22 Illumination of Beacons and Buoys. (January, 
too small and not sufficiently important to allow of the 
erection of a lighthouse with the necessary expensive appa- 
ratus and shelter for the keepers; some other expedient 
must then be found, and recourse is had to the mooring of a 
buoy, or the erection of a light ironwork beacon. It is evi- 
dent that the value of these substitutes is but small, as they 
are visible by day only, and that the advantages to be 
derived from their illumination are all important; for it 
must be borne in mind that a vessel when in mid ocean is 
perfectly safe compared with her position when nearing the 
coast. The dangers, too, increase with the time of being 
out of sight of land, because a seaman on a long voyage 
may, by fogs and cloudy weather, be prevented the verifica- 
tion of his course by solar or lunar observations, thus 
rendering it difficult for him to determine accurately the 
situation of a dangerous point. The erection of suitable 
sea marks is, therefore, a matter affecting equally our 
foreign commerce and our coast service. 
The sources of illumination for beacons and buoys, Mr. 
Thomas Stevenson, C.E., in his recently published work on 
‘** Lighthouse Illumination,” considers the following :— 
‘‘rst. The adoption of apparent or borrowed lights. 
“and. The use of dipping lights for indicating the position 
of shoals, by depressing the lamp and apparatus, so as 
to cover with the light the ground that is dangerous. 
“3rd. The conduction either of voltaic, magnetic, or fric- 
tional electricity, or that produced by the efflux of steam, 
through wires, submarine, or where practicable, suspended 
in the air, so as to produce a spark either with or without 
vacuum tubes, or by means of an electro-magnet and the 
deflagration of mercury. 
“ath. The conduction of gas from the shore in sub- 
marine pipes. 
“sth. Self-acting electrical apparatus, produced by the 
action of sea-water or otherwise at the beacon itself, so as to 
require no connection with the shore.” 
This last method was suggested by Mr. T. Stevenson, in 
the ‘‘ Transactions of the Society of Arts for 1866 ;” and it 
is worthy of note that Mr. A. Bain, in 1848, produced and 
maintained a steady light off Brighton by the use of sea- 
water as the meee Gd fluid of the galvanic arrangement. 
The use of apparent or borrowed light is now,, from 
its simplicity, almost generally known. It consists of a 
certain combination of prisms contained in a lantern 
erected on the sunken rock, for producing the divergence of 
parallel rays emitted by a distant lamp placed on the shore. 
