States

STATES.) One or other of them is usually employed when first class signals are required and space is available for the accom modation of the necessary plant. Both are compressed-air in struments, but differ somewhat in operation. The diaphone, in its modern form the more powerful instrument, was invented in Canada by J. P. Northey, about 1903, and consists of a piston reciprocating in a cylinder, around both of which are cut circum ferential slots or ports. The piston is fitted with an operating head to which air is admitted, first on one side and then on the other, for giving to it the reciprocating motion. As the slots in the two units pass and repass one another, air is being admitted through them to produce the impulses or sound waves, and upon the number of times these ports open and close each second de 'The Flamborough Head rocket was superseded by a siren fog signal in 1908.

pends the note. In the case of the siren the piston revolves in the cylinder instead of having a reciprocating motion, but other wise the principle of air admission is the same in both instruments. Each is fitted with a trumpet-horn or resonator and the working air-pressure is 3o lb. per sq.in. for the diaphone and 25 lb. per sq.in. for the siren. The diaphone note is usually about 18o vibrations per sec. or F sharp in the tenor clef ; it terminates with a quick descending note termed the "grunt" at the end of each blast. This grunt is a valuable distinctive feature as it can some times be heard when the remainder of the signal is inaudible.

To provide the air for these instruments compressing machin ery and large capacity air storage receivers are required. In recent. Trinity House installations two instruments with their axes ap proximately 12o° apart horizontally are fitted for distributing the sound over a wide arc. In the diaphone installations at Flam borough, Hartland and Skerries (1927) the mouths of the two trumpets are placed on a common vertical axis with their centres half a wave-length apart, to give effect to the theory propounded by the third Lord Rayleigh that vertical dispersion of sound was by this means avoided. A similar arrangement has also been

introduced at some French stations.

The siren in a primitive form was invented by John Robison (1739-1805). Cagniard de la Tour evolved the disc form and gave it the name of siren. The first steam siren was patented by Brown of New York. The cylindrical form and the centrifugal governor now commonly used are due to G. H. Slight.

Nautophones.

A form of aerial-acoustic instrument called the Nautophone has recently been devised which consists of an electrically vibrated diaphragm sounding a high note. In its pres ent state of development (1929) it compares, as regards intensity, with a reed horn.

Submarine Bell and Oscillator 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. Submerged bells are used principally in connection with light-vessels and are struck by clappers actuated by pneumatic or electrical mechanism. They have also been fitted to buoys and beacon structures, and placed on the sea bed. In the first case the bell is actuated by the motion of the buoy and in the others by electric current, transmitted by cable from the shore.

The oscillator or electromagnetic submarine fog-signal is actuated electrically from the lightship to which it is attached or from the light station with which it is connected. The instru ment, which came into use during the World War, comprises a vibrating diaphragm of large dimensions, and its principle of operation is similar to the working of a telephone. It sends out a high note to which can be given a characteristic code notation. The instrument has been fitted in several light-vessels in Euro pean and American waters. The underwater range of the oscillator has been known to exceed 5o m. as compared with io m. for the bell.