the decreased pressure cannot force so much mercury into the tube.
Construction of the Barometer.—A common form of the barometer is a siphon tube, the short arm of which is enlarged to a bulb, and this constitutes the mercury cistern. The barometric column is the distance between the upper and the lower surface of the mercury.
The Weather Bureau pattern is more complicated. The tube has a caliber usually about 0.25 inch. The cistern or tank containing the mercury consists of a short cylindrical glass tube. The cover is a piece of boxwood perforated to receive the barometer tube and flanged to fit the cylindrical glass section of the cistern. The lower part of the cistern is a broad ring, of boxwood, flanged and fitted to the lower edge of the glass cylinder. To the lower part of the boxwood ring a kidskin bag is attached. The mercury fills the bag and reaches nearly to the top of the glass cylinder. The cistern fits snugly into a cylindrical metal box. A plug within the cylindrical box, operated by a screw at the bottom, partly supports the bag of mercury. The construction may be likened to a glass tube projecting vertically from the mouth of a rubber bag filled with water. Pressure on the bag forces water up the tube; release of pressure causes it to lower in the tube.
The object of the leather bag and the plug is two-fold; it enables the observer to raise or to lower the mercury in the cistern so that the surface touches the ivory point which is the end of the scale, thereby giving a more accurate reading; it also enables the observer to close the cistern and tube, so that the barometer may be carried in any position without permitting the mercury to escape from the tube, or air to enter it.
The cistern of this pattern of barometer is commonly known as the Fortin cistern. It was improved by Henry J. Green. The improved pattern is used by the Weather Bureau. The Tuch cistern, also used by the Weather Bureau, has a piston bottom that can be raised and lowered by a thumb-screw. The Weather Bureau pattern of the barometer made by the Taylor Instrument Companies has a similar device. In each, a stopper pressed against the mouth of the tube secures the mercury so that the barometer may be transported with a minimum of risk.
