ter case purified acetylene is held in solution
under high pressure in a reservoir containing an asbestos composition
saturated with acetone.

The light-sources of beacons have had the same history as those of other
navigation lights. Many of these are automatic in operation, sometimes
being controlled by clockwork. During the last twenty years the
gas-mantle has been very generally applied to beacon-lights. In the
latter part of the nineteenth century a mineral-oil lamp was devised
with a permanent wick made by forming upon a thick wick a coating of
carbon. The operation is such that this is not consumed and it prevents
further burning of the wick.

The optical apparatus of navigation lights has undergone many
improvements in the past century. The early lights were not equipped
with either reflecting or refracting apparatus. In 1824 Drummond devised
a scheme for reflecting light in order that a distant observer might
make a reading upon the point where the apparatus was being operated by
another person. He was led by his experiments to suggest the application
of mirrors to lighthouses. His device was essentially a parabolic mirror
similar to the reflectors now widely used in automobile head-lamps,
search-lights, etc. He employed the lime-light as a source of light and
was enthusiastic over the results obtained. His discussion published in
1826 indicates that little practical work had been done up to that time
toward obtaining beams or belts of light by means of optical apparatus.
However, lighthouse records show that as early as 1763 small silvered
plane glasses were set in plaster of Paris in such a manner as to form a
partially enveloping reflector. Spherical reflectors were introduced in
about 1780 and parabolic reflectors about ten years later.

All the earlier lights were "fixed," but as it is desirable that the
mariner be able to distinguish one light from another, the revolving
mechanism evolved. By its agency characteristic flashes are obtained and
from the time interval the light is recognized. The first revolving
mechanism was installed in 1783. The early flashing lights were obtained
by means of revolving reflectors which gathered the light and directed
it in the form of a beam or pencil. The type of parabolic reflector now
in use does not differ essentially from that of an automobile head-lamp,
excepting that it is larger.

Lenses appear to have been introduced in the latter part of the
nineteenth centu