rapidly either by a small whirling machine, or by attaching it to a string and swivel and whirling it like a sling. When this whirling is done in a shady place excellent results are obtained. Renou and Craig placed the thermometer in a thin metallic enclosure or shelter, and whirled the latter. Wild established the thermometer