+-------------+---------+-------------------------------------+
| | | Vertical Angles of Optics. |
| | | (Ordinary Dimensions.) |
| | Focal +-------------+-----------------------+
| Order. |Distance,| | Holophotal Optics. |
| | mm. | Dioptric +-------+-------+-------+
| | | Belt only. | Lower | Lens. | Upper |
| | | |Prisms.| |Prisms.|
+-------------+---------+-------------+-------+-------+-------+
| Hyper-Radial| 1330 | 80° | 21° | 57° | 48° |
| 1st order | 920 |92°, 80°, 58°| 21° | 57° | 48° |
| 2nd " | 700 | 80° | 21° | 57° | 48° |
| 3rd " | 500 | 80° | 21° | 57° | 48° |
| Small 3rd | | | | | |
| order | 375 | 80° | 21° | 57° | 48° |
| 4th order | 250 | 80° | 21° | 57° | 48° |
| 5th " | 187.5 | 80° | 21° | 57° | 48° |
| 6th " | 150 | 80° | 21° | 57° | 48° |
+-------------+---------+-------------+-------+-------+-------+
Lenses of small focal distance are also made for buoy and beacon
lights.
[Illustration: FIG. 42.--Cape Naturaliste Apparatus.]
[Illustration: FIG. 43.--Île Vierge Apparatus.]
_Light Intensities._--The powers of lighthouse lights in the British
Empire are expressed in terms of standard candles or in "lighthouse
units" (one lighthouse unit = 1000 standard candles). In France the
unit is the "Carcel" = .952 standard candle. The powers of burners and
optical apparatus, then in use in the United Kingdom, were carefully
determined by actual photometric measurement in 1892 by a committee
consisting of the engineers of the three general lighthouse boards,
and the values so obtained are used as the basis for calculating the
intensities of all British lights. It was found that the intensities
determined by photometric measurement were considerably less than the
values given by the theoretical calculations formerly employed. A
deduction of 20% was made from the mean experimental results obtained
to compensate for loss by absorption in the lantern glass, variations
in effects obtained by different men in working the burners and in the
illuminating quality of oils, &c. The resulting reduced values are
termed "service" intensities.
As has been explained above, the effect of a dioptric apparatus is to
condense the light rays, and the measure of this condensation is the
ratio between the vertical divergence and the vertical angle of the
optic in the case of fixed lights. In flashing lights the ratio of
vertical condensation must be multiplied by the ratio between the
horizontal divergence and the horizontal angle of the panel. The loss
of light by absorption in passing through the glass and by refraction
varies from 10% to 15%. For apparatus containing catadioptric elements
a larger deduction must be made.
The intensity of the flash emitted from a dioptric apparatus, showing
a white light, may be found approximately by the empirical formula I =
PCVH/vh, where I = intensity of resultant beam, P = service intensity
of flame, V = vertical angle of optic, v = angle of mean vertical
divergence, H = horizontal angle of panel, h = angle of mean
horizontal divergence, and C = constant varying between .9 and .75
according to the description of apparatus. The factor H/h must be
eliminated in the case of fixed lights. Deduction must also be made in
the case of coloured lights. It should, however, be pointed out that
photometric measurements alone can be relied upon to give accurate
values for lighthouse intensities. The values obtained by the use of
Allard's formulae, which were largely used before the necessity for
actual photometric measurements came to be appreciated, are
considerably in excess of the true intensities.
[Illustration: FIG. 43A.--Île Vierge Apparatus and Lantern. Plan at
focal plane.]
_Optical Calculations._--The mathematical theory of optical apparatus
for lighthouses and formulae for the calculations of profiles will be
found in the works of the Stevensons, Chance, Allard, Reynaud, Ribière
and others. Particulars of typical lighthouse apparatus will be found
in tables VI. and VII.
4. ILLUMINANTS.--The earliest form of illuminant used for lighthouses
was a fire of coal or wood set in a brazier or grate erected on top of
the lighthouse tower. Until the end of the 18th and even into the 19th
century this primitive illuminant continued to be almost the only one in
use. The coal fire at the Isle of May light continued until 1810 and
that at St Bees lighthouse in Cumberland till 1823. Fires are stated to
have been used on the two towers of Nidingen, in the Kattegat, until
1846. Smeaton was the first to use any form of illuminant other than
coal fires; he placed within the lantern of his Eddystone lighthouse a
chandelier holding 24 tallow candles each of which weighed 2/5 of a
lb. and emitted a light of 2.8 candle power. The aggregate illuminating
power was 67.2 candles and the consumption at the rate of 3.4 lb. per
hour.
_Oil._--Oil lamps with flat wicks were used in the Liverpool
lighthouses as early as 1763. Argand, between 1780 and 1783, perfected
his cylindrical wick lamp which provides a central current of air
through the burner, thus allowing the more perfect combustion of the
gas issuing from the wick. The contraction in the diameter of the
glass chimney used with wick lamps is due to Lange, and the principle
of the multiple wick burner was devised by Count Rumford. Fresnel
produced burners having two, three and four concentric wicks. Sperm
oil, costing 5s. to 8s. per gallon, was used in English lighthouses
until 1846, but about that year colza oil was employed generally at a
cost of 2s. 9d. per gallon. Olive oil, lard oil and coconut oil have
also been used for lighthouse purposes in various parts of the world.
_Mineral Oil Burners._--The introduction of mineral oil, costing a
mere fraction of the expensive animal and vegetable oils,
revolutionized the illumination of lighthouses. It was not until 1868
that a burner was devised which successfully consumed hydrocarbon
oils. This was a multiple wick burner invented by Captain Doty. The
invention was quickly taken advantage of by lighthouse authorities,
and the "Doty" burner, and other patterns involving the same
principle, remained practically the only oil burners in lighthouse use
until the last few years of the 19th century.
The lamps used for supplying oil to the burner are of two general
types, viz. those in which the oil is maintained under pressure by
mechanical action and constant level lamps. In the case of single
wick, and some 2-wick burners, oil is supplied to the burner by the
capillary action of the wick alone.
The mineral oils ordinarily in use are petroleum, which for lighthouse
purposes should have a specific gravity of from .820 to .830 at 60° F.
and flashing point of not less than 230° F. (Abel close test), and
Scottish shale oil or paraffin with a specific gravity of about .810
at 60° F. and flash point of 140° to 165° F. Both these varieties may
be obtained in England at a cost of about 6½d. per gallon in bulk.
_Coal Gas_ had been introduced in 1837 at the inner pier light of
Troon (Ayrshire) and in 1847 it was in use at the Heugh lighthouse
(West Hartlepool). In 1878 cannel coal gas was adopted for the Galley
Head lighthouse, with 108-jet Wigham burners. Sir James Douglass
introduced gas burners consisting of concentric rings, two to ten in
number, perforated on the upper edges. These give excellent results
and high intensity, 2600 candles in the case of the 10-ring burner
with a flame diameter at the focal plane of 5(5/8) in. They are still
in use at certain stations. The use of multiple ring and jet gas
burners is not being further extended. Gas for lighthouse purposes
generally requires to be specially made; the erection of gas works at
the station is thus necessitated and a considerable outlay entailed
which is avoided by the use of oil as an illuminant.
_Incandescent Coal Gas Burners._--The invention of the Welsbach mantle
placed at the disposal of the lighthouse authorities the means of
producing a light of high intensity combined with great focal
compactness. For lighthouse purposes other gaseous illuminants than
coal gas are as a rule more convenient and economical, and give better
results with incandescent mantles. Mantles have, however, been used
with ordinary coal gas in many instances where a local supply is
available.
[Illustration: FIG. 44.--"Chance" Incandescent Oil Burner, with 85 mm.
diameter mantle.]
_Incandescent Mineral Oil Burners._--Incandescent lighting with
high-flash mineral oil was first introduced by the French Lighthouse
Service in 1898 at L'Île Penfret lighthouse. The burners employed are
all made on the same principle, but differ slightly in details
according to the type of lighting apparatus for which they are
intended. The principle consists in injecting the liquid petroleum in
the form of spray mixed with air into a vaporizer heated by the mantle
flame or by a subsidiary heating burner. A small reservoir of
compressed air is used--charged by means of a hand pump--for providing
the necessary pressure for injection. On first ignition the vaporizer
is heated by a spirit flame to the required temperature. A reservoir
air pressure of 125 lb. per sq. in. is employed, a reducing valve
supplying air to the oil at from 60 to 65 lb. per sq. in. Small
reservoirs containing liquefied carbon dioxide have also been employed
for supplying the requisite pressure to the oil vessel.
The candle-power of apparatus in which ordinary multiple wick burners
were formerly employed is increased by over 300% by the substitution
of suitable incandescent oil burners. In 1902 incandescent oil burners
were adopted by the general lighthouse authorities in the United
Kingdom. The burners used in the Trinity House Service and some of
those made in France have the vaporizers placed over the flame. In
other forms, of which the "Chance" burner (fig. 44) is a type, the
vaporization is effected by means of a subsidiary burner placed under
the main flame.
Particulars of the sizes of burner in ordinary use are given in the
following table.
+--------------------+------------------+-------------------+
| Diameter of Mantle.|Service Intensity.|Consumption of oil.|
| | | Pints per hour. |
+--------------------+------------------+-------------------+
| 35 mm. | 600 candles. | .50 |
| 55 mm. | 1200 " | 1.00 |
| 85 mm. | 2150 " | 2.25 |
|Triple mantle 50 mm.| 3300 " | 3.00 |
+--------------------+------------------+-------------------+
The intrinsic brightness of incandescent burners generally may be
taken as being equivalent to from 30 candles to 40 candles per sq. cm.
of the vertical section of the incandescent mantle.
In the case of wick burners, the intrinsic brightness varies,
according to the number of wicks and the type of burner from about 3.5
candles to about 12 candles per sq. cm., the value being at its
maximum with the larger type of burner. The luminous intensity of a
beam from a dioptric apparatus is, _ceteris paribus_, proportional to
the intrinsic brightness of the luminous source of flame, and not of
the total luminous intensity. The intrinsic brightness of the flame of
oil burners increases only slightly with their focal diameter,
consequently while the consumption of oil increases the efficiency of
the burner for a given apparatus decreases. The illuminating power of
the condensed beam can only be improved to a slight extent, and, in
fact, is occasionally decreased, by increasing the number of wicks in
the burner. The same argument applies to the case of multiple ring and
multiple jet gas burners which, notwithstanding their large total
intensity, have comparatively small intrinsic brightness. The economy
of the new system is instanced by the case of the Eddystone bi-form
apparatus, which with the concentric 6-wick burner consuming 2500
gals. of oil per annum, gave a total intensity of 79,250 candles.
Under the new régime the intensity is 292,000 candles, the oil
consumption being practically halved.
_Incandescent Oil Gas Burners._--It has been mentioned that
incandescence with low-pressure coal gas produces flames of
comparatively small intrinsic brightness. Coal gas cannot be
compressed beyond a small extent without considerable injurious
condensation and other accompanying evils. Recourse has therefore been
had to compressed oil gas, which is capable of undergoing compression
to 10 or 12 atmospheres with little detriment, and can conveniently be
stored in portable reservoirs. The burner employed resembles the
ordinary Bunsen burner with incandescent mantle, and the rate of
consumption of gas is 27.5 cub. in. per hour per candle. A reducing
valve is used for supplying the gas to the burner at constant
pressure. The burners can be left unattended for considerable periods.
The system was first adopted in France, where it is installed at eight
lighthouses, among others the Ar'men Rock light, and has been extended
to other parts of the world including several stations in Scotland and
England. The mantles used in France are of 35 mm. diameter. The 35 mm.
mantle gives a candle-power of 400, with an intrinsic brightness of 20
candles per sq. cm.
The use of oil gas necessitates the erection of gas works at the
lighthouse or its periodical supply in portable reservoirs from a
neighbouring station. A complete gas works plant costs about £800. The
annual expenditure for gas lighting in France does not exceed £72 per
light where works are installed, or £32 where gas is supplied from
elsewhere. In the case of petroleum vapour lighting the annual cost of
oil amounts to about £26 per station.
_Acetylene._--The high illuminating power and intrinsic brightness of
the flame of acetylene makes it a very suitable illuminant for
lighthouses and beacons, providing certain difficulties attending its
use can be overcome. At Grangemouth an unattended 21-day beacon has
been illuminated by an acetylene flame for some years with
considerable success, and a beacon light designed to run unattended
for six months was established on Bedout Island in Western Australia
in 1910. Acetylene has also been used in the United States, Germany,
the Argentine, China, Canada, &c., for lighthouse and beacon
illumination. Many buoys and beacons on the German and Dutch coasts
have been supplied with oil gas mixed with 20% of acetylene, thereby
obtaining an increase of over 100% in illuminating intensity. In
France an incandescent burner consuming acetylene gas mixed with air
has been installed at the Chassiron lighthouse (1902). The French
Lighthouse Service has perfected an incandescent acetylene burner with
a 55 mm. mantle having an intensity of over 2000 candle-power, with
intrinsic brightness of 60 candles per sq. cm.
_Electricity._--The first installation of electric light for
lighthouse purposes in England took place in 1858 at the South
Foreland, where the Trinity House established a temporary plant for
experimental purposes. This installation was followed in 1862 by the
adoption of the illuminant at the Dungeness lighthouse, where it
remained in service until the year 1874 when oil was substituted for
electricity. The earliest of the permanent installations now existing
in England is that at Souter Point which was illuminated in 1871.
There are in England four important coast lights illuminated by
electricity, and one, viz. Isle of May, in Scotland. Of the former St
Catherine's, in the Isle of Wight, and the Lizard are the most
powerful. Electricity was substituted as an illuminant for the then
existing oil light at St Catherine's in 1888. The optical apparatus
consisted of a second-order 16-sided revolving lens, which was
transferred to the South Foreland station in 1904, and a new second
order (700 mm.) four-sided optic with a vertical angle of 139°,
exhibiting a flash of .21 second duration every 5 seconds substituted
for it. A fixed holophote is placed inside the optic in the dark or
landward arc, and at the focal plane of the lamp. This holophote
condenses the rays from the arc falling upon it into a pencil of small
angle, which is directed horizontally upon a series of reflecting
prisms which again bend the light and throw it downwards through an
aperture in the lantern floor on to another series of prisms, which
latter direct the rays seaward in the form of a sector of fixed red
light at a lower level in the tower. A somewhat similar arrangement
exists at Souter Point lighthouse.
The apparatus installed at the Lizard in 1903 is similar to that at St
Catherine's, but has no arrangement for producing a subsidiary sector
light. The flash is of .13 seconds duration every 3 seconds. The
apparatus replaced the two fixed electric lights erected in 1878.
[Illustration: FIG. 45.--Isle of May Apparatus.]
The Isle of May lighthouse, at the mouth of the Firth of Forth, was
first illuminated by electricity in 1886. The optical apparatus
consists of a second-order fixed-light lens with reflecting prisms,
and is surrounded by a revolving system of vertical condensing prisms
which split up the vertically condensed beam of light into 8 separate
beams of 3° in azimuth. The prisms are so arranged that the apparatus,
making one complete revolution in the minute, produces a group
characteristic of 4 flashes in quick succession every 30 seconds (fig.
45). The fixed light is not of the ordinary Fresnel section, the
refracting portion being confined to an angle of 10°, and the
remainder of the vertical section consisting of reflecting prisms.
In France the old south lighthouse at La Hève was lit by electricity
in 1863. This installation was followed in 1865 by a similar one at
the north lighthouse. In 1910 there were thirteen important coast
lights in France illuminated by electricity. In other parts of the
world, Macquarie lighthouse, Sydney, was lit by electricity in 1883;
Tino, in the gulf of Spezia, in 1885; and Navesink lighthouse, near
the entrance to New York Bay, in 1898. Electric apparatus were also
installed at the lighthouse at Port Said in 1869, on the opening of
the canal; Odessa in 1871; and at the Rothersand, North Sea, in 1885.
There are several other lights in various parts of the world
illuminated by this agency.
Incandescent electric lighting has been adopted for the illumination
of certain light-vessels in the United States, and a few small harbour
and port lights, beacons and buoys.
Table VI. gives particulars of some of the more important electric
lighthouses of the world.
_Electric Lighthouse Installations in France._--A list of the thirteen
lighthouses on the French coast equipped with electric light
installations will be found in table VI. It has been already mentioned
that the two lighthouses at La Hève were lit by electric light in 1863
and 1865. These installations were followed within a few years by the
establishment of electricity as illuminant at Gris-Nez. In 1882 M.
Allard, the then director-general of the French Lighthouse Service,
prepared a scheme for the electric lighting of the French littoral by
means of 46 lights distributed more or less uniformly along the
coast-line. All the apparatus were to be of the same general type, the
optics consisting of a fixed belt of 300 mm. focal distance, around
the outside of which revolved a system of 24 faces of vertical lenses.
These vertical panels condensed the belt of fixed light into beams of
3° amplitude in azimuth, producing flashes of about ¾ sec. duration.
To illuminate the near sea the vertical divergence of the lower prisms
of the fixed belt was artificially increased. These optics are very
similar to that in use at the Souter Point lighthouse, Sunderland. The
intensities obtained were 120,000 candles in the case of fixed lights
and 900,000 candles with flashing lights. As a result of a nautical
inquiry held in 1886, at which date the lights of Dunkerque, Calais,
Gris-Nez, La Canche, Baleines and Planier had been lighted, in
addition to the old apparatus at La Hève, it was decided to limit the
installation of electrical apparatus to important landfall lights--a
decision which the Trinity House had already arrived at in the case of
the English coast--and to establish new apparatus at six stations
only. These were Créac'h d'Ouessant (Ushant), Belle-Île, La Coubre at
the mouth of the river Gironde, Barfleur, Île d'Yeu and Penmarc'h. At
the same time it was determined to increase the powers of the existing
electric lights. The scheme as amended in 1886 was completed in
1902.[2]
All the electrically lit apparatus, in common with other optics
established in France since 1893, have been provided with mercury
rotation. The most recent electric lights have been constructed in the
form of twin apparatus, two complete and distinct optics being mounted
side by side upon the same revolving table and with corresponding
faces parallel. It is found that a far larger aggregate candle-power
is obtained from two lamps with 16 mm. to 23 mm. diameter carbons and
currents of 60 to 120 amperes than with carbons and currents of larger
dimensions in conjunction with single optics of greater focal
distance. A somewhat similar circumstance led to the choice of the
twin form for the two very powerful non-electric apparatus at Île
Vierge (figs. 43 and 43A) and Ailly, particulars of which will be seen
in table VII.
Several of the de Meritens magneto-electric machines of 5.5 K.W., laid
down many years ago at French electric lighthouse stations, are still
in use. All these machines have five induction coils, which, upon the
installation of the twin optics, were separated into two distinct
circuits, each consisting of 2½ coils. This modification has enabled
the old plants to be used with success under the altered conditions of
lighting entailed by the use of two lamps. The generators adopted in
the French service for use at the later stations differ materially
from the old type of de Meritens machine. The Phare d'Eckmühl
(Penmarc'h) installation serves as a type of the more modern
machinery. The dynamos are alternating current two-phase machines, and
are installed in duplicate. The two lamps are supplied with current
from the same machine, the second dynamo being held in reserve. The
speed is 810 to 820 revolutions per minute.
The lamp generally adopted is a combination of the Serrin and Berjot
principles, with certain modifications. Clockwork mechanism with a
regulating electromagnet moves the rods simultaneously and controls
the movements of the carbons so that they are displaced at the same
rate as they are consumed. It is usual to employ currents of varying
power with carbons of corresponding dimensions according to the
atmospheric conditions. In the French service two variations are used
in the case of twin apparatus produced by currents of 60 and 120
amperes at 45 volts with carbons 14 mm. and 18 mm. diameter, while in
single optic apparatus currents of 25, 50 and 100 amperes are utilized
with carbon of 11 mm., 16 mm. and 23 mm. diameter. In England fluted
carbons of larger diameter are employed with correspondingly increased
current. Alternating currents have given the most successful results
in all respects. Attempts to utilize continuous current for lighthouse
arc lights have, up to the present, met with little success.
The cost of a first-class electric lighthouse installation of the most
recent type in France, including optical apparatus, lantern, dynamos,
engines, air compressor, siren, &c., but not buildings, amounts
approximately to £5900.
_Efficiency of the Electric Light._--In 1883 the lighthouse
authorities of Great Britain determined that an exhaustive series of
experiments should be carried out at the South Foreland with a view to
ascertaining the relative suitability of electricity, gas and oil as
lighthouse illuminants. The experiments extended over a period of more
than twelve months, and were attended by representatives of the chief
lighthouse authorities of the world. The results of the trials tended
to show that the rays of oil and gas lights suffered to about equal
extent by atmospheric absorption, but that oil had the advantage over
gas by reason of its greater economy in cost of maintenance and in
initial outlay on installation. The electric light was found to suffer
to a much larger extent than either oil or gas light per unit of power
by atmospheric absorption, but the infinitely greater total intensity
of the beam obtainable by its use, both by reason of the high luminous
intensity of the electric arc and its focal compactness, more than
outweighed the higher percentage of loss in fog. The final conclusion
of the committee on the relative merits of electricity, gas or oil as
lighthouse illuminants is given in the following words: "That for
ordinary necessities of lighthouse illumination, mineral oil is the
most suitable and economical illuminant, and that for salient
headlands, important landfalls, and places where a very powerful light
is required electricity offers the greater advantages."
5. MISCELLANEOUS LIGHTHOUSE EQUIPMENT. _Lanterns._--Modern lighthouse
lanterns usually consist of a cast iron or steel pedestal, cylindrical
in plan, on which is erected the lantern glazing, surmounted by a
domed roof and ventilator (fig. 41). Adequate ventilation is of great
importance, and is provided by means of ventilators in the pedestal
and a large ventilating dome or cowl in the roof. The astragals
carrying the glazing are of wrought steel or gun-metal. The astragals
are frequently arranged helically or diagonally, thus causing a
minimum of obstruction to the light rays in any vertical section and
affording greater rigidity to the structure. The glazing is usually
¼-in. thick plate-glass curved to the radius of the lantern. In
situations of great exposure the thickness is increased. Lantern roofs
are of sheet steel or copper secured to steel or cast-iron rafter
frames. In certain instances it is found necessary to erect a grille
or network outside the lantern to prevent the numerous sea birds,
attracted by the light, from breaking the glazing by impact. Lanterns
vary in diameter from 5 ft. to 16 ft. or more, according to the size
of the optical apparatus. For first order apparatus a diameter of 12
ft. or 14 ft. is usual.
_Lightning Conductors._--The lantern and principal metallic structures
in a lighthouse are usually connected to a lightning conductor carried
either to a point below low water or terminating in an earth plate
embedded in wet ground. Conductors may be of copper tape or
copper-wire rope.
_Rotating Machinery._--Flashing-light apparatus are rotated by
clockwork mechanism actuated by weights. The clocks are fitted with
speed governors and electric warning apparatus to indicate variation
in speed and when rewinding is required. For occulting apparatus
either weight clocks or spring clocks are employed.
_Accommodation for Keepers, &c._--At rock and other isolated stations,
accommodation for the keepers is usually provided in the towers. In
the case of land lighthouses, dwellings are provided in close
proximity to the tower. The service or watch room should be situated
immediately under the lantern floor. Oil is usually stored in
galvanized steel tanks. A force pump is sometimes used for pumping oil
from the storage tanks to a service tank in the watch-room or lantern.
6. UNATTENDED LIGHTS AND BEACONS.--Until recent years no unattended
lights were in existence. The introduction of Pintsch's gas system in
the early 'seventies provided a means of illumination for beacons and
buoys of which large use has been made. Other illuminants are also in
use to a considerable extent.
_Unattended Electric Lights._--In 1884 an iron beacon lighted by an
incandescent lamp supplied with current from a secondary battery was
erected on a tidal rock near Cadiz. A 28-day clock was arranged for
eclipsing the light between sunrise and sunset and automatically
cutting off the current at intervals to produce an occulting
characteristic. Several small dioptric apparatus illuminated with
incandescent electric lamps have been made by the firm of Barbier
Bénard et Turenne of Paris, and supplied with current from batteries
of Daniell cells, with electric clockwork mechanism for occulting the
light. These apparatus have been fitted to beacons and buoys, and are
generally arranged to automatically switch off the current during the
day-time. They run unattended for periods up to two months. Two
separate lenses and lamps are usually provided, with lamp changer,
only one lamp being in circuit at a time. In the event of failure in
the upper lamp of the two the current automatically passes to the
lower lamp.
[Illustration: FIG. 46.--Garvel Beacon.]
_Oil-gas Beacons._--In 1881 a beacon automatically lighted by
Pintsch's compressed oil gas was erected on the river Clyde, and large
numbers of these structures have since been installed in all parts of
the world. The gas is contained in an iron or steel reservoir placed
within the beacon structure, refilled by means of a flexible hose on
the occasions of the periodical visits of the tender. The beacons,
which remain illuminated for periods up to three months are charged to
7 atmospheres. Many lights are provided with occulting apparatus
actuated by the gas passing from the reservoir to the burner
automatically cutting off and turning on the supply. The Garvel beacon
(1899) on the Clyde is shown in fig. 46. The burner has 7 jets, and
the light is occulting. Since 1907 incandescent mantle burners for oil
gas have been largely used for beacon illumination, both for fixed and
occulting lights.
Acetylene has also been used for the illumination of beacons and other
unattended lights.
_Lindberg Lights._--In 1881-1882 several beacons lighted automatically
by volatile petroleum spirit on the Lindberg-Lyth and Lindberg-Trotter
systems were established in Sweden. Many lights of this type have
subsequently been placed in different parts of the world. The volatile
spirit lamp burns day and night. Occultations are produced by a screen
or series of screens rotated round the light by the ascending current
of heated air and gases from the lamp acting upon a horizontal fan.
The speed of rotation of the fan cannot be accurately adjusted, and
the times of occultation therefore are liable to slight variation. The
lights run unattended for periods up to twenty-one days.
_Benson-Lee Lamps._--An improvement upon the foregoing is the
Benson-Lee lamp, in which a similar occulting arrangement is often
used, but the illuminant is paraffin consumed in a special burner
having carbon-tipped wicks which require no trimming. The flame
intensity of the light is greater than that of the burner consuming
light spirit. The introduction of paraffin also avoids the danger
attending the use of the more volatile spirit. Many of these lights
are in use on the Scottish coast. They are also used in other parts of
the United Kingdom, and in the United States, Canada and other
countries.
_Permanent Wick Lights._--About 1891 the French Lighthouse Service
introduced petroleum lamps consuming ordinary high-flash lighthouse
oil, and burning without attention for periods of several months. The
burners are of special construction, provided with a very thick wick
which is in the first instance treated in such a manner as to cause
the formation of a deposit of carbonized tar on its exposed upper
surface. This crust prevents further charring of the wick after
ignition, the oil becoming vaporized from the under side of the crust.
Many fixed, occulting and flashing lights fitted with these burners
are established in France and other countries. In the case of the
occulting types a revolving screen is placed around the burner and
carried upon a miniature mercury float. The rotation is effected by
means of a small Gramme motor on a vertical axis, fitted with a speed
governor, and supplied with current from a battery of primary cells.
The oil reservoir is placed in the upper part of the lantern and
connected with the burner by a tube, to which is fitted a constant
level regulator for maintaining the burning level of the oil at a
fixed height. In the flashing or revolving light types the arrangement
is generally similar, the lenses being revolved upon a mercury float
which is rotated by the electric motor. The flashing apparatus
established at St Marcouf in 1901 has a beam intensity of 1000
candle-power, and is capable of running unattended for three months.
The electric current employed for rotating the apparatus is supplied
by four Lalande and Chaperon primary cells, coupled in series, each
giving about 0.15 ampere at a voltage of 0.65. The power required to
work the apparatus is at the maximum about 0.165 ampere at 0.75 volt,
the large surplus of power which is provided for the sake of safety
being absorbed by a brake or governor connected with the motor.
_Wigham Beacon Lights._--Wigham introduced an oil lamp for beacon and
buoy purposes consisting of a vertical container filled with ordinary
mineral oil or paraffin, and carrying a roller immediately under the
burner case over which a long flat wick passes. One end of the wick is
attached to a float which falls in the container as the oil is
consumed, automatically drawing a fresh portion of the wick over the
roller. The other end of the wick is attached to a free counterweight
which serves to keep it stretched. The oil burns from the convex
surface of the wick as it passes over the roller, a fresh portion
being constantly passed under the action of the flame. The light is
capable of burning without attention for thirty days. These lights are
also fitted with occulting screens on the Lindberg system. The
candle-power of the flame is small.
7. LIGHT-VESSELS.--The earliest light-vessel placed in English waters
was that at the Nore in 1732. The early light-ships were of small size
and carried lanterns of primitive construction and small size
suspended from the yard-arms. Modern light-vessels are of steel, wood
or composite construction. Steel is now generally employed in new
ships. The wood and composite ships are sheathed with Muntz metal. The
dimensions of English light-vessels vary. The following may be taken
as the usual limits:
Length 80 ft. to 114 ft.
Beam 20 ft. to 24 ft.
Depth moulded 13 ft. to 15 ft. 6 in.
Tonnage 155 to 280.
The larger vessels are employed at outside and exposed stations, the
smaller ships being stationed in sheltered positions and in estuaries.
The moorings usually consist of 3-ton mushroom anchors and 1(5/8) open
link cables. The lanterns in common use are 8 ft. in diameter,
circular in form, with glazing 4 ft. in height. They are annular in
plan, surrounding the mast of the vessel upon which they are hoisted
for illumination, and are lowered to the deck level during the day.
Fixed lanterns mounted on hollow steel masts are now being used in
many services, and are gradually displacing the older type. The first
English light-vessel so equipped was constructed in 1904. Of the 87
light-vessels in British waters, including unattended light-vessels,
eleven are in Ireland and six in Scotland. At the present time there
are over 750 light-vessels in service throughout the world.
Until about 1895 the illuminating apparatus used in light-vessels was
exclusively of catoptric form, usually consisting of 21 in. or 24 in.
silvered parabolic reflectors, having 1, 2 or 3-wick mineral oil
burners in focus. The reflectors and lamps are hung in gimbals to
preserve the horizontal direction of the beams.
The following table gives the intensity of beam obtained by means of a
type of reflector in general use:
_21-in. Trinity House Parabolic Reflector_
Service Intensity
of Beam.
Burners 1 wick "Douglass" 2715 candles
" 2 " (Catoptric) 4004 "
" 2 " (Dioptric) 6722 "
" 3 " 7528 "
In revolving flashing lights two or more reflectors are arranged in
parallel in each face. Three, four or more faces or groups of
reflectors are arranged around the lantern in which they revolve, and
are carried upon a turn-table rotated by clockwork. The intensity of
the flashing beam is therefore equivalent to the combined intensities
of the beams emitted by the several reflectors in each face. The first
light-vessel with revolving light was placed at the Swin Middle at the
entrance to the Thames in 1837. Group-flashing characteristics can be
produced by special arrangements of the reflectors. Dioptric apparatus
is now being introduced in many new vessels, the first to be so fitted
in England being that stationed at the Swin Middle in 1905, the
apparatus of which is gas illuminated and gives a flash of 25,000
candle-power.
Fog signals, when provided on board light-vessels are generally in the
form of reed-horns or sirens, worked by compressed air. The
compressors are driven from steam or oil engines. The cost of a modern
type of English light-vessel, with power-driven compressed air siren,
is approximately £16,000.
In the United States service, the more recently constructed vessels
have a displacement of 600 tons, each costing £18,000. They are
provided with self-propelling power and steam whistle fog signals. The
illuminating apparatus is usually in the form of small dioptric lens
lanterns suspended at the mast-head--3 or more to each mast, but a few
of the ships, built since 1907, are provided with fourth-order
revolving dioptric lights in fixed lanterns. There are 53
light-vessels in service on the coasts of the United States with 13
reserve ships.
_Electrical Illumination._--An experimental installation of the
electric light placed on board a Mersey light-vessel in 1886 by the
Mersey Docks and Harbour Board proved unsuccessful. The United States
Lighthouse Board in 1892 constructed a light-vessel provided with a
powerful electric light, and moored her on the Cornfield Point station
in Long Island Sound. This vessel was subsequently placed off Sandy
Hook (1894) and transferred to the Ambrose Channel Station in 1907.
Five other light-vessels in the United States have since been provided
with incandescent electric lights--either with fixed or occulting
characteristics--including Nantucket Shoals (1896), Fire Island
(1897), Diamond Shoals (1898), Overfalls Shoal (1901) and San
Francisco (1902).
_Gas Illumination._--In 1896 the French Lighthouse Service completed
the construction of a steel light-vessel (Talais), which was
ultimately placed at the mouth of the Gironde. The construction of
this vessel was the outcome of experiments carried out with a view to
produce an efficient light-vessel at moderate cost, lit by a dioptric
flashing light with incandescent oil-gas burner. The construction of
the Talais was followed by that of a second and larger vessel, the
Snouw, on similar lines, having a length of 65 ft. 6 in., beam 20 ft.
and a draught of 12 ft., with a displacement of 130 tons. The cost of
this vessel complete with optical apparatus and gasholders, with
accommodation for three men, was approximately £5000. The vessel was
built in 1898-1899.[3] A third vessel was constructed in 1901-1902 for
the Sandettié Bank on the general lines adopted for the preceding
examples of her class, but of the following increased dimensions:
length 115 ft.; width at water-line 20 ft. 6 in.; and draught 15 ft.,
with a displacement of 342 tons (fig. 47). Accommodation is provided
for a crew of eight men. The optical apparatus (fig. 48) is dioptric,
consisting of 4 panels of 250 mm. focal distance, carried upon a
"Cardan" joint below the lens table, and counter-balanced by a heavy
pendulum weight. The apparatus is revolved by clockwork and
illuminated by compressed oil gas with incandescent mantle. The
candle-power of the beam is 35,000. The gas is contained in three
reservoirs placed in the hold. The apparatus is contained in a 6-ft.
lantern constructed at the head of a tubular mast 2 ft. 6 in.
diameter. A powerful siren is provided with steam engine and boiler
for working the air compressors. The total cost of the vessel,
including fog signal and optical apparatus, was £13,600. A vessel of
similar construction to the Talais was placed by the Trinity House in
1905 on the Swin Middle station. The illuminant is oil gas. Gas
illuminated light-vessels have also been constructed for the German
and Chinese Lighthouse Service.
_Unattended Light-vessels._--In 1881 an unattended light-vessel,
illuminated with Pintsch's oil gas, was constructed for the Clyde, and
is still in use at the Garvel Point. The light is occulting, and is
shown from a dioptric lens fitted at the head of a braced iron lattice
tower 30 ft. above water-level. The vessel is of iron, 40 ft. long, 12
ft. beam and 8 ft. deep, and has a storeholder on board containing oil
gas under a pressure of six atmospheres capable of maintaining a light
for three months. A similar vessel is placed off Calshot Spit in
Southampton Water, and several have been constructed for the French
and other Lighthouse Services. The French boats are provided with deep
main and bilge keels similar to those adopted in the larger gas
illuminated vessels. In 1901 a light-vessel 60 ft. in length was
placed off the Otter Rock on the west coast of Scotland; it is
constructed of steel, 24 ft. beam, 12 ft. deep and draws 9 ft. of
water (fig. 49). The focal plane is elevated 25 ft. above the
water-line, and the lantern is 6 ft. in diameter. The optical
apparatus is of 500 mm. focal distance and hung in gimbals with a
pendulum balance and "Cardan" joint as in the Sandettié light-vessel.
The illuminant is oil gas, with an occulting characteristic. The
storeholder contains 10,500 cub. ft. of gas at eight atmospheres,
sufficient to supply the light for ninety days and nights. A bell is
provided, struck by clappers moved by the roll of the vessel. The cost
of the vessel complete was £2979. The Northern Lighthouse
Commissioners have four similar vessels in service, and others have
been stationed in the Hugli estuary, at Bombay, off the Chinese coasts
and elsewhere. In 1909 an unattended gas illuminated light-vessel
provided with a dioptric flashing apparatus was placed at the Lune
Deep in Morecambe Bay. It is also fitted with a fog bell struck
automatically by a gas operated mechanism.
[Illustration: FIG. 47.--Sandettié Lightship.]
[Illustration: FIG. 48.--Lantern of Sandettié Lightship.]
_Electrical Communication of Light-vessels with the
Shore._--Experiments were instituted in 1886 at the Sunk light-vessel
off the Essex coast with the view to maintaining telephonic
communication with the shore by means of a submarine cable 9 m. in
length. Great difficulties were experienced in maintaining
communication during stormy weather, breakages in the cable being
frequent. These difficulties were subsequently partially overcome by
the employment of larger vessels and special moorings. Wireless
telegraphic installations have now (1910) superseded the cable
communications with light-vessels in English waters except in four
cases. Seven light-vessels, including the four off the Goodwin Sands,
are now fitted for wireless electrical communication with the shore.
In addition many pile lighthouses and isolated rock and island
stations have been placed in electrical communication with the shore
by means of cables or wireless telegraphy. The Fastnet lighthouse was,
in 1894, electrically connected with the shore by means of a
non-continuous cable, it being found impossible to maintain a
continuous cable in shallow water near the rock owing to the heavy
wash of the sea. A copper conductor, carried down from the tower to
below low-water mark, was separated from the cable proper, laid on the
bed of the sea in a depth of 13 fathoms, by a distance of about 100
ft. The lighthouse was similarly connected to earth on the opposite
side of the rock. The conductor terminated in a large copper plate,
and to the cable end was attached a copper mushroom. Weak currents
were induced in the lighthouse conductor by the main current in the
cable, and messages received in the tower by the help of electrical
relays. On the completion of the new tower on the Fastnet Rock in 1906
this installation was superseded by a wireless telegraphic
installation.
8. DISTRIBUTION AND DISTINCTION OF LIGHTS, &c.--_Methods of
Distinction._--The following are the various light characteristics which
may be exhibited to the mariner:--
_Fixed._--Showing a continuous or steady light. Seldom used in modern
lighthouses and generally restricted to small port or harbour lights. A
fixed light is liable to be confused with lights of shipping or other
shore lights.
_Flashing._[4]--Showing a single flash, the duration of darkness always
being greater than that of light. This characteristic or that
immediately following is generally adopted for important lights. The
French authorities have given the name _Feux-Eclair_ to flashing lights
of short duration.
_Group-Flashing._--Showing groups of two or more flashes in quick
succession (not necessarily of the same colour) separated by eclipses
with a larger interval of darkness between the groups.
_Fixed and Flashing._--Fixed light varied by a single white or coloured
flash, which may be preceded and followed by a short eclipse. This type
of light, in consequence of the unequal intensities of the beams, is
unreliable, and examples are now seldom installed although many are
still in service.
_Fixed and Group-Flashing._--Similar to the preceding and open to the
same objections.
_Revolving._--This term is still retained in the "Lists of Lights"
issued by the Admiralty and some other authorities to denote a light
gradually increasing to full effect, then decreasing to eclipse. At
short distances and in clear weather a faint continuous light may be
observed. There is no essential difference between revolving and
flashing lights, the distinction being merely due to the speed of
rotation, and the term might well be abandoned as in the United States
lighthouse list.
_Occulting._--A continuous light with, at regular intervals, one sudden
and total eclipse, the duration of light always being equal to or
greater than that of darkness. This characteristic is usually exhibited
by fixed dioptric apparatus fitted with some form of occulting
mechanism. Many lights formerly of fixed characteristic have been
converted to occulting.
_Group Occulting._--A continuous light with, at regular intervals,
groups of two or more sudden and total eclipses.
_Alternating._--Lights of different colours (generally red and white)
alternately without any intervening eclipse. This characteristic is not
to be recommended for reasons which have already been referred to. Many
of the permanent and unwatched lights on the coasts of Norway and Sweden
are of this description.
_Colour._--The colours usually adopted for lights are white, red and
green. White is to be preferred whenever possible, owing to the great
absorption of light by the use of red or green glass screens.
[Illustration: FIG. 49.--Otter Rock Light-vessel.]
_Sectors._--Coloured lights are often requisite to distinguish cuts or
sectors, and should be shown from fixed or occulting light apparatus and
not from flashing apparatus. In marking the passage through a channel,
or between sandbanks or other dangers, coloured light sectors are
arranged to cover the dangers, white light being shown over the fairway
with sufficient margin of safety between the edges of the coloured
sectors next the fairway and the dangers.
_Choice of Characteristic and Description of Apparatus._--In
determining the choice of characteristic for a light due regard must
be paid to existing lights in the vicinity. No light should be placed
on a coast line having a characteristic the same as, or similar to,
another in its neighbourhood unless one or more lights of dissimilar
characteristic, and at least as high power and range, intervene. In
the case of "landfall lights" the characteristic should differ from
any other within a range of 100 m. In narrow seas the distance between
lights of similar characteristic may be less. Landfall lights are, in
a sense, the most important of all and the most powerful apparatus
available should be installed at such stations. The distinctive
characteristic of a light should be such that it may be readily
determined by a mariner without the necessity of accurately timing the
period or duration of flashes. For landfall and other important coast
stations flashing dioptric apparatus of the first order (920 mm. focal
distance) with powerful burners are required. In countries where the
atmosphere is generally clear and fogs are less prevalent than on the
coasts of the United Kingdom, second or third order lights suffice for
landfalls having regard to the high intensities available by the use
of improved illuminants. Secondary coast lights may be of second,
third or fourth order of flashing character, and important harbour
lights of third or fourth order. Less important harbours and places
where considerable range is not required, as in estuaries and narrow
seas, may be lighted by flashing lights of fourth order or smaller
size. Where sectors are requisite, occulting apparatus should be
adopted for the main light; or subsidiary lights, fixed or occulting,
may be exhibited from the same tower as the main light but at a lower
level. In such cases the vertical distance between the high and the
low light must be sufficient to avoid commingling of the two beams at
any range at which both lights are visible. Such commingling or
blending is due to atmospheric aberration.
_Range of Lights._--The range of a light depends first on its
elevation above sea-level and secondly on its intensity. Most
important lights are of sufficient power to render them visible at the
full geographical range in clear weather. On the other hand there are
many harbour and other lights which do not meet this condition.
The distances given in lists of lights from which lights are
visible--except in the cases of lights of low power for the reason
given above--are usually calculated in nautical miles as seen from a
height of 15 ft. above sea-level, the elevation of the lights being
taken as above high water. Under certain atmospheric conditions, and
especially with the more powerful lights, the glare of the light may
be visible considerably beyond the calculated range.
TABLE III.--_Distances at which Objects can be seen at Sea,
according to their Respective Elevations and the Elevation of the
Eye of the Observer._ (A. Stevenson.)
+--------+------------+---------------------+
| |Distances in| |Distances in|
|Heights |Geographical|Heights |Geographical|
|in Feet.|or Nautical |in Feet.|or Nautical |
| | Miles. | | Miles. |
+--------+------------+--------+------------+
| 5 | 2.565 | 110 | 12.03 |
| 10 | 3.628 | 120 | 12.56 |
| 15 | 4.443 | 130 | 13.08 |
| 20 | 5.130 | 140 | 13.57 |
| 25 | 5.736 | 150 | 14.02 |
| 30 | 6.283 | 200 | 16.22 |
| 35 | 6.787 | 250 | 18.14 |
| 40 | 7.255 | 300 | 19.87 |
| 45 | 7.696 | 350 | 21.46 |
| 50 | 8.112 | 400 | 22.94 |
| 55 | 8.509 | 450 | 24.33 |
| 60 | 8.886 | 500 | 25.65 |
| 65 | 9.249 | 550 | 26.90 |
| 70 | 9.598 | 600 | 28.10 |
| 75 | 9.935 | 650 | 29.25 |
| 80 | 10.26 | 700 | 30.28 |
| 85 | 10.57 | 800 | 32.45 |
| 90 | 10.88 | 900 | 34.54 |
| 95 | 11.18 | 1000 | 36.28 |
| 100 | 11.47 | | |
+--------+------------+--------+------------+
EXAMPLE: A tower 200 ft. high will be visible 20.66 nautical miles
to an observer, whose eye is elevated 15 ft. above the water; thus,
from the table:
15 ft. elevation, distance visible 4.44 nautical miles
200 " " 16.22 "
-----
20.66 "
_Elevation of Lights._--The elevation of the light above sea-level
need not, in the case of landfall lights, exceed 200 ft., which is
sufficient to give a range of over 20 nautical miles. One hundred and
fifty feet is usually sufficient for coast lights. Lights placed on
high headlands are liable to be enveloped in banks of fog at times
when at a lower level the atmosphere is comparatively clear (e.g.
Beachy Head). No definite rule can, however, be laid down, and local
circumstances, such as configuration of the coast line, must be taken
into consideration in every case.
_Choice of Site._--"Landfall" stations should receive first
consideration and the choice of location for such a light ought never
to be made subservient to the lighting of the approaches to a port.
Subsidiary lights are available for the latter purpose. Lights
installed to guard shoals, reefs or other dangers should, when
practicable, be placed seaward of the danger itself, as it is
desirable that seamen should be able to "make" the light with
confidence. Sectors marking dangers seaward of the light should not
be employed except when the danger is in the near vicinity of the
light. Outlying dangers require marking by a light placed on the
danger or by a floating light in its vicinity.
[Illustration: FIG. 50.--Spar Gas Buoy.]
9. ILLUMINATED BUOYS.--_Gas Buoys._ Pintsch's oil gas has been in use
for the illumination of buoys since 1878. In 1883 an automatic
occulter was perfected, worked by the gas passing from the reservoir
to the burner. The lights placed on these buoys burn continuously for
three or more months. The buoys and lanterns are made in various forms
and sizes. The spar buoy (fig. 50) may be adopted for situations where
strong tides or currents prevail. Oil gas lights are frequently fitted
to Courtenay whistling (fig. 51) and bell buoys.
In the ordinary type of gas buoy lantern the burner employed is of the
multiple-jet, Argand ring, or incandescent type. Incandescent mantles
have been applied to buoy lights in France with successful results.
Since 1906, and more recently the same system of illumination has been
adopted in England and other countries. The lenses employed are of
cylindrical dioptric fixed-light form, usually 100 mm. to 300 mm.
diameter. Some of the largest types of gas-buoy in use on the French
coast have an elevation from water level to the focal plane of over 26
ft. with a beam intensity of more than 1000 candles. A large gas-buoy
with an elevation of 34 ft. to the focal plane was placed at the
entrance to the Gironde in 1907. It has an incandescent burner and
exhibits a light of over 1500 candles. Oil gas forms the most
trustworthy and efficient illuminant for buoy purposes yet introduced,
and the system has been largely adopted by lighthouse and harbour
authorities.
There are now over 2000 buoys fitted with oil gas apparatus, in
addition to 600 beacons, light-vessels and boats.
_Electric Lit Buoys._--Buoys have been fitted with electric light,
both fixed and occulting. Six electrically lit spar-buoys were laid
down in the Gedney channel, New York lower bay, in 1888. These were
illuminated by 100 candle-power Swan lamps with continuous current
supplied by cable from a power station on shore. The wear and tear of
the cables caused considerable trouble and expense. In 1895
alternating current was introduced. The installation was superseded by
gas lit buoys in 1904.
_Acetylene and Oil Lighted Buoys._--Acetylene has been extensively
employed for the lighting of buoys in Canada and in the United States;
to a less extent it has also been adopted in other countries. Both the
low pressure system, by which the acetylene gas is produced by an
automatic generator, and the so-called high pressure system in which
purified acetylene is held in solution in a high pressure gasholder
filled with asbestos composition saturated with acetone, have been
employed for illuminating buoys and beacons. Wigham oil lamps are also
used to a limited extent for buoy lighting.
_Bell Buoys._--One form of clapper actuated by the roll of the buoy
(shown in fig. 52) consists of a hardened steel ball placed in a
horizontal phosphor-bronze cylinder provided with rubber buffers.
Three of these cylinders are arranged around the mouth of the fixed
bell, which is struck by the balls rolling backwards and forwards as
the buoy moves. Another form of bell mechanism consists of a fixed
bell with three or more suspended clappers placed externally which
strike the bell when the buoy rolls.
[Illustration: FIG. 51.--Courtenay's Automatic Whistling Buoy.
A, Cylinder, 27 ft. 6 in. long.
B, Mooring shackle.
C, Rudder.
D, Buoy.
E, Diaphragm.
F, Ball valves.
G, Air inlet tubes.
H, Air (compressed outlet tube to whistle).
I, Compressed air inlet to buoy.
K, Manhole.
L, Steps.
N, Whistle.]
10. FOG SIGNALS.--The introduction of coast fog signals is of
comparatively recent date. They were, until the middle of the 19th
century, practically unknown except so far as a few isolated bells and
guns were concerned. The increasing demands of navigation, and the
application of steam power to the propulsion of ships resulting in an
increase of their speed, drew attention to the necessity of providing
suitable signals as aids to navigation during fog and mist. In times
of fog the mariner can expect no certain assistance from even the
most efficient system of coast lighting, since the beams of light from
the most powerful electric lighthouse are frequently entirely
dispersed and absorbed by the particles of moisture, forming a sea fog
of even moderate density, at a distance of less than a ¼ m. from the
shore. The careful experiments and scientific research which have been
devoted to the subject of coast fog-signalling have produced much that
is useful and valuable to the mariner, but unfortunately the practical
results so far have not been so satisfactory as might be desired,
owing to (1) the very short range of the most powerful signals yet
produced under certain unfavourable acoustic conditions of the
atmosphere, (2) the difficulty experienced by the mariner in judging
at any time how far the atmospheric conditions are against him in
listening for the expected signal, and (3) the difficulty in locating
the position of a sound signal by phonic observations.
[Illustration: FIG. 52.--Buoy Bell.]
_Bells and Gongs_ are the oldest and, generally speaking, the least
efficient forms of fog signals. Under very favourable acoustic
conditions the sounds are audible at considerable ranges. On the other
hand, 2-ton bells have been inaudible at distances of a few hundred
yards. The 1893 United States trials showed that a bell weighing 4000
lb. struck by a 450 lb. hammer was heard at a distance of 14 m. across
a gentle breeze and at over 9 m. against a 10-knot breeze. Bells are
frequently used for beacon and buoy signals, and in some cases at
isolated rock and other stations where there is insufficient
accommodation for sirens and horns, but their use is being gradually
discontinued in this country for situations where a powerful signal
is required. Gongs, usually of Chinese manufacture, were formerly in
use on board English light-ships and are still used to some extent
abroad. These are being superseded by more powerful sound instruments.
_Explosive Signals._--Guns were long used at many lighthouse and
light-vessel stations in England, and are still in use in Ireland and
at some foreign stations. These are being gradually displaced by other
explosive or compressed air signals. No explosive signals are in use
on the coasts of the United States. In 1878 sound rockets charged with
gun-cotton were first used at Flamborough Head and were afterwards
supplied to many other stations.[5] The nitrated gun-cotton or tonite
signals now in general use are made up in 4 oz. charges. These are
hung at the end of an iron jib or pole attached to the lighthouse
lantern or other structure, and fired by means of a detonator and
electric battery. The discharge may take place within 12 ft. of a
structure without danger. The cartridges are stored for a considerable
period without deterioration and with safety. This form of signal is
now very generally adopted for rock and other stations in Great
Britain, Canada, Newfoundland, northern Europe and other parts of the
world. An example will be noticed in the illustration of the Bishop
Rock lighthouse, attached to the lantern (fig. 13). Automatic hoisting
and firing appliances are also in use.
_Whistles._--Whistles, whether sounded by air or steam, are not used
in Great Britain, except in two instances of harbour signals under
local control. It has been objected that their sound has too great a
resemblance to steamers' whistles, and they are wasteful of power. In
the United States and Canada they are largely used. The whistle
usually employed consists of a metallic dome or bell against which the
high-pressure steam impinges. Rapid vibrations are set up both in the
metal of the bell and in the internal air, producing a shrill note.
The Courtenay buoy whistle, already referred to, is an American
invention and finds favour in the United States, France, Germany and
elsewhere.
_Reed-Horns._--These instruments in their original form were the
invention of C. L. Daboll, an experimental horn of his manufacture
being tried in 1851 by the United States Lighthouse Board. In 1862 the
Trinity House adopted the instrument for seven land and light-vessel
stations. For compressing air for the reed-horns as well as sirens,
caloric, steam, gas and oil engines have been variously used,
according to local circumstances. The reed-horn was improved by
Professor Holmes, and many examples from his designs are now in use in
England and America. At the Trinity House experiments with fog signals
at St Catherine's (1901) several types of reed-horn were experimented
with. The Trinity House service horn uses air at 15 lb. pressure with
a consumption of .67 cub. ft. per second and 397 vibrations. A small
manual horn of the Trinity House type consumes .67 cub. ft. of air at
5 lb. pressure. The trumpets of the latter are of brass.
_Sirens._--The most powerful and efficient of all compressed air fog
signals is the siren. The principle of this instrument may be briefly
explained as follows:--It is well known that if the tympanic membrane
is struck periodically and with sufficient rapidity by air impulses or
waves a musical sound is produced. Robinson was the first to construct
an instrument by which successive puffs of air under pressure were
ejected from the mouth of a pipe. He obtained this effect by using a
stop-cock revolving at high speed in such a manner that 720 pulsations
per second were produced by the intermittent escape of air through the
valves or ports, a smooth musical note being given. Cagniard de la
Tour first gave such an instrument the name of siren, and constructed
it in the form of an air chamber with perforated lid or cover, the
perforations being successively closed and opened by means of a
similarly perforated disk fitted to the cover and revolving at high
speed. The perforations being cut at an angle, the disk was
self-rotated by the oblique pressure of the air in escaping through
the slots. H. W. Dove and Helmholtz introduced many improvements, and
Brown of New York patented, about 1870, a steam siren with two disks
having radial perforations or slots. The cylindrical form of the siren
now generally adopted is due to Slight, who used two concentric
cylinders, one revolving within the other, the sides being perforated
with vertical slots. To him is also due the centrifugal governor
largely used to regulate the speed of rotation of the siren. Over the
siren mouth is placed a conical trumpet to collect and direct the
sound in the desired direction. In the English service these trumpets
are generally of considerable length and placed vertically, with bent
top and bell mouth. Those at St Catherine's are of cast-iron with
copper bell mouth, and have a total axial length of 22 ft. They are 5
in. in diameter at the siren mouth, the bell mouth being 6 ft. in
diameter. At St Catherine's the sirens are two in number, 5 in. in
diameter, being sounded simultaneously and in unison (fig. 53). Each
siren is provided with ports for producing a high note as well as a
low note, the two notes being sounded in quick succession once every
minute. The trumpet mouths are separated by an angle of 120° between
their axes. This double form has been adopted in certain instances
where the angle desired to be covered by the sound is comparatively
wide. In Scotland the cylindrical form is used generally, either
automatically or motor driven. By the latter means the admission of
air to the siren can be delayed until the cylinder is rotating at full
speed, and a much sharper sound is produced than in the case of the
automatic type. The Scottish trumpets are frequently constructed so
that the greater portion of the length is horizontal. The Girdleness
trumpet has an axial length of 16 ft., 11 ft. 6 in. being horizontal.
The trumpet is capable of being rotated through an angle as well as
dipped below the horizon. It is of cast-iron, no bell mouth is used,
and the conical mouth is 4 ft. in diameter. In France the sirens are
cylindrical and very similar to the English self-driven type. The
trumpets have a short axial length, 4 ft. 6 in., and are of brass,
with bent bell mouth. The Trinity House has in recent years
reintroduced the use of disk sirens, with which experiments are still
being carried out both in the United Kingdom and abroad. For
light-vessels and rock stations where it is desired to distribute the
sound equally in all directions the mushroom-head trumpet is
occasionally used. The Casquets trumpet of this type is 22 ft. in
length, of cast-iron, with a mushroom top 6 ft. in diameter. In cases
where neither the mushroom trumpet nor the twin siren is used the
single bent trumpet is arranged to rotate through a considerable
angle. Table IV. gives particulars of a few typical sirens of the most
recent form.
[Illustration: FIG. 53.--St Catherine's Double-noted Siren.]