Weather Bureau barometers are provided with a glass-paneled containing box, the front and right side of which swing open. Many observers prefer a plain board mounting. Marine barometers are usually contained in a box with an outrigger which permits them to be removed to a position convenient for reading.
The box or supporting board must be mounted so as to be vertical in all meridians. Metal eyelets, or hangers, accompany the supporting box or board. When in place, there should be no “wiggle” or dead motion. The hangers will be found in such a position that the barometer swings in the middle of the lower ring.
When the barometer is removed from the packing case it should be lifted, cistern uppermost, and laid on a table or bench to be unwrapped. Until it is finally in position it should be moved about cistern uppermost. When the wrappings are removed, it should be carried cistern uppermost to the support and turned carefully top end up. The cistern end should be put within its supporting ring before it is hung upon the hook of the support. If the box or the supporting board has been accurately leveled, the cistern will swing freely in the supporting ring. The centering screws in the ring may then be turned until each barely touches the cistern box. In case the screws are lost, pegs of soft wood, whittled to the right size, will answer temporarily. The case of the barometer should turn freely on the swivel, but there should be no dead motion.
The Care of the Barometer.—Except in unusual cases, a mercurial barometer should be kept indoors in a position where the temperature is as nearly uniform as possible. At temperatures materially below 10° F the readings of barometers side by side may vary enough to give concern to a conscientious observer. When the temperature is materially below zero, F, at an altitude of 5000 feet, more or less, the readings are often of uncertain value. The moral is obvious. Uniform and constant conditions are necessary for uniform results.
The compensation for capillarity is usually corrected by adjustment of the scale. Mercury does not “wet” glass; therefore the surface of the tube not only tends to retard the rise of the column, but prevents the mercury from assuming a level surface at the top. The rounded surface is the meniscus, the shape of which changes from time to time, as pressure varies.
