_The vibrations of the fork have by contact induced similar vibrations in the wooden table or panel which reinforce the primary tone._ Experiment III Place the handle of the vibrating tuning-fork on a small upturned empty box, or, better still, in contact with the body of a violin, and the sound will be stronger than in the previous experiment, because to the vibrations of the wood are added the vibrations of the air enclosed in the box or the violin.

_To the resonance of the wood has been added the sympathetic resonance of the confined air._ Experiment IV Hold the vibrating fork over the mouth of an empty fruit-jar and there will probably be little or no reinforcement; but gently pour in water, thereby shortening the air column within the jar, and the sound of the fork will be gradually intensified until at a certain point it becomes quite loud.

If you pour in still more water the sound will gradually become feebler.

This shows that _for every tone an air column of a certain size most powerfully reinforces that tone_.
Experiment V As a sequence to the last experiment, take two fruit-jars of the same size, and, having learned to what point to fill them for the greatest resonance, fill one jar (after warming it) to the required point with hot water, the other with cold water, and you will find that the resonance of the heated, therefore expanded, air is much less than the denser air of the cold jar.

This shows that _the degree of density of the air affects its resonance_.
Experiment VI To demonstrate the resonance of the oral cavity, apart from the voice, hold a vibrating tuning-fork before the open mouth.