evening or early next morning so as to record the maximum for the next day.
To insure accuracy of readings the thermometer should not be higher than the eye of the observer; and to avoid error of refraction by the lens front, the observer should stand squarely in front of the end of the mercury column. An error of refraction may amount to half a degree.
The “setting” of the maximum thermometer is accomplished by whirling it around a stud and bearing at the end opposite the bulb; centrifugal motion forces the mercury past the stricture, and back into the bulb.
The maximum thermometer of the Weather Bureau type should rest in a nearly horizontal position, the bulb slightly higher than the farther end of the tube. Ordinarily this will prevent any back flow. Occasionally, however, a maximum thermometer fails to leave the mercury in the bore at the point of maximum expansion; for reasons not fully explained the column of mercury is drawn back toward the bulb as the temperature falls. A maximum thermometer of this sort is known in Weather Bureau cant as a “retreater.” The retreating of the column of mercury may be overcome by a slight increase in the elevation of the bulb. When, however, it is discovered that the retreating is habitual, the thermometer should be returned to the maker.
Some maximum thermometers set easily; others require to be whirled vigorously. Observers differ in their likes and dislikes. If the column of mercury moves very easily there is always danger of error. If the tube is held in a horizontal position there is the possibility of a slight retreat of the column. And if the bulb is elevated there is always the possibility that it may slide toward the far end of the tube. The maximum thermometer readings are not trustworthy for any except maximum temperatures. When set, it should agree with the reset minimum thermometer.
It is advisable to bring the thermometer very carefully to a vertical position when the reading is made. This corrects at once any sliding of the column that may have occurred. But there is always danger that a part of the mercury in the tube may flow into the bulb when the thermometer is brought to