holes so that it may be adjusted to compensate corrections which are constant, especially capillarity. The inch is divided into tenths and subdivided into twentieths. The scales of commercial barometers are usually without compensation adjustments.
The vernier enables the observer to read the height of the column to two one-thousandths of an inch, and to estimate it to a one-thousandth part. On commercial barometers the vernier enables the observer to read accurately to the one-hundredth part of an inch. A rack and pinion moved by a milled screw enable the observer to adjust the vernier to the height of the mercury.
The thermometer set into the metal case of the Weather Bureau barometer is always a standard instrument, whether carrying a certificate or not. The scale is etched on the tube to single degrees, but it may be read to half-degrees in accordance with the temperature corrections which are calculated to half-degrees.
The Installation of the Barometer.—When a barometer is sent from the manufacturer, or is issued from the Weather Bureau, it is pretty certain to be in good order and ready for installation. It is an almost universal custom to wrap the instrument first in tissue paper, then in cotton flannel, and finally in stout wrapping paper. The packing case should be so large that the elasticity of the packing material will compensate any jar that may occur from ordinary handling. Barometers sent out by the Weather Bureaus are usually packed in cases designed for the purpose. Ordinary precaution suggests that the cover of the packing case should be fastened with screws and not with nails.
When a barometer is sent by messenger it should be sent either in a leather case or a box designed for the purpose, with the handle so placed that it must be carried cistern uppermost. It should not be allowed to rest with the end on the floor of a moving vehicle.
Before the barometer is removed from the packing case, the position most advantageous for it should be determined. A wall or partition that is easily shaken should be avoided. A position on a window frame or near the corner of a room is often the best available. A position where the temperature is not subject to sudden change is very desirable.
