When the dark intervals occur, the rays from the flame which are then
pointing toward the obscure spaces, have their direction so altered
laterally as to pass into the adjoining bright places; and so increase the
power of the luminous flashes. A revolving light, though supplied by a
flame of the same strength as a fixed, will thus necessarily be raised to
a higher degree; for it does not lose its power by diffusing the rays
constantly over the whole horizon, but gathers them up into a number of
separate beams of greater intensity.
The lights made to revolve by means of clockwork, were fed with mineral
oil, a refined kerosine; and the refraction was caused by highly polished
metallic reflectors.
This visit to the Sand Beach Beacon was quite instructive; since we viewed
there a practical application of an important principle in optics, based
on the reflection of light.
On Wednesday morning, the first beams of the new-born day had just
appeared, when the yacht continued her voyage on Lake Huron. After a
course of nearly twenty-two miles, we approached _Saginaw Bay_--the
largest indentation on the western lake shore--comprising a width of
thirty miles and a length of sixty miles.
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