Fig. 45.
2. The ordinary syphon barometer (Fig. 46) consists of a bent tube attached to a piece of wood, and furnished with a screw v. The atmospheric pressure acts on the mercury at d, and the difference between the level of the mercury in the two arms of the syphon is the height of the barometer. To find the true height of the barometer the screw is turned till the shorter column stands at a opposite zero.
3. The aneroid barometer is made by exhausting the air from a small round metal box. This box is closed by a flexible lid of metal which, being elastic, yields to changes in the atmospheric pressure. To the lower end of the lid a spring is attached which runs downwards to the floor of the box and resists the atmospheric pressure. The movements thus produced by variations in pressure are magnified by a rack and pinion, and so communicated to a long index which moves over a graduated scale.
Fig. 46
Fig. 47.
Fortin Barometer.
a—Attached thermometer. b—Screw of vernier. c—Screw for setting level of cistern.
The standard Kew and Fortin barometers are both cistern barometers, the mercury in the inverted tube communicating with the mercury in a cistern below.
4. The Kew pattern barometer has a closed cistern below, the area of which being accurately known, the inches on the scale are not real inches, but inches of pressure, i.e. inches so shortened as to compensate for the rise of the mercury in the cistern. This compensation is necessary inasmuch as changes in atmospheric pressure affect the level of the mercury in the cistern as well as of that in the tube.