Harmonic wave apparatus, 18th century.
© Science Museum / Science & Society Picture Library
This demonstrates the effect of combining two waves (interference). A row of bras rods are cut so that their tops form a wave shape. A mahogany template in the form of another wave is slid underneath the rods. Where the crests of the two waves are above each other, the waves reinforce each other and the amplitude of the combined wave is greater than that of either of the two original waves. When the crest of one wave aligns with the trough of the other, the waves cancel each other out.