takes its name from two Greek words meaning "without fluid." It consists of a circular box of thin metal, with corrugated sides, a cross section of which is shown in Fig. 1. One side of the box, or vacuum chamber, as it is called, is firmly fixed to the base of the instrument. The air is partly removed from the box and it is sealed air-tight.
Variations in the atmospheric pressure cause corresponding changes in the position of the elastic upper face of the vacuum chamber, and these changes are transmitted by a system of delicate levers to a pointer which moves around a graduated dial. The readings of this dial are made to correspond to those of a standard mercury barometer.
These instruments are very delicate and will show a variation in reading on being raised from the floor to a table; for the atmospheric pressure varies not only with the weather but also with the elevation above sea level. If an aneroid barometer is carried in an elevator from the street floor to the top floor of a high building, the decrease of air pressure from floor to floor is readily seen It is on account of this decrease in atmospheric pressure that the pressure of illuminating gas is apparently greater at the top of a building than at the base. The pressure forcing the gas from the tube is the difference between the pressure in the pipes and the pressure of the air. Since the air pressure is greatest at the base of the building, the excess pressure of the gas is greater at the top.
The barograph (Fig. 3) is an aneroid barometer with attachments that make a permanent record of its readings.
Fig. 2
FIG.3.
casting changes in the weather. The relation of barometric readings to the state of the weather may be stated as follows.
- A rising barometer precedes fair weather.
- A falling barometer precedes foul weather.
- A sudden fall in the barometer precedes a storm.
- An unchanging high barometer indicates settled fair weather.
