that all substances expand and contract at the same rate. Different solids expand and contract at different rates, and the same is true of liquids. For example, copper expands and contracts much more than platinum, and alcohol more than water, for the same change of temperature.
28. In Fig. 4 is shown an apparatus for exhibiting the linear expansion of a solid body. A metal rod a is fixed at one end by a screw b, the other end passing freely through the fork c held in the post, and pressing against the short arm of the indicator f. The rod is heated as shown, and its elongation causes the indicator to move along the arc de.
29. An illustration of surface expansion is afforded nearly every day in machine shops, particularly in locomotive shops, where piston rods, crankpins, etc. are shrunk in and tires shrunk on their centers. In shrinking on a tire, it is bored a little smaller than the wheel center. The tire is then heated until it is expanded enough to allow it to slide over the wheel center. It is then cooled with cold water, when it contracts, tending to regain its original size, but is prevented by the wheel center, which is a trifle larger. This causes the tire to hug the center with great force, and prevents it from coming off.
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30. Cubic expansion may be illustrated by means of a Gravesandes' ring, which consists of a brass ball a, Fig. 5, that at ordinary temperatures passes freely through the ring m, of very nearly the same diameter. When the ball
