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Show ! ' ' I . ' 166 SENSATION OF AIR. We are not authorized to say that it moves without any friction; but its friction is only the friction of particles ; and, with moderate velocities, the resist· ance of air rubbing on air is very small. The atmospheric air is at once the most delicate and the most powerful of all springs. It actually yields to the touch of a sunbeam, and yet it can cleave rocks, and shake the surfaces of countries to pieces in earthquakes. It is more nice in the detection of pressure than any instrument that we can contrive, and no thermometer can measure heat with nearly the precision of an air one.. The a~r is, indeed, not only fine beyond all sensatiOn, but 1t is the immediate object of all the senses. It is the air which the eye sees, the ear hears, the nose scents, and the finger touches. We know nothing of what sight might be in a vacuum, or space where there were no air, because the eye would be destroyed if it were in such a place, even though the apparatns were so contrived as that the operation of breathing could still be carried on. Once remove the pressure of the atmosphere, and the fluids of the eye would burst the vessels and coats, and there would be an end of its curious structure, as well as its power of seeing. Smell and taste are not in the air, but still the fragrance and the sapidity are." melted or dis~olved in air," before we can perceive them; and m those ~nternal parts of the body which we may suppose that the atmospheric air does not reach, we have no perception of any thing like either smell or taste. Then as to hearing, it is the air that we hear. Air is the instrument, and the only 'instrument of sound; and if it were taken away, all nature would be as dumb as a little bell is when it is tolled or struck within an exhausted receiver. Indeed, it not only requires air, but it requires some body or substance of air to produce a sound that can be heard; for we are not able, by even the best air-pump, to exhaust ... ; w' ~ .. • • AIR EVERYWHERE. 167 even the smallest vessel completely of air, as there must always be as much remaining as has spring enough to raise the valve of the pump. Then as to touching, if we touched things themselves, and not the air, they would stick to our fingers, or our fingers would stick to them. The mean pressure of the air is about fifteen pounds on every square inch of surface; and so, if even the strongest man were to grasp a stick without air between it and his hand, he would never be able to unclasp his hand and let it go. As little could a man walk i-f there were no air between his feet and the ground. If there were no air, each foot of a full-grown man, if the sole were entirely on the ground, would be pressed to the ground by a weight of about four hundred pounds; and thus the man could never lift a foot, but would stand on the earth, as still as an earth-fast stone. The little ridges of papillre that are on the palm and fingers of a healthy hand, and also on the sole of a well-kept foot, contribute to the ease with which the hands and the feet can be separated from that which they touch, by the air that is lodged in the little hollo.ws between ; and though by close squeezing the stdes of the fingers may be made to stick together the fronts or tips of the fingers never can. ' If there were not atmospheric air in the interstices between all substances, nothing which had a base, or surface, of any size that could be placed in contact with another, would fall. In that case a man would not need to h~ng his hat on the peg ; he would only need to push It to the wall, and it would remain there. So also he might stick himself to the wall, or lie down on the ceiling on his back and l?ok. down o!l the company below. Indeed, it would ~~gn~fy but little where he lay down; for be it where tt nught, assuredly he would never be able to rise up again. If it were not that the air always comes between |
