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 indices are set by the use of a small magnet which accompanies the thermometer. The poles of the magnet are hollow-ground, so as to fit closely to the tube.

In many respects the Six thermometer is preferable for ordinary uses. It is not so likely to be broken as the regular Weather Bureau thermometers; it is very readily set; and it is more nearly “fool-proof” than the delicate Weather Bureau instruments. It is not so sensitive as the Weather Bureau thermometers; it is slow in registering; and the indices are occasionally caught in the mercury from which they are separated with difficulty. A violent jar may break the hair-like appendages that cause the indices to register. If this happens to the index in the right-hand tube its repair by a thermometer maker is possible; if in the left-hand tube the case is hopeless.

In selecting a thermometer of this type one should note first whether or not the readings of the two tubes are the same. When a thermometer has lain edgewise, or on its side, for a number of days—and this may occur when it is in transit on a railway—a flow of the liquid past the mercury, from one tube to the other, may take place. As a result the readings on the two sides do not coincide. An expert in the mechanics of thermometry can make the necessary adjustment, but it should be done by an expert and not an experimenter. In selecting a Six thermometer, a comparison of the readings of the two sides should be the first care.

The Six thermometer may not be quite up to the standard of accuracy. If the error is small the thermometer needs not be condemned, however, for an allowance can be made therefor.