The barograph records values of atmospheric pressure, giving the changes over time, on which weather forecasts are based. It consists of eight stacked aneroid chambers, used to enhance the movement of the lever connected to the top. The movements of the lever are transmitted to the short arm of an angle lever. The long arm of the latter is a flexible rod, thin and light, ending with a small metal container filled with ink. The ink is aniline mixed with glycerine.
The size of the different parts of the levers are such that the excursion of the pen is amplified to match that of a mercury barometer: an inch on paper equals one millimeter of mercury column. The pen is lightly pressed on the paper wrapped around a brass drum that, by means of a clockwork, through a pinion and a gear, rotates with uniform motion. The gear ratio is calculated so that the paper advances four centimeters per day and the drum makes a complete turn in a week.
The clock is wound by means of a key passing in a hole on the upper surface of the drum. A second hole on the same face is used to adjust the advance or delay of the escapement, as in any watch.
The paper is traced horizontally and vertically. Horizontal lines correspond to millimeters of Hg and the vertical ones to hors. The latter are arched to follow the movement of the pen. The top is marked with weekdays. A spring button allows to adjust the pen, when it does not touch the paper or has excessive pressure, which would affect the sensitivity of the instrument.
BAROGRAPH for use onboard
The barograph for use onboard continuously records the values of atmospheric pressure, to obtain trends for weather forecasting. To dampen the effects of vibrations on board and then make it suitable for use in navigation, the Hydrographic Institute of the Navy made a change to the instrument. The capsules were inserted in a cylinder full of oil, without altering the sensitivity of the instrument.