The long lever, the pen arm, carries a pen which presses lightly on the ruled paper wrapped around the drum. A milled screw adjusts the pressure of the pen on the paper, and a switch enables the observer to throw the pen off or on at pleasure. The drum makes one revolution per week. The record sheets are ruled with horizontal lines representing inches, halves, and twentieths. Arcs of circles, having radii equal to the length of the pen arm, divide the record sheet into midday and midnight periods, and two-hour intervals. Eight o’clock Monday morning is the normal time for changing the record sheets used by most observers. These contain day and month spaces for record-
Barograph.
ing dates. In Weather Bureau practise the sheets are changed on the 1st, 8th, 15th, 22d and 29th days of the month.
The clocks used in most barographs are watch movements of the finest type. It is desirable to have the clock oiled and cleaned once a year. The clock is practically dust-proof, being within the drum, and protected also by the glass case which incloses the mechanism of the barograph.
In replacing the record sheet the observer must look carefully to two things; the lower edge of the record sheet must fit closely to the collar at the lower edge of the drum; the lines of equal pressure must match precisely. A failure to conform to these requirements leads to incorrect records.
