« ForrigeFortsett »
of a degree ; in other words, they might be considered as almost absolutely correct.
135. A good deal of annoyance arises from the frameworks of thermometers being made of unsuitable materials. When any wood except well-seasoned boxwood is used, it soon warps on exposure to the weather, and, the tube breaking, the thermometer is rendered useless. Porcelain scales also occasionally go to pieces, and the tubes are at the same time frequently snapped. The best frameworks are those made of zinc.
136. Box for Thermometers.—In order to ascertain the temperature of the air, it is necessary that thermometers be protected from the direct and reflected rays of the sun, and at the same time have the benefit of a free circulation of air. No arrangement can completely fulfil both these conditions. For
the circulation of the air be quite unimpeded, the thermometers are unduly exposed to radiation. All, therefore, that can be secured is a fair compromise between protection and circulation. The best and cheapest contrivance yet devised to meet these requirements is the Louvre-Boarded Box for Thermometers, constructed by Mr Thomas Stevenson, C.E., Edinburgh, and now extensively used by the observers of the Scottish Meteorological Society, and other meteorologists. A figure of the box, fig. 15, is here given, with the door let down to show the hanging of the thermometers inside. Fig. 16 shows the simple and ingenious page method by which the louvre-boards are fixed.
Fig. 16. 137. The box is screwed to four posts firmly fixed in the ground, and these posts and the box itself are painted white, being the colour which absorbs least of the sun's heat. The posts are of such a length that when the minimum thermometer is hung in its place it is exactly four feet from the ground. This height is an essential point in the arrangements of the observatory, owing to the very great differences which frequently obtain between the temperature of the air at a height of four feet, as compared with the temperature at the surface of the earth, and at intermediate points.
138. Placing of the Thermometer-Box.—The box should be placed at some distance from walls, or other objects likely to be heated by the sun ; in an open space; and over old grass to which the sun has free access during the greater part of the day. For if it is placed on the north side of walls or buildings, the thermometers do not indicate a sufficiently high day temperature nor a sufficiently low night temperature for the average of the district where they are placed. And if it is placed over black soil, which is more highly heated during day, and cooled to a greater degree during night than grass, the maximum temperature will be too high, and the minimum too low.
139. Directions for taking Observations with the Thermometers.—Having let down the lid of the box as in fig. 15, the chief thing to attend to is, to take all the observations before touching one of the instruments. The dry and wet bulbs of the hygrometer are to be read first, so that their temperature may not be affected by the heat of the person standing near them. The minimum thermometer is read by noting down the degree on the scale at which the end of the index farthest from the bulb is lying. Beginners sometimes "read off” the degree at which the top of the column of spirit is at the time, which gives not the lowest temperature that has occurred during the night, but the temperature at the time of observation. The maximum thermometer is read by noting the degree at which the end of the index next the bulb is lying, if it is Rutherford's maximum ; but in the case of the other two maximum thermometers described above, the reading is taken from the point on the scale at which the end of the mercury furthest from the bulb is lying. When this is done the maximum and minimum thermometers should be “set” as already explained, and the box then shut up.
140. Mean Daily Temperature.—If the thermometer be observed once each hour, or twenty-four times a-day, and the sum of the observations be divided by 24, we shall obtain the mean temperature of that day. Observations of this nature, extending over considerable periods, have been made at Leith, Rothesay, Inverness, Greenwich, Rome, Calcutta, Madras, Bombay, several places in the Russian Empire, and in other parts of the world, and the hourly means for each month have been calculated and published. These tables show that there are two times in the day when the temperature is at the mean, occurring generally in winter between 9 and 10 in the morning, and between 9 and 10 in the evening, and in summer about an hour earlier.
141. It might have been supposed that the daily minimum would have occurred at the rising of the sun, or just before its rays had begun to heat the air; but observation is unani
before the sun rises. Within the tropics, and in temperate regions during summer, it occurs about half an hour before sunrise; but in temperate regions during winter generally two hours, and at some places as much as three hours, before sunrise. As causes concerned in raising the temperature of the air before the sun's rays begin directly to influence it, may be mentioned—(1) heat reflected from the upper regions of the air, already heated as well as lighted up by the sun before it appears above the horizon ; (2) heat liberated from the deposition of dew during the night; and (3) the slight influx of air from the warmer east towards the place of greatest cold, as evidenced by the daily fluctuations of the barometer, perhaps contributes in a small degree to the same result.
142. Within the tropics, and in temperate regions during winter-in other words, where the daily range of temperature is small—the maximum temperature occurs about an hour and a half after the sun has passed the meridian at noon. But in temperate climates during summer the maximum does not take place till from 24 P.M. to 3 P.M.
143. To this general law of the time of its occurrence there are two interesting exceptions. At many places in tropical countries it occurs about noon, or sometimes a little earlier, when the temperature at the coast is lowered by the seabreeze, which begins to blow before noon, and during the rainy season the temperature at this hour is still further lowered by the clouds which then begin to overspread the sky. At the Hospice of the Great St Bernard, 8174 feet above the sea, the daily maximum temperature occurs within an hour after twelve o'clock. The air being comparatively rare at this great height, the effects of solar and terrestrial radiation are more immediately felt than at lower levels. Hence, though the heat received by bodies exposed to the direct rays of the sun at noon is very great, yet the dispersion of the heat by terrestrial radiation is so rapid that in an hour after the sun has passed the meridian, the temperature of the air begins to fall. At the Great St Bernard,
at 3} P.M., the temperature is 1°.0 lower than it is about noon, whereas at lower situations it is from 10.5 to 20.0 higher.
144. In Melville Island, in the Arctic Ocean, there is no trace of a daily period during the absence of the sun in December and January; at Fort Bowen, the highest occurs in December at 9 A.M., and the lowest at 7 P.M., but at this place in January there is no trace of a daily period; at Igloolik, in North America, there is a daily fluctuation, but it is indistinctly marked. On the other hand, at MatotschinSchar, in Spitzbergen, the daily range of temperature is well marked—the maximum during December occurring at 9 P.M., and the minimum at 8 A.M. ; and during January, the maximum at 3 A.M., and two minima at 9 A.M. and 9 P.M. ; or taking the mean of the whole period when the sun is invisible at this place, the maximum occurs at 8 P.m., and the minimum at 8 A.M.
145. Sir David Brewster has investigated the hourly observations of the three Scottish stations, and drawn many interesting and important conclusions from them. By dividing the mean annual curve of daily temperature into four portions, at the points representing the two daily means and the two extremes, he finds that four portions have a striking similarity to parabolas, calculated on the supposition of the temperatures being the abscissæ of paraholas, and the hours the ordinates. The coincidence between the observed and calculated results is so remarkable that the calculated parabolic temperatures never differ from the real temperatures more than one quarter of a degree of Fahrenheit.
146. He also conclusively establishes an important modification of the law of the daily march of temperature, consequent on the form of the visible horizon at the place of observation. If a hill rises to the north of the place, by which the sun's rays are never obstructed, it can exercise little or no influence on the thermometer; but if one or more hills obstruct the sun's rays after it has risen above the true horizon, that obstruction affects the temperature at the place