154. =Pressure of the Atmosphere.=--The amount of the pressure of the atmosphere is very readily estimated, the mode of doing which I will speak of in another part of this chapter. It presses with a weight of fifteen pounds on every square inch. Suppose that you extend your outspread hand horizontally in the air. You feel no pressure upon it, but there is a pressure of some two or three hundred pounds of air upon it. If your hand be five inches long and three broad it presents a surface of fifteen square inches, on every one of which the atmosphere is pressing with the weight of fifteen pounds. That is, there is a pressure on the upper surface of your hand of a column of air weighing 225 pounds. So, also, on the lid of a box only thirty inches square, there is a pressure of 13,500 pounds. The whole pressure on the body of a man of common size is about fifteen tons. But why is it that the lid of the box is not broken in, your hand not borne down, and your body not crushed? It is simply from the fact, shown in the previous chapter in regard to liquids, and in this in regard to aeriform substances, that the pressure is equal in all directions. The lid and the outspread hand are therefore balanced by an upward pressure equal to the downward, and the body has the pressure on all sides the same. If the air could be removed from within the box the lid would be crushed in; if from under the hand, that would be borne down; and if from one side of the body, the body would be forced violently in that direction till it met with an opposing pressure. But besides this equal pressure of the air on all sides, there is air within the pores and interstices of all bodies that are not very dense, and its particles are subject to the same laws as are those on the outside. All this can be made clear to you by the air-pump. [Illustration: Fig. 94.] 155. =Air-pump.=--In Fig. 94 you have a representation of an air-pump as commonly arranged. At _a a_ are two pump-barrels, the pistons in which are worked by means of the handle, _b_. Those pumps are very nicely made, and the frame-work, _d e d e_, to which they are attached, is very strong and firm, so that the pumps may work evenly. There is a large, smooth, metallic plate, _f_. At _c_ is a bell-shaped glass vessel, close at the top, but open at the bottom, the edge of which is ground very true, so that it may fit exactly on the metallic plate. In the middle of the plate is an opening which leads to the pump-barrels, and it is through this that the air is pumped out of the glass receiver, _c_. If we wish to let the air in after we have pumped it out we loosen the screw at _g_, for from the opening here is a passage to the opening in the middle of the plate. [Illustration: Fig. 95.]