The pressure of the air at the tops of mountains sometimes decreases very much, and it is not sufficiently dense for perfect respiration, as many people find. Some climbers suffer from bleeding at the nose, etc., at great altitudes. This is occasioned by the action of the heart, which pumps with great force, and the outward pressure upon the little veins being so much less than usual, they give way. [Illustration: Fig. 53.—Diver under water.] Many important instruments depend upon atmospheric pressure. The most important of these is the pump, which will carry us to the consideration of water and FLUIDS generally. The fire-engine is another example, but we will now proceed to explain the diving-bell already referred to. Fig. 52 represents the experiment of the diving-bell, which is so simple, and is explained below. It belongs to the same category of experiments as those relating to the pressure of air and compression of gas. Two or three flies have been introduced into the glass, and they prove by their buzzing about that they are quite at their ease in the rather confined space. The DIVING-BELL in a crude form appears to have been used as early as 1538. It was used by two Greeks in the presence of the Emperor Charles V., and numerous spectators. In the year 1720 Doctor Halley improved the diving-bell, which was a wooden box or chamber open at the bottom. Air casks were used to keep the inmate supplied with air. The modern diving-bell was used by Smeaton in 1788, and was made of cast iron. It sinks by its own weight. The pressure of the air inside is sufficient to keep the water out. Air being easily compressed, it is always pumped in to keep the hollow iron “bell” full, and to supply the workmen. There are inventions now in use by which the diver carries a supply of air with him on his back, and by turning a tap can supply himself for a long time at a distance from the place of descent, and thus is able to dispense with the air-tube from the boat at the surface. This apparatus was exhibited at the Crystal Palace some years ago. [Illustration: Fig. 54.—The Hand Fire-Engine.] THE PUMP. We have seen in the case of the Water Barometer that the pressure of the air will sustain a column of water about thirty feet high. So the distance between the lower valve and the reservoir or cistern must not be more than thirty-two feet, practically the distance is about twenty-five feet in pumps. We can see by the illustration that the working is much the same as in the air-pump. The suction pipe B is closed by the valve C, the cylinder D and spout E are above, the piston rod F lifts the air-tight piston in which is a valve H. When the piston is raised the valve C opens and admits the water into the cylinder. When the piston is depressed the valve C is closed, the water already in forces H open, and passing through the piston, reaches the cylinder and the spout (fig. 55).