The “Torricellian vacuum” is the empty space above the column of mercury in the barometer which we will proceed to describe. Air has a certain weight or pressure which is sufficient to raise a column of mercury thirty inches. We will prove this by illustration. Take a bent tube and fill it with mercury; the liquid will stand equally high in both arms, in consequence of the ratio of equilibrium in fluids, of which we shall read more when we come to consider Water. So the two columns of mercury are in equilibrium. (See A.) Now stop the arm a with a cork, and take out half the mercury. It will remain in one arm only. Remove the cork, and the fluid will fall in both arms, and remain in equilibrio. If a long bent glass tube be used, the arms being thirty-six inches high, the mercury will fall to a point _c_, which measures 29·9 inches from the bottom. If the tube be a square inch in bore, we have 29·9 cubic inches of mercury, weighing 14⅘ lbs., balancing a column of air one square inch thick and as high as the atmosphere. So the mercury and the column of air must weigh the same. Thus every square inch on the earth supports a weight of (nearly) 15 lbs (figs. 48 and 50). [Illustration: Fig. 48.—Air Pressure.] [Illustration: Fig. 49.—The Barometer.] The barometer invented by Pascal, working on the investigations of Torricelli, is a very simple and useful instrument. Fill a tube with mercury from which all moisture has been expelled, and turn it over in a dish of mercury; the mercury will rise to a certain height (30 inches), and no higher in vacuo. When the pressure of the air increases the mercury rises a little, and falls when the pressure is removed. Air charged with aqueous vapour is lighter than dry air, so a fall in the mercury indicates a certain amount of water-vapour in the air, which may condense and become rain. The action of mercury is therefore used as a weather-glass, by which an index-point shows the movements of the fluid, by means of a wheel over which a thread passes, sustaining a float and a counterpoise. When the mercury rises the float goes up, and the weight falls, and turns the wheel by means of the thread. The wheel having a pointer on the dial tells us how the mercury moves. This _weather-glass_ is the usual _syphon barometer_ with the float on the surface and a weight (fig. 50). [Illustration: Fig. 50.—Syphon barometer.] The Syphon Barometer is a bent tube like the one already shown, with one limb much shorter than the other.