247. =Man a Tool-Making Animal.=--Though there is no actual saving of power in the tools and machines which man uses, yet so great are the advantages which he reaps from them, that more than two thousand years ago a philosopher thought that man could not be better distinguished from brutes than by calling him a tool-making animal. If the distinction was so striking in the time of Aristotle, when tools and machines were so few in number and so rudely contrived, and so few of the sources of power were appropriated by man to his use, how much more striking is it now, with all the variety and perfection of instruments and machinery, and with the ever-extending appropriation of the sources of power furnished by the elements. The power which air and water and gravitation give is applied constantly with more and more variety and effect; and the appropriation of that mighty source of power, steam, is wholly a modern invention. CHAPTER XII. SOUND. 248. =What Sound is.=--Sound is such a vibration of substances as can, on being transmitted to the ear, act upon the sense of hearing. I say _such_ a vibration, because there may be vibrations which will not produce the sensation of sound, Vibrations which are either very slow or very quick will not do it. Thus if a plate of metal or a string make less than 15 or more than 48,000 vibrations in a second, no effect is produced upon the ear. The capacity of hearing differs, however, in different persons, so that although few can hear vibrations which are beyond the range which I have mentioned, there are many whose capacity falls much within it either at one end or both ends of the scale. The range for animals is not the same as that for man. Thus the lion and the elephant can hear a sound when the vibrations are too infrequent to make any impression upon our ears; while small animals have a susceptibility in the organ of hearing for vibrations so quick that we can not hear them, and at the same time are not susceptible to the slower vibrations. How far the range varies in different animals has not been ascertained to any extent. [Illustration: Fig. 182.] 249. =The Vibration of Sounding Bodies Manifest to the Senses.=--If we place the hand upon a large bell that has been struck we can feel the vibration. If we strike one of the ends of a tuning-fork upon some hard body we can see the vibration, as represented in Fig. 182 by the dotted lines. If we look in upon the strings of a piano as it is played, the vibration of the larger strings is very observable to the eye. If we rub the edge of a drinking-glass so as to produce a musical sound, the water which is in it will be thrown into waves from the vibration of the glass. [Illustration: Fig. 183.]