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top of the tube, and the temperature begins to fall, the elastic force of the enclosed air pushes back the mercury in the tube, thus causing it to act like a common thermometer. If the mercury be heated till it rises nearly to the top of the tube, and be then allowed to cool, the height to which it registers may be ascertained.
129. Minimum Thermometers.—Rutherford's Minimum, fig. 14, is the best. In this thermometer the fluid used is
spirit of wine, in which there is immersed a steel index. When in use, it is hung horizontally. As the temperature falls, the spirit drags the index with it, but when the spirit rises it freely passes the index and leaves it lying at the lowest point to which it had been dragged, thus registering the greatest cold. This thermometer is set by bringing the index close up to the top of the spirit, by raising the bulb end of the instrument, or by a magnet.
130. There are no instruments for meteorological purposes so liable to go wrong as spirit thermometers, owing to part of the spirit evaporating and settling in the top of the tube. This frequently happens with spirit thermometers exposed on grass ; indeed I have seen them out of order to the extent of 3°, and more rarely 8, or upwards. Such thermometers ought therefore to be frequently examined, to ascertain whether any of the fluid has lodged in the top of the tube. This remark applies also to spirit thermometers which are exposed for common purposes outside windows. These I have often seen with a quantity of spirit equal to 12° detached from the column and settled in the top of the tube, my attention having been drawn to them by the accounts of coldSiberian almost in intensity-alleged to prevail in certain
places. It is thus that many of our severest frosts find their way into the newspapers among other wonders. Spirit thermometers kept in the shade are also liable to the same derangement, but to a much less degree, because they are better protected from great heat. Generally the quantity of spirit evaporated is small, though sometimes it amounts to one degree. Observers should give special attention to this point, and the more especially so because it is by the temperature recorded by these thermometers that the chief elements of climate are determined.
131. How to Unite the Broken Column of Spirit Thermometers.-Fortunately spirit thermometers may be easily set right when the column of spirit chances to separate. Let the thermometer be taken in the hand by the end farthest from the bulb, raised above the head, and then forcibly swung down towards the feet; the object being, on the principle of centrifugal force, to send down the detached portion of spirit till it unites with the column. A few throws or swinging strokes will generally be sufficient, after which the thermometer should be placed in a slanting position, to allow the rest of the spirit still adhering to the sides of the tube to drain down to the column. But another method must be adopted if the portion of spirit in the top of the tube be small. Heat should then be applied slowly and cautiously to the end of the tube where the detached portion of spirit is lodged; this being turned into vapour by the heat, will condense on the surface of the unbroken column of spirit. Care should be taken that the heat is not too quickly applied, for if this be done the tube will break and the instrument be destroyed. The best and safest way to apply the requisite amount of heat is to bring the end of the tube slowly down towards a minute flame from a gas burner; or if gas is not to be had, a piece of heated metal will serve instead.
132. It is evident that if mercurial minimum thermometers were constructed, this source of error would be obviated. Great and partially successful efforts have been made to supply this desideratum. The Mercurial Minimum Thermometer of Casella may be referred to as a triumph of science and glassblowing. It is, however, too sensitive, and the mercury too easily shaken along the tube, to be recommended for general use,—some of them being so delicately adjusted that the greatest care and most expert manipulation is required to work them. Hence the best minimum thermometers are spirit thermometers, for they really leave nothing to be desired if the most ordinary vigilance be exercised by the observer in setting them right when the column happens to separate.
133. In Hicks's Maximum and Minimum Thermometers, both are combined into one column; but this composite instrument is inferior to the ordinary maximum and minimum thermometers made separate from each other. This is particularly apparent in registering extreme temperatures of short duration, because the tube is filled with two fluids, mercury and spirit, which, having different capacities for heat, expand unequally.
134. No thermometer ought to be used which has not been previously compared with some standard instrument, such as the Kew standard adopted in Great Britain, so that its errors, if any, at the different points of the scale may be ascertained. This is especially necessary in purchasing cheap instruments, many of which I have had occasion to reject as being from 1° to 30 wrong. The amount of the error frequently varies at different points of the scale ; thus one I compared was correct at 60°, 3° too low at 45°, and 6° too low at 32°. Errors are also found, though rarely, in first-class thermometers. Thus, I recently compared a number of high-priced thermometers, every one of which was from 10.2 to 10.7 too high. But it would be unjust to omit paying a well-deserved compliment to opticians for the high degree of excellence and refinement now attained in the construction of meteorological instruments. I have compared many hundreds, and could name firms, none of whose thermometers, at or above a certain price, have been found to vary from the standard more than half a degree through the scale, and many of them not more than the tenth
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.