TABLE IV._Encyclopaedia Britannica, 11th Edition, "Lightfoot, Joseph" to "Liquidation" Volume 16, Slice 6

+-----------------------+----------------------+----------+---------+----------------+--------------------+

+-----------------------+----------------------+-----+----+---------+-------+--------+--------------------+

| | |High.|Low.| | High. | Low. | |

|St Catherine's (Trinity|Two 5-in. cylindrical,| 295 | 182| 25 | 32 | 16 |The air consumption |

|Girdleness (N.L.C) |7-in. cylindrical | 234 | 100| 30 | 130 | 26 | |

|Casquets (Trinity |7-in. disk siren, | .. | 98| 25 | .. | 36 | |

|French pattern siren |6-in. cylindrical | 326 | .. | 28 | 14 | .. |A uniform note of |

+-----------------------+----------------------+-----+----+---------+-------+--------+--------------------+

Since the first trial of the siren at the South Foreland in 1873 a

very large number of these instruments have been established both at

lighthouse stations and on board light-vessels. In all cases in Great

Britain and France they are now supplied with air compressed by steam

or other mechanical power. In the United States and some other

countries steam, as well as compressed air, sirens are in use.

_Diaphones._--The diaphone is a modification of the siren, which has

been largely used in Canada since 1903 in place of the siren. It is

claimed that the instrument emits a note of more constant pitch than

does the siren. The distinction between the two instruments is that in

the siren a revolving drum or disk alternately opens and closes

elongated air apertures, while in the diaphone a piston pulsating at

high velocity serves to alternately cover and uncover air slots in a

cylinder.

_The St Catherine's Experiments._--Extensive trials were carried out

during 1901 by the Trinity House at St Catherine's lighthouse, Isle of

Wight, with several types of sirens and reed-horns. Experiments were

also made with different pattern of trumpets, including forms having

elliptical sections, the long axis being placed vertically. The

conclusions of the committee may be briefly summarized as follows: (1)

When a large arc requires to be guarded two fixed trumpets suitably

placed are more effective than one large trumpet capable of being

rotated. (2) When the arc to be guarded is larger than that

effectively covered by two trumpets, the mushroom-head trumpet is a

satisfactory instrument for the purpose. (3) A siren rotated by a

separate motor yields better results than when self-driven. (4) No

advantage commensurate with the additional power required is obtained

by the use of air at a higher pressure than 25 lb. per sq. in. (5) The

number of vibrations per second produced by the siren or reed should

be in unison with the proper note of the associated trumpet. (6) When

two notes of different pitch are employed the difference between these

should, if possible, be an octave. (7) For calm weather a low note is

more suitable than a high note, but when sounding against the wind and

with a rough and noisy sea a high note has the greater range. (8) From

causes which cannot be determined at the time or predicted beforehand,

areas sometimes exist in which the sounds of fog signals may be

greatly enfeebled or even lost altogether. This effect was more

frequently observed during comparatively calm weather and at no great

distance from the signal station. (It has often been observed that the

sound of a signal may be entirely lost within a short distance of the

source, while heard distinctly at a greater distance and at the same

time.) (9) The siren was the most effective signal experimented with;

the reed-horn, although inferior in power, is suitable for situations

of secondary importance. (No explosive signals were under trial during

the experiments.) (10) A fog signal, owing to the uncertainty

attending its audibility, must be regarded only as an auxiliary aid to

navigation which cannot at all times be relied upon.

_Submarine Bell Signals._--As early as 1841 J. D. Colladon conducted

experiments on the lake of Geneva to test the suitability of water as

a medium for transmission of sound signals and was able to convey

distinctly audible sounds through water for a distance of over 21 m.,

but it was not until 1904 that any successful practical application of

this means of signalling was made in connexion with light-vessels.

There are at present (1910) over 120 submarine bells in service,

principally in connexion with light-vessels, off the coasts of the

United Kingdom, United States, Canada, Germany, France and other

countries. These bells are struck by clappers actuated by pneumatic or

electrical mechanism. Other submerged bells have been fitted to buoys

and beacon structures, or placed on the sea bed; in the former case

the bell is actuated by the motion of the buoy and in others by

electric current, transmitted by cable from the shore. In some cases,

when submarine bells are associated with gas buoys or beacons, the

compressed gas is employed to actuate the bell striking mechanism. To

take full advantage of the signals thus provided it is necessary for

ships approaching them to be fitted with special receiving mechanism

of telephonic character installed below the water line and in contact

with the hull plating. The signals are audible by the aid of ear

pieces similar to ordinary telephone receivers. Not only can the bell

signals be heard at considerable distances--frequently over 10 m.--and

in all conditions of weather, but the direction of the bell in

reference to the moving ship can be determined within narrow limits.

The system is likely to be widely extended and many merchant vessels

and war ships have been fitted with signal receiving mechanism.

The following table (V.) gives the total numbers of fog signals of

each class in use on the 1st of January 1910 in certain countries.

Encyclopaedia Britannica, 11th Edition, "Lightfoot, Joseph" to "Liquidation" Volume 16, Slice 6 · 第 17 / 50 题