_Answer._ Because the air surrounds us on all sides, and presses just as much in one direction as it does in another, so that the pressures in different directions just balance each other, or are _in equilibrium_; but if you disturb this balance, for example, by sucking the air from a tube closed at one end, it will cling to your tongue; or if you take a thick piece of leather under ordinary conditions it will not adhere to anything, but if it be thoroughly wet and pressed hard against the surface of a smooth stone, so as to force out the air from under it, the stone, as nearly all school-boys know, can be lifted up if a string is attached to the leather; or if the air be sucked out of a tube, one end of which is inserted in a liquid, the latter will be forced up the tube. These phenomena are due to the pressure of the atmosphere in the first case on one side of the person’s tongue, pressing it against the mouth of the tube; in the second, to the same pressure on the top of the leather, causing it to adhere to the stone; and in the last, to the weight of the air pressing on the surface of the liquid, forcing it into the vacuum in the tube. [Illustration: _Fig. 7._ Scale ¹⁄₄.] QUESTION 10. _What is the amount of the pressure of the atmosphere, and how is it measured?_ _Answer._ It is usually measured by the pressure on one square inch of surface, which at the earth’s surface is 15 pounds.[3] If, for example, we have a cylinder, _A_, fig. 7, with an air-tight piston, _B_, fitted to it whose area is just one square inch, if we exhaust the air through the tube _C_ from the cylinder above the piston, the air will press against the under side of the piston so that, if no power is required to overcome its friction in the cylinder, the pressure of the air will raise a weight of 15 pounds. The pressure of the air varies, however, as you ascend or descend from the surface of the earth, because as you go up on a mountain or in a balloon the layer of air above you becomes thinner, and, therefore, its weight and consequent pressure are diminished; and as you descend, as in a deep mine, the layer is thicker, and its pressure consequently greater. [3] In common practice it is generally taken at 15 lbs. per square inch, but the average atmospheric pressure is more accurately 14.7 pounds. QUESTION 11. _What is steam?_