145. =Centre of Gravity in Floating Bodies.=--The same principles which apply to the centre of gravity in bodies standing on a firm basis apply also to floating bodies. That the centre of gravity may be low in a loaded vessel the heavy part of the cargo is put underneath, and generally ballast of stone or iron is necessary for the same purpose. In large flat-boats, the base of support being extensive, there is not the same need of taking care that the centre of gravity be low. If a ship be laden in part with an article which will dissolve in water, there is much danger, if the ship should leak, that this portion of the cargo shall be dissolved and be pumped out with the bilge-water, thus altering the trim of the vessel, or removing the centre of gravity from over the middle line, and bringing it too far forward, or carrying it too far back, making the ship wholly unmanageable. Four large English ships, in part loaded with saltpetre, were supposed to be lost from this cause in 1809 off the Isle of France. The immense ice-islands, or icebergs, which float about in summer in the polar regions, by melting irregularly often change the place of their centre of gravity, and in turning over present one of the most sublime spectacles in nature. A mountain of ice, extending high in the air and deep in the sea, suddenly turns over, and produces a rolling of the ocean which is often felt at the distance of many leagues. CHAPTER IX. PNEUMATICS. 146. =What Pneumatics Teaches.=--As Hydrostatics treats of the pressure and equilibrium of liquids, Pneumatics treats of the same in air and the gases, or aeriform substances. The name comes from the Greek word πνευμα, meaning air, breath, spirit. [Illustration: Fig. 92.] 147. =Air Material and has Weight.=--That air is a material substance has been already proved to you, for it was shown in § 46 that it has impenetrability, one of the essential properties of matter. It has extension also, for bodies of air can be obtained in various shapes confined in vessels, so that we can speak of cubes and spheres of air; and besides, the ultimate atoms (§ 15) of air must have shape or extension. That air has weight can be proved by weighing it as you would any other substance. Let a hollow globe, A, Fig. 92, having a neck with a stop-cock, B, be emptied of air and weighed. If now you open the stop-cock, and so let in the air, the other beam of the scale will rise, because the globe is heavier than it was before. The additional weight required to make the scales balance will indicate the weight of the air which the globe contains. It is one eight hundredth (1/800) of the weight of the same volume of water. How the globe can be emptied of the air will be shown in another part of this chapter.