=75. Baudin's Alcohol Minimum Thermometer.=--This instrument resembles Rutherford's thermometer in appearance; its indications are given by the expansion and contraction of alcohol, and its minimum temperature is likewise registered by a glass index being pulled back and left behind by the alcohol, as in Rutherford's instrument. There is, however, a great improvement in Baudin's instrument; for whilst Rutherford's thermometer can only register in a horizontal position, Baudin's can be used either horizontally or vertically, as necessity may require. This important change is effected in the following manner:--Instead of the index in the thermometer being loose and free to run up and down according to the position in which the instrument is held, as in Rutherford's, the index in the new instrument is made to fit the bore of the tube as nearly tight as possible, so much so that in holding the thermometer even upside down, or shaking it, the index will not shift from its position; but, inasmuch as a minimum thermometer with an immoveable index could not be set when required for observation, and would consequently be useless, the inventor has introduced behind the index a piece of solid glass, about one-and-a-half inch in length, which moves freely in the alcohol. The addition of the weight of this piece of glass on the top of the index, when turned upside down, forces the index down to the edge of the alcohol; and it is there left, as in the case of the ordinary Rutherford's thermometer. It is, therefore, by turning the thermometer upside down, and letting the moveable piece of glass fall on the index, that the index is driven to the end of the alcohol; after this operation the thermometer is hung up either horizontally or vertically, and will then be ready for use. The index, although immoveable _per se_, is by the alcohol drawn back, as in the ordinary minimum, and its indications are read off on the scale from the top of the index. =76. Mercurial Minima Thermometers desirable.=--Alcohol does not expand equally for equal increments of heat, consequently errors are likely to exist in the scale indications unless the graduations are very accurately--not necessarily equally--made. On this account, as well as from the volatility of alcohol, and the intervention of gaseous partitions in the tube, a good and thoroughly reliable minimum thermometer was for a long time a desideratum. It was desirable to obtain a thermometer which should register the lowest temperature by mercury, the fluid in general use for meteorological thermometers. Several instruments have recently been invented to meet this requirement, which are suitable and satisfactory for land purposes, but one well adapted for use on board ship is still very much wanted. For very low temperatures, alcohol thermometers will always be required; as mercury freezes at -40 deg. F, and contracts very irregularly much before this point, while alcohol has never yet been frozen.