Page:Handbook of Meteorology.djvu/196

 Instead of the fasteners described above, clamps, sometimes called the Townsend supports, may be used to hold the thermometers. The clamps are fastened to the board support and permit the setting of the thermometers. The clamps are issued as a part of station equipment.

Care and Adjustment of Thermometers.—The thermometers, being exposed to the weather, accumulate dust; the metal parts may become tarnished or even rusty. It is advisable to use a soft camel's hair brush for removing the dust, and this should be done two or three times a week. When occasion requires, a polishing brush may be used on the metal parts. It is more desirable to prevent than to remove rust and tarnish.

Unless the maximum thermometer becomes a retreater, it is not likely to get out of order. Even if drifting snow blown into the shelter incrusts it, no damage is likely to result. It is better to allow the snow to melt off than to attempt to remove it by force.

If a maximum thermometer has not been set for a long time, a break in the column which refuses to unite may result. The same may occur if the moisture has not been wholly expelled during the roasting process. In such a case it is usually possible to drive the space to the small chamber at the end of the tube. It may be driven into the bulb; if this is done the break is likely to work back into the column again. If the instrument is held in a vertical position, bulb down, at a distance of 1 or 2 inches from a table, and is allowed to fall with vertical blows so as to hit a thickness of blotting paper placed on the table, the broken space gradually displaces the mercury until it reaches the top of the column.

A break in the column of mercury in the tube is not necessarily a defect; it is only when the break will not close—that is, when it leaves an open space—that error in the reading results. In such a case the thermometer should be discarded, or else returned to the maker for repair.

The minimum thermometer is usually out of order when it is received at its destination. The index may be fast at the top of the tube, or in the bulb; most likely the alcohol column is broken, a half dozen or more bubbles occurring; possibly some of the alcohol is lodged in the chamber at the farther end of the tube.