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.