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because 32° on Fahrenheit's scale is the zero of the Centigrade scale ; and to convert Fahrenheit into Centigrade, first subtract 32°, multiply by 5, and divide by 9. In Table V. the Centigrade thermometer is compared with Reaumur's and Fahrenheit's from 140° to -40° F.
124. Self-Registering Thermometers. The most important temperatures in their relation to climate and most other inquiries are the highest which occur during the day and the lowest which occur during the night; and to record these, various thermometers have been devised, well known as maximum and minimum thermometers.
125. Maximum Thermometers. — The maximum thermometers generally used are Rutherford's, Phillips's, and Negretti and Zambra's. Rutherford's Maximum Thermometer has a movable steel index on the top of the mercurial column. When the instrument is in use, it is hung horizontally, and as the temperature rises, the mercury pushes the index before it, but as it falls the index is left, thus registering the highest temperature. It is set by bringing the steel index to the surface of the mercury by means of a magnet; or by simply holding the instrument upright, and thus permitting the index to fall gently down on the mercury, shaking or tapping it slightly if required. The objection to this thermometer consists in its liability to get out of order by the index oxidising, and then getting plunged into the mercury; and so certain is this to take place in a few years at most, that the instrument cannot be recommended. The end of the index next the mercury is sometimes covered with a coating of glass, by which its tendency to oxidisation is prevented. A maximum thermometer thus constructed forms perhaps the best that has yet been invented, because the index is not easily shaken out of its position, and it may be set with a magnet without requiring to be removed from the hook to which it is attached, thus lessening the risk of breakage.
126. In Phillips's Maximum Thermometer, fig. 13, a portion of the mercurial column is detached and kept separate from the other part by a minute air-bubble. When in position it is hung horizontally; then as the temperature rises the whole column moves along the scale, but when it begins
Fig. 13. to fall the detached portion is left behind at the point to which it had been pushed, thus registering the greatest heat. This is an excellent thermometer. Care, however, should be taken to select one in which the detached portion is little more than 1} inch in length; for if it is longer, its weight in the tube makes it easily shaken out of its place; and if shorter, it is so troublesome to set, that in striking it so as to send down the detached part to the mercury, the risk of breaking it is considerable.
127. In Negretti and Zambra's Maximum Thermometer, the tube is bent at the part of the tube near the bulb, and the bore of the tube is contracted at the angle. It is hung horizontally; with a rising temperature the column is pushed along the scale, but when the temperature begins to fall, the column of mercury breaks at the angle where the bore is narrowed, thus leaving the mercury in the tube at the highest point to which it has been driven. It is also an excellent thermometer; but since the detached portion of mercury in the tube is always of very considerable length, and therefore of some weight, it is liable to be shifted out of its place when shaken by accident, or by the wind during storms.
128. In selecting a Phillips's, or a Negretti and Zambra's thermometer for the registration of very high temperatures, such as those recorded by thermometers exposed to the sun's rays, care ought to be taken to see that the thermometer registers sufficiently high. For these thermometers only register properly up to a certain point; the reason being, that there is almost always a portion of air, however small, in the top of the column, and hence when the mercury has risen to near the
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 19.7 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