general view of the
subject is likely to prove of more interest than particular details.

In consequence of the rotundity of the earth, the distance at which a
beacon light ceases to be visible depends upon its elevation. The
height to which a lighthouse may be carried is a simple question of
expense. The greater part of the pharos of the Romans were much higher
than the most celebrated modern towers. Yet, as it respects optical
effect, the feeble rays which were diffused from the wood or
coal-fires at their summits, could never have traversed the thick fogs
which in all climates occasionally overspread the lower regions of the
atmosphere.

Nevertheless, as to the strength of the light, the modern lighthouses
were, until lately, little superior to the ancient. At the time of the
erection of the Eddystone lighthouse civil engineering was greatly in
advance of practical optics. That noble structure was lighted by
tallow candles, without reflectors or the aid of any kind of apparatus
for concentrating the light. 'For more than half a century this feeble
light was all that directed the mariner in the very high-road of
commerce.' So late as the year 1811 it was lighted with twenty-four
wax candles. In 1812 the Lizard lighthouse, certainly one of the most
important in the kingdom, was maintained with coal-fires. The
Bidstone, a leading light to the port of Liverpool, was furnished with
an enormous spout lamp, with a wick twelve inches in width, the smoke
from which was so great as to completely darken the upper surface of
its reflector. The first important improvement was the introduction of
that admirable invention the Argand Lamp, with a double stream of air.
Four or five of these lamps would doubtless give as much light as the
large fires kept by the Romans; but if those lamps are furnished with
reflecting mirrors, the luminous effect is prodigiously increased.

The light of inflamed bodies spreads itself equally in all directions.
One portion is absorbed by the ground, another is dissipated in space.
The navigator, whose route we are anxious to enlighten, profits only
by the rays that proceed in a horizontal direction, or nearly so, from
the lamp to the sea. But such of the horizontal rays as are directed
towards the land are of course entirely lost to the purposes of the
lighthouse. This zone of horizontal rays forms not only a very small
portion of the total light, but has also the serious inconvenience of
becoming