much weaker by divergence, so as to convey to a distance but
a very feeble light. To destroy this divergence, and to profit by all
the light of the lamp, was the task to be accomplished, before
lighthouses could be rendered useful to the full extent.

The application to this purpose of deep metallic mirrors, known under
the name of parabolic mirrors, has been found effectual to the
purposes required. When a lamp is placed in the focus of such a
mirror, all the rays which emanate from it are reflected from the
polished surface, and converge in one direction: their original
divergence is destroyed, and they form, as they issue from the
apparatus, a cylinder of light, parallel with the axis of the mirror.
This light would be transmitted with undiminished brilliancy to a
great distance, did not the atmosphere absorb a portion of it.

It must, however, be admitted, that this method is not free from
defect. It is true, we direct towards the horizon of the sea, a vast
number of rays, which would have been lost upon the ground, in space,
or landward; we also destroy the primitive divergence of those rays
which fall within the range of the seaman; but the cylinder of
reflected light is of no greater size than that of the mirror; the
zone which it illuminates has precisely the same dimensions, at
whatever distance, and, unless we employ a number of similar
reflectors differently inclined, there will be a number of large
spaces in the horizon completely obscure, from which the pilot will
never see any signal whatever. This serious objection has been removed
by imparting, by means of clock-work, a uniform rotatory motion to the
reflector. The collection of rays proceeding from the mirror is thus
directed to all the points of the horizon in succession. Every vessel
perceives the signal-light during one instant, and immediately after
it is seen to disappear; and if, in a great extent of coast, the
different lights revolve in different times, the various signals
become thus individualized. According to the interval of time, which
elapses between two successive appearances or eclipses of the light,
does the sailor recognize the part of the coast which is in view: he
is thus no longer liable to mistake a planet, or a star of the first
magnitude, at its rising or setting, or a fire lighted on the coast by
fishermen, charcoal-burners, &c. for the light of the lighthouse;
mistakes, which have often led to the most deplorable wrecks.

Th