Page:Handbook of Meteorology.djvu/195

 a vertical position; this is most likely to occur in hot weather when the column is long.

Taking everything into consideration, a maximum thermometer that requires a moderately vigorous whirling is preferable. The instrument with which the observer can work to best advantage is the one with which he should be provided.

The minimum thermometer is for the purpose of registering the lowest temperature between settings. It is exposed in a nearly horizontal position with the bulb slightly higher than the opposite end of the tube. Inasmuch as winter minima are sometimes lower than the freezing temperature of mercury, and because the instrument here described requires a transparent column, minimum thermometers usually contain alcohol instead of mercury. The alcohol of American Weather Bureau thermometers usually is colored; in most foreign made instruments it is uncolored. In the matter of visibility the gain of a colored liquid is material, but coloring matter is not essential. In spite of care, a precipitation of the coloring matter occasionally may occur, and this is likely to cause a slight constant error. The space in the tube above the alcohol contains air, more or less saturated with alcohol vapor.

The essential feature of the minimum thermometer is a small black index within the bore and also within the liquid. As a lowering temperature contracts the liquid column, the cohesion of its surface drags the index toward the bulb. When the liquid expands, however, it flows around the index without moving it. The index therefore shows the lowest temperature between settings. The minimum temperature is read at the end of the index farthest from the bulb. The temperature may be read from the minimum thermometer at any time, reading from the end of the column of alcohol, and not from the index.

The minimum thermometer is usually attached to a strip of brass, bent so that the instrument is held about an inch from the board support. It is fastened so that the end containing the bulb may be swung to an inverted position. The maximum thermometer is fastened to the same support. The stud on which it whirls is about 2 inches long. The free end rests on a pin which is removed when the thermometer is set.