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The brass ball and brass ring shown in Figure 43 are called the expansion ball and ring.

Heat both ball and ring for a short time.

The latter is suspended by two dolphins, instead of the chains generally in use, whose tails are united, and attached to a small ball and ring.

Now take the ball and ring into sunlight, or the light of a lamp or candle, holding them so that the shadow of the ring is as thin as possible.

Move the ball and ring so that the shadow slowly returns to its first position.