_Eddystone, New Lighthouse (J. N. Douglass)._--The site selected for
the new tower is 120 ft. S.S.E. from Smeaton's lighthouse, where a
suitable foundation was found, although a considerable section of the
lower courses had to be laid below the level of low water. The
vertical base is 44 ft. in diameter and 22 ft. in height. The tower
(figs. 5 and 6) is a concave elliptic frustum, and is solid, with the
exception of a fresh-water tank, to a height of 25 ft. 6 in. above
high-water level. The walls above this level vary in thickness from 8
ft. 6 in. to 2 ft. 3 in. under the gallery. All the stones are
dovetailed, both horizontally and vertically, on all joint faces, the
stones of the foundation course being secured to the rock by Muntz
metal bolts. The tower contains 62,133 cub. ft. of granite, weighing
4668 tons. The height of the structure from low water ordinary spring
tides to the mean focal plane is 149 ft. and it stands 133 ft. above
high water. The lantern is a cylindrical helically framed structure
with domed roof. The astragals are of gun-metal and the pedestal of
cast iron. The optical apparatus consists of two superposed tiers of
refracting lens panels, 12 in each tier of 920 mm. focal distance. The
lenses subtend an angle of 92° vertically. The 12 lens panels are
arranged in groups of two, thus producing a group flashing light
showing 2 flashes of 1½ seconds' duration every half minute, the
apparatus revolving once in 3 minutes. The burners originally fitted
in the apparatus were of 6-wick pattern, but these were replaced in
1904 by incandescent oil vapour burners. The intensity of the combined
beam of light from the two apparatus is 292,000 candles. At the time
of the completion of the lighthouse two bells, weighing 2 tons each
and struck by mechanical power, were installed for fog-signalling
purposes. Since that date an explosive gun-cotton fog signal has been
erected, the bells being removed. At a lower level in the tower are
installed 2 21-in. parabolic silvered reflectors with 2-wick burners,
throwing a fixed light of 8000 candle-power over a danger known as the
Hand Deeps. The work of preparing the foundation was begun on the 17th
of July 1878, the foundation stone being laid by the late duke of
Edinburgh on the 19th of August 1879. The last stone was laid on the
1st of June 1881, and the light was exhibited for the first time on
the 18th of May 1882. The upper portion of Smeaton's tower, which was
removed on completion of the new lighthouse, was re-erected on
Plymouth Hoe, where it replaced the old Trinity House sea mark. One of
the principal features in the design of the new Eddystone lighthouse
tower is the solid vertical base. This construction was much
criticized at the time, but experience has proved that heavy seas
striking the massive cylindrical structure are immediately broken up
and rush round to the opposite side, spray alone ascending to the
height of the lantern gallery. On the other hand, the waves striking
the old tower at its foundation ran up the surface, which presented a
curved face to the waves, and, unimpeded by any projection until
arriving at the lantern gallery, were partially broken up by the
cornice and then spent themselves in heavy spray over the lantern. The
shock to which the cornice of the gallery was exposed was so great
that stones were sometimes lifted from their beds. The new Eddystone
tower presents another point of dissimilarity from Smeaton's
structure, in that the stones forming the floors consist of single
corbels built into the wall and constituting solid portions thereof.
In Smeaton's tower the floors consisted of stone arches, the thrust
being taken by the walls of the tower itself, which were strengthened
for the purpose by building in chains in the form of hoops (fig. 7).
The system of constructing corbelled stone floors was first adopted by
R. Stevenson in the Bell Rock lighthouse (fig. 8).
[Illustration: FIG. 6.--Plan of Entrance Floor, Eddystone Lighthouse.]
[Illustration: FIG. 7.--Floor, Smeaton's Eddystone Lighthouse.]
_Bell Rock Lighthouse_ (fig. 9).--The Bell Rock, which lies 12 m. off
the coast of Forfarshire, is exposed to a considerable extent at low
water. The tower is submerged to a depth of about 16 ft. at high water
of spring tides. The rock is of hard sandstone. The lighthouse was
constructed by Robert Stevenson and is 100 ft. in height, the solid
portion being carried to a height of 21 ft. above high water. The work
of construction was begun in 1807, and finished in 1810, the light
being first exhibited in 1811. The total weight of the tower is 2076
tons. A new lantern and dioptric apparatus were erected on the tower
in 1902. The focal plane of the light is elevated 93 ft. above high
water.
[Illustration: FIG. 8.--Floor, Stevenson's Bell Rock Lighthouse.]
_Skerryvore Lighthouse_ (fig. 10).--The Skerryvore Rocks, 12 m. off
the island of Tyree in Argyllshire, are wholly open to the Atlantic.
The work, designed by Alan Stevenson, was begun in 1838 and finished
in 1844. The tower, the profile of which is a hyperbolic curve, is 138
ft. high to the lantern base, 42 ft. diameter at the base, and 16 ft.
at the top. Its weight is 4308 tons. The structure contains 9 rooms in
addition to the lantern chamber. It is solid to a height of 26 ft.
above the base.
_Heaux de Brehat Lighthouse._--The reef on which this tower is
constructed lies off the coast of Brittany, and is submerged at high
tide. The work was carried out in 1836-1839. The tower is circular in
plan with a gallery at a height of about 70 ft. above the base. The
tower is 156 ft. in height from base to lantern floor.
_Haut Banc du Nord Lighthouse._--This tower is placed on a reef at the
north-west extremity of the Île de Ré, and was constructed in
1849-1853. It is 86 ft. in height to the lantern floor.
_Bishop Rock Lighthouse._--The lighthouse on the Bishop Rock, which is
the westernmost landfall rock of the Scilly Islands, occupies perhaps
a more exposed situation than any other in the world. The first
lighthouse erected there was begun in 1847 under the direction of N.
Douglass. The tower consisted of a cast and wrought iron openwork
structure having the columns deeply sunk into the rock. On the 5th of
February 1850, when the tower was ready for the erection of the
lantern and illuminating apparatus, a heavy storm swept away the whole
of the structure. This tower was designed for an elevation of 94 ft.
to the focal plane. In 1851 the erection of a granite tower, from the
designs of James Walker, was begun; the light was first exhibited in
1858. The tower (fig. 11) had an elevation to the focal plane of 110
ft., the lower 14 courses being arranged in steps, or offsets, to
break up the force of the waves. This structure also proved
insufficient to withstand the very heavy seas to which it was exposed.
Soon after its completion the 5-cwt. fog bell, fixed to the lantern
gallery 100 ft. above high-water mark, was washed away, together with
the flagstaff and ladder. The tower vibrated considerably during
storms, and it was found that some of the external blocks of granite
had been split by the excessive stress to which they had been exposed.
In 1874 the tower was strengthened by bolting continuous iron ties to
the internal surfaces of the walls. In 1881, when further signs of
damage appeared, it was determined to remove the upper storey or
service room of the lighthouse, and to case the structure from its
base upwards with granite blocks securely dovetailed to each other and
to the existing work. At the same time it was considered advisable to
increase the elevation of the light, and place the mean focal plane of
the new apparatus at an elevation of 146 ft. above high-water mark.
The work was begun in 1883, and the new apparatus was first
illuminated on the 25th of October 1887. During the operation of
heightening the tower it was necessary to install a temporary light,
consisting of a cylindrical lightship lantern with catoptric
apparatus; this was raised from time to time in advance of the
structure as the work proceeded. The additional masonry built into the
tower amounts approximately to 3220 tons. Profiting by the experience
gained after the construction of the new Eddystone tower, Sir J. N.
Douglass decided to build the lower portion of the improved Bishop
Rock tower in the form of a cylinder, but with considerably increased
elevation (figs. 12 and 13). The cylindrical base is 40 ft. in
diameter, and rises to 25 ft. above high-water mark. The lantern is
cylindrical and helically framed, 14 ft. in diameter, the glazing
being 15 ft. in height. The optical apparatus consists of two
superposed tiers of lenses of 1330 mm. focal distance, the lenses
subtending a horizontal angle of 36° and a vertical angle of 80°. The
apparatus consists of 5 groups of lenses each group producing a double
flashing light of one minute period, the whole apparatus revolving
once in five minutes. The maximum aggregate candle-power of the flash
is 622,000 candles. A gun-cotton explosive fog signal is attached to
the lantern. The cost of the various lighthouses on the Bishop Rock
has been as follows:
1. Cast iron lighthouse £12,500 0 0
2. Granite lighthouse 34,559 18 9
3. Improved granite lighthouse 64,889 0 0
[Illustration: FIG. 9.--Bell Rock.
FIG. 10.--Skerryvore.
FIG. 11.--Bishop Rock.