1872.] Electricity. 275 
requisite machinery and apparatus for the production of the electric light have 
been erected midway between the two lighthouses. The ele¢tricity is 
generated by one of Professor Holmes’s magneto-electric machines, worked by 
a small horizontal condensing engine. There are four of these engines, two 
being for the service of each lighthouse—one used ordinarily, but in times of 
fog both. They make 400 revolutions per minute, effecting with this speed an 
alternation in the direction of the currents of 6400 times in the same period. 
The electrical currents are sent from the machines by underground wires to 
the lantern of each lighthouse. The steam-engine, boiler, and magneto- 
electric machines are all duplicated in case of accident or want of repair to any 
part. The supply of water for the steam-engines is obtained from a well sunk 
through the chalk a depth of 280 feet to the high water level of the sea. This 
well, although the water is remarkably pure and free from salt, is curiously 
affected by the action of the tide. During each flood-tide the well is quite dry, 
but throughout each ebb-tide there is an abundant supply of water. The 
optical apparatus in each lantern is of the dimensions of a third order for fixed 
light, but has been especially designed and manufactured for the purpose of 
the electric light. From the high lighthouse 246 degrees of the arc surface is 
illuminated, and from the low lighthouse the arc illuminated is 106 degrees, 
The landward arc of the light, instead of being waste light to the mariner, is 
also utilised, and is in each case carefully gathered up, and by reflecting prisms 
arranged on each side of the main apparatus, equally distributed over the 
portion of the surface illuminated by the latter, thus adding very considerably 
to the power. Each apparatus is provided with an efficient oil-lamp as a 
further precaution in case of accident. 
M. Trouvé has found a new application of electricity to medical purposes in 
his eleétric probe for bullet wounds. This probe consists of three distinct 
parts, the battery, the probe, and the indicator; and the adaptation is founded 
on the conduétibility of metals. The battery is an ordinary zinc-carbon 
element, the exciting liquid being bisulphide of mercury. The probe is flexible 
or rigid, and is a conduétor. The indicator has in its interior a very small 
electro-magnet with a vibrator and two small rods of steel, very sharp and 
insulated from each other; and as soon as these points, which are in con- 
nection with the battery, touch any metallic substance, the vibrator begins to 
move. The surgeon with this apparatus can even distinguish the different 
metals from one another. If the metal is lead, the trembler vibrates regularly ; 
if, however, it is iron or copper, the trembler has a jerky movement. Iron may 
be distinguished from copper by its action upon the needle of a galvanometer. 
Dr. Joule has given a note of some experiments on the polarisation by 
frictional electricity of platina plates, either immersed in water or in alternate 
series with wet silk. The charge was only diminished one-half after an 
interval of an hour-and-a-quarter. It was ascertained both in quality and 
quantity by transmitting it through a delicate galvanometer. He suggested 
that a condenser on this principle might be useful for the observation of 
atmospheric electricity. 
Some investigations of a novel kind relating to the dire@ive power of large 
steel magnets, bars of magnetised soft iron, and of galvanic coils in their 
action on external small magnets, have been brought forward by Dr. Airey, 
C.B., P.R.S., Astronomer Royal, and James Stuart, M.A. The experiments 
show the distribution of force in the sphere of magnets, and a marked difference 
was observed between the behaviour of an ordinary steel magnet and that of a 
galvanic coil without a core; for whereas in the instance of the steel magnet 
the focus lies within, and some distance from, the end of the bar, the focus of 
the coil was found to be at the centre of the end of the coil. It would be 
interesting to know if this be so, for it might be supposed that the focus is a 
little distant from the flush end, perhaps little more than one-twelfth part of 
the diameter of the coil’s wire from it. The presumption would be that the 
rings of the coil have, unless clasping a core, each a magnetic action of their 
own, besides their combined aétion, and that it is the last ring which deter- 
mines the focus alluded to. The apparatus used for the experiments is a bar 
