1872.] Illumination of Beacons and Buoys. 25 
a few miles’ distance. But there is a corresponding diffi- 
culty. Neither Holmes’s nor Wilde’s machine can be 
employed, because, as the light is produced by the rapid 
consumption of the carbon-points, there is involved the 
requirement of lamp machinery too delicate to withstand 
the buffets of the waves. Mr. Stevenson therefore decided 
on the employment of vacuum tubes, or the electric spark 
without carbons. In order to increase the intensity of the 
light, Professor Swan suggested the use of a Ruhmkorff’s 
induction coil and condenser. From the experiments insti- 
tuted between the 2nd and 13th of January, 1866, Mr. T. 
Stevenson deduced some important results, which were 
communicated to the Royal Scottish Society of Arts. 
Platinum electrodes were employed, and the primary cur- 
rent was kept passing for a week without any sensible waste 
of the metallic points. To put these results into practice 
was the next step, and a submarine cable. was procured 
by the Commissioners of Northern Lights. But the condi- 
tions of working a cable were not so well known as now, 
and the intense secondary current could not be made to 
take the desired path. Mr. Siemens was then appealed 
to, and he submitted a very ingenious device by which 
the extra current, as it is called, from the coils of an electro- 
magnet, situated at the buoy or beacon, in the primary cir- 
cuit, was utilised in the production of the spark. The 
electro-magnet was, made to work its own contact-lever, 
and the luminous effect was increased by forming one 
electrode of a vessel of mercury, renewed by the action of 
the eleCtro-magnet on a small pump. This arrangement 
was placed at Granton Harbour, and although the light was 
vivid and the current easily managed, insuperable diffi- 
culties arose from the deposition of mercurial vapours upon 
the apparatus. Experiments with the induCtion-coil were, 
therefore, resumed, and following some improvements sug- 
gested by Professor P. G. Tait, were attended with success. 
The battery and break were retained on shore at Granton 
pier-head, while the cable extended a distance of half-a-mile 
to Trinity Pier, where two induction coils with condensers 
and the optical apparatus were placed. The coils together 
contained about eight miles of wire, and the spark was 
induced by sixteen Bunsen elements, two additional cells 
being used for working the break. The light was very vivid 
and striking. With the exception, however, of some 
enquiries made by the Trinity House, which led to the repe- 
tition of the experiments in 1869, to the present nothing has 
been done, although the recent improvements in electrical 
VOL. IT.” (N-S:) E 
