THE RATTRAY HEAD LIGHTHOUSE.
A very exposed Scottish rock tower. It is unique because a full-powered siren fog-signal is installed therein.
Each signal is mounted in a domed house built of concrete, the mouth of the trumpet extending from the crown of the roof. Within the house is an air-receiver 9 feet in height by 4½ feet in diameter, of about 140 cubic feet capacity, which receives the compressed air transmitted through the piping from the compressing-station. It also contains the automatic apparatus whereby the signal is brought into action at the stipulated intervals, so as to produce the requisite sound characteristic. This is a self-winding clockwork mechanism which admits and cuts off the supply of air to the trumpets, its chief feature being that the clock is wound up by the compressed air itself, so that it is entirely free from human control. However, as a breakdown even with the best-designed and most-carefully-tended machinery cannot be circumvented entirely, there is a duplicate electrical mechanism, also automatically controlled from the power-generating station, the electric cables for which are laid in the pipe trenches. This acts as an emergency control.
By courtesy of Messrs. D. and C. Stevenson.
SULE SKERRY LIGHT.
A lonely light of Scotland. The nearest land is the Butt of Lewis, 30 miles distant.
The two signals are not sounded simultaneously; neither are they alike nor of the same tone. The north signal gives a single blast of high tone, lasting five seconds, and then is silent for 175 seconds. On the south side the siren gives a double note, although there are three blasts—viz., high, low, high—corresponding to the letter R of the Morse code. The notes are sounded for two seconds, with similar intervening periods of silence, and silence for 170 seconds between the groups. The complete signal from the two stations is given once in three minutes, the north signal commencing to sound ninety seconds after the south signal has ceased. The high note corresponds to the fourth E in the musical compass, there being 38,400 vibrations per minute; while the low note is tuned to the third D in the musical compass, with 16,800 vibrations per minute. The notes are purposely timed more than an octave apart and made discordant, as thereby the sound is more likely to attract attention and to be readily distinguished.
About eighteen minutes are required to bring the apparatus into operation—that is, to start compressing and to raise the pressure of the air to the requisite degree—but, as fogs descend upon the Clyde with startling suddenness, the signals may be started within five minutes of the fog-alarm. The air-reservoirs are kept charged to the working pressure, the machinery being run once or twice for a short time every week for this purpose and to keep the plant in working order.
Up to this time it had been the practice to place the siren in close proximity to the air-compressing machinery, but the installation at Ailsa Craig proves conclusively that this is not essential to success; also it demonstrates the fact that a number of signals can be operated reliably and effectively from a central station. Indeed, this Scottish plant aroused such widespread interest that the Pulsometer Engineering Company of Reading, who had acquired Professor Holmes’s patents and who carried out the above installation, received several inquiries from abroad with regard to its suitability for similar situations. In one instance the compressed air was to be transmitted for a distance of nearly four miles.