. Elementary physics and chemistry: first stage . Fig. 53.—The paper does not fall off though the glass is full of water. REASONS AND RESULTS. Air exerts Pressure.—Even'thing which has weight can exert pressure. The pressure depends first of all upon what we have learned to call the density of a thing. For instance, if you carry a piece of iron upon your shoulder you arc pressed down more than by a piece of wood of the same size. But the pressure also depends, of course, upon the amount of material you are bearing. Thus, a sack of wool borne upon your head would exert more pressure than an iro
. Elementary physics and chemistry: first stage . Fig. 53.—The paper does not fall off though the glass is full of water. REASONS AND RESULTS. Air exerts Pressure.—Even'thing which has weight can exert pressure. The pressure depends first of all upon what we have learned to call the density of a thing. For instance, if you carry a piece of iron upon your shoulder you arc pressed down more than by a piece of wood of the same size. But the pressure also depends, of course, upon the amount of material you are bearing. Thus, a sack of wool borne upon your head would exert more pressure than an iron nail. You will be able to understand, then, that though the air is so light 'compared with other substances, a large quantity of it would be ver\' heavy and would exert a ver}' great pressure. Now, the air above us extends up- wards from the surface of the earth for many miles ; and in consequence of this it presses very heavily upon every- thing. You will see in the next chapter how this pressure is measured, but there are many simple ways which show that it is real. The reason why a leathern sucker is difficult to pull off an object upon which it has been placed is that the air is pressing upon the outside of the sucker but not upon the inside (Fig. 55). When a liquid is drunk through a tube, the vessel containing the liquid mtist be open to the air or else it cannot be obtained. You may see that this is so by examining a baby's feeding bottle. If the stopper of
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