This instrument has displaced the siren among the stations upon the St. Lawrence River. The general type of apparatus has a piston 4½ inches in diameter, and uses 11 pounds of air per second during the sounding of the blast. But at more important stations a far larger and more powerful class of apparatus is used, the diaphone at Cape Race having a piston 8½ inches in diameter and using 27 feet of air per second while sounding. This does not indicate the limit of size, however, since the builders of this terrible noise-producer are experimenting with an apparatus having a piston 14 inches in diameter. The sound issuing from such a huge apparatus would be almost as deafening as the report of a big gun and should succeed in warning a mariner several miles away.
The atmosphere, however, plays many strange pranks with the most powerful sound-producing instruments. To-day, for instance, a fog-signal may be heard at a distance of ten miles; to-morrow it will fail to be audible more than a mile away. This aberration of sound is extraordinary and constitutes one of the unsolved problems of science. Innumerable investigations have been made with the object of finding the cause of this erratic action, but no conclusive explanation has been forthcoming. Another strange trick is that, while a sound may be audible at distances of two and four miles during a fog, it fails to strike the ear at three miles. It is as if the sound struck the water at a range of two miles, bounded high into the air, and again fell upon the water at four miles, giving a second leap to hit the water again farther on, in much the same way as a thin flat stone, when thrown horizontally into the water, will hop, skip, and jump over the surface. This trick renders the task of the lighthouse engineer additionally exasperating and taxes his ingenuity to the utmost, as it appears to baffle completely any attempt towards its elimination.
Recently another ingenious and novel system has been perfected by Messrs. D. and C. Stevenson. This is an acetylene gun which acts automatically. Hitherto an unattended fog-signal--except the bell-buoy tolled by the movement of the waves, which is far from satisfactory, or the whistling buoy, which is operated upon the same lines and is equally ineffective except at very short range--has found little favour. The objections to the bell and whistle buoys are the faintness of the sounds, which may be drowned by the noises produced on the ship herself; while, if the wind is blowing away from the vessel, she may pass within a few feet of the signal, yet outside its range. Thus it will be recognized that the fog-gun serves to fill a very important gap in connection with the warning of seafarers during thick weather.