rious Frenchman, Fresnel.

In a first-class 'dioptric' apparatus the light emanates from a lamp
with several concentric wicks, the flame of which, being kindled by a
very active draught, attains to great intensity. In fixed lights the
lenses refract the rays issuing from the lamp so as to cause them to
form a luminous sheet which grazes the sea-horizon. In revolving
lights the lenses gather up the rays into distinct beams, resembling
the spokes of a wheel, which sweep over the sea and strike the eye of
the mariner in succession.

It is not for clear weather that the greatest strengthening of the
light is intended, for here it is not needed. Nor is it for densely
foggy weather, for here it is ineffectual. But it is for the
intermediate stages of hazy, snowy, or rainy weather, in which a
powerful light can assert itself, while a feeble one is extinguished.
The usual first-order lamp is one of four wicks, but Mr. Douglass, the
able and indefatigable engineer of the Trinity House, has recently
raised the number of the wicks to six, which produce a very noble
flame. To Mr. Wigham, of Dublin, we are indebted for the successful
application of gas to lighthouse illumination. In some lighthouses
his power varies from 28 jets to 108 jets, while in the lighthouse of
Galley Head three burners of the largest size can be employed, the
maximum number of jets being 324. These larger powers are invoked
only in case of fog, the 28-jet burner being amply sufficient for
clear weather. The passage from the small burner to the large, and
from the large burner to the small, is made with ease, rapidity, and
certainty. This employment of gas is indigenous to Ireland, and the
Board of Trade has exercised a wise liberality in allowing every
facility to Mr. Wigham for the development of his invention.

The last great agent employed in lighthouse illumination is
electricity. It was in this Institution, beginning in 1831, that
Faraday proved the existence and illustrated the laws of those induced
currents which in our day have received such astounding development.
In relation to this subject Faraday's words have a prophetic ring. 'I
have rather,' he writes in 1831, 'been desirous of discovering new
facts and new relations dependent on magneto-electric induction than
of exalting the force of those already obtained, being assured that
the latter would find their full development hereafter.' The labours
of Holmes, of the Paris Alliance C