removed to a position materially lower, allowance should be made for the increase in pressure. Unless this precaution is observed, the pressure may be great enough to force mercury through the joints of the cistern.
The following instructions concerning the removal from position are issued by the Weather Bureau: “When moved about, the cistern end should be carried uppermost. To turn the barometer tube-end up, bring it gradually to a horizontal position, watching for a small bubble at the cistern. This should not be large, nor should it be absent, in which case there may be serious pressure from within, tending to force the mercury out of the cistern. If necessary the adjusting screw should be turned so that the bubble is not larger than the space within which a dime can be placed. If there is an air vent, as in the Tuch cistern, as soon as the mercury is raised to the top of the cistern, close the air vent tight and continue screwing up the cistern until the top of the column reaches the summit of the opening in the metal tube. Avoid raising the mercury in the cistern until the tube is entirely filled with mercury. Do not strain the screw if it turns hard; mercury may have leaked from the cistern and there may not be enough to fill the tube." If no air has entered the upper end of the tube, when the barometer is inclined about half way the mercury will rise to the top of the tube with a slight but distinct click; and when the instrument is nearly horizontal a bubble should appear at the cistern.”
The foregoing cautions apply to Fortin type barometers chiefly, but will apply in some respects to other types. In the installation, removal, and care of other barometers, the directions of the makers should be followed. Marine barometers are provided with tubes a considerable portion of which is constricted to prevent the “pumping” of the mercury which the motion of the vessel would otherwise cause. The constriction prevents the vacuous part of the tube from filling quickly. The barometer must be inclined gradually, waiting until the flow ceases. By the time it is inclined about 40 degrees the mercury will have filled the tube. It can then be inverted and moved about in that position. Because the cistern is partly filled only, a marine barometer is easily put out of adjustment during transportation.
