There are certain words, such as Wet, Change, Fair, Dry, &c., on the face of the dial, but they are only conventional, the real test of the weather being the direction in which the hand moves. For example, if with a west wind the hand moves from Dry towards Fair, rain may be expected; whereas, if it should move from Wet to Change with an east wind, we may reasonably think that fine weather is coming.
The whole cause of this revolution of the hand may be found in the weight of the atmosphere.
It is found that a column of water thirty feet high, or a column of mercury thirty inches high, is exactly equal in weight to a column of air of the same diameter, but some forty odd miles high, so that the two columns precisely balance each other.
Suppose, then, the water or mercury to be placed in tubes closed at the top and open at the bottom, the water or mercury will exactly balance the air, and will not escape from the tubes. It necessarily follows that if the air be heavier than usual, it will force the liquid higher into the tubes, and, if it be lighter than usual, will allow them to fall lower. This is the principle of the Barometer.
The mechanism of the hand and dial is shown in the diagram which occupies the centre of the illustration. For convenience, sake the mercury column is mostly employed, but several Water Barometers, some thirty feet in length, have been constructed.
On the left hand is seen a boy engaged in sucking an egg. The plan employed is simple enough. A tolerably large hole is made at one end, and a very small one at the other. The yolk having been broken up by a long needle, or similar implement, the larger hole is placed to the lips, and, suction being used, the contents pass into the mouth.
Were it not for the hole at the end opposite the mouth, it would be impossible to extract the contents, but the air rushes through the aperture, and so forces out the contents of the egg.
Above is a representation of the way in which Sugar-cane is sucked. The reader probably knows that the Sugar-cane, like the wheat-stem, has knots at certain intervals, which divide the cane into a number of separate parts.
There is quite an art in sucking the Sugar-cane. If a joint be cut off, and the lips applied to the end, not a drop of the sweet juice would be extracted. But if a notch be cut close to the joint, as shown in the illustration, the air can gain access, and then the juice flows easily enough.