Fixed-cistern barometers have not been favorably considered by meteorologists. The objection to them on the whole is not well founded. If the scale has been compensated a fixed-cistern barometer will meet all the requirements of accuracy demanded by ordinary meteorological measurements. It is more serviceable for use at sea than a cistern of the Fortin type. It is less likely to injury in transportation.
The necessity of a compensated scale may be understood from the following facts. The sectional area of the cistern is about fifty times the sectional area of the tube. If the atmospheric pressure increases, say, from 29.00 inches to 30.00 inches the rise of 1 inch in the tube is balanced by a fall of 0.02 inch in the cistern. The true height of the column therefore is 30.02 inches. When the sectional areas of both cistern and tube have been accurately determined, an empiric scale compensated for the instrument may be engraved to meet the requirements of accuracy. Should the tube be broken, however, a new scale will be required inasmuch as the caliber of tubes varies considerably.
A barometer with a fixed cistern, made by Schneider Brothers, is highly regarded among officials of the Weather Bureau. A feature of this barometer is the facility with which the mercury can be made secure within the cistern and tube, so that the instrument will not lose its adjustment.
Fixed-cistern barometers may not meet the requirements of precision measurements so well as instruments of the Fortin type of cistern; but for marine purposes or for field work, their simplicity of construction and stability commend instruments of this character. When securely adjusted, they may be transported over rough wagon roads and carried in any position without especial care.
The tube of the barometer is inclosed in a case with the necessary openings which permit the height of the mercury column to be read. Weather Bureau barometers are of the “gun-barrel” type, tube and cistern being inclosed in a cylindrical metal case. Openings, or “windows” are cut in the sides so that the top of the column of mercury is always in sight.
The scale of Weather Bureau barometers is on the left side of the window. It is a strip of white metal, with slotted screw
