=30. Principle of.=--If some mercury, or any other fluid, be poured into a tube of glass, bent in the form of =U=, and open at both ends, it will rise to the same height in both limbs, the tube being held vertically. If mercury be poured in first, and then water upon it at one end, these liquids will not come to the same level; the water will stand much higher than the mercury. If the height of the mercury, above the line of meeting of the fluids, be one inch, that of the water will be about thirteen-and-a-half inches. The explanation of this is, that the two columns balance each other. The pressure of the atmosphere in each limb is precisely similar; but the one column stands so much higher than the other, because the fluid of which it is composed is so much lighter, bulk for bulk, than the other. If one end of the tube be hermetically closed, the other limb be cut off within a few inches of the bend, and the tube carefully filled with mercury; by placing it in a vertical position, the mercury will fall, if the closed limb be long enough, until it is about thirty inches higher than that in the exposed limb, where it will remain. Here the atmosphere presses upon the short column; but not upon the long one. It is this pressure, therefore, which maintains the difference of level. In fact, it forms a barometer without a cistern, the short limb answering the purpose of a cistern. The first barometers on this principle were devised by the celebrated philosopher, Dr. Hook, as described in the next section. 31. DIAL, OR WHEEL BAROMETERS. The familiar household "Weather Glasses" are barometers on the syphon principle. The portions of the two limbs through which the mercury will rise and fall with the varying pressure of the atmosphere are made of precisely the same diameter; while the part between them is contracted. On the mercury, in the exposed limb, rests a round float of ivory or glass; to this a string is attached and passed over and around a brass pulley, the other end carrying another lighter weight. The weight resting on the mercury rises and falls with it. On the spindle of the pulley, which passes through the frame and centre of the dial-plate, is fixed a light steel hand, which revolves as the pulley turns round. When the mercury falls for a decrease of atmospheric pressure, it rises by the same quantity in the short tube, and pushes up the float, the counterpoise falls, and thus moves the hand or pointer to the left. When the pressure increases, the pointer is drawn in a similar manner to the right. [Illustration: Fig. 18.] [Illustration: Fig. 19.] [Illustration: Fig. 20.]