1. _The Dial or wheel barometer_ consists of a bent tube A B, the open end of which supports an ivory float B. This, as it rises and falls with the mercury, by means of the rack C turns a wheel, in the axle of which a needle is fixed. The needle turns in one direction, or the other as the mercury rises or falls (Fig. 45); the dial is divided by comparing it with a standard barometer. As the ordinary variations of the barometer are from 28 to 31 inches, the circumference of the wheel is made exactly 1½ inches, and thus the float B will rise or fall 1½ inches for a rise or fall of 3 inches in the barometer. [Illustration: 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. [Illustration: FIG. 46.] 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. 5. In the _Fortin barometer_ (Fig. 47) the cistern has a pliable base of leather, which can be raised or lowered by means of a screw. The upper part of the cistern is made of glass, a piece of ivory indicating the zero of the scale. Before taking a reading, the level of the mercury must always be set exactly to this point by means of the screw. The Fortin is the most sensitive form of barometer, and the adjustment required in order to take a reading is easily performed.