pressed against a strip of wood or metal dipping into the water. If the
strip is connected with a much larger wooden or metallic surface in the
water the sound is heard much more distinctly. Now, the sides of a ship
form a very large collecting surface, and at the distance of several
miles from such a water siren as might be constructed, we feel quite
sure that, above the noise of engines and flapping sails, above the far
more troublesome noise of waves striking the ship's side, the musical
note of the distant siren would be heard, giving warning of a dangerous
neighborhood. In considering this problem, you must remember that
Messrs. Colladon and Sturn heard distinctly the sound of a bell struck
underwater at the distance of nearly nine miles, the sound being
communicated by the water of Lake Geneva."

The next portion of the lecture discussed the great value of a rapid
recurrence of effects, the obtaining of sound by means of a rapid
intermission of light rays on selenium joined up in an electric circuit
being instanced as an example. Then recent experiments on the refractive
power of ebonite were detailed--the rough results tending to give
greater weight to Clerk-Maxwell's electro-magnetic theory of light. The
index of refraction of ebonite was found by Profs. Ayrton and Perry to
be roughly 1.7. Clerk-Maxwell's theory requires that the square of this
number should be equal to the electric specific inductive capacity of
the substance. For ebonite this electric constant varies from 2.2 to 3.5
for different specimens, the mean of which is almost exactly equal to
the square of 1.7.

* * * * *