There are certain cases of obscuration of lights by fog in which bells and gongs are continually sounded. These convey information of danger, but do not identify its position. The same principle which gives numerical accuracy to light-houses, and even the same mechanism, may be made to operate during fogs with equal effect on sounds. Thus, by striking the gong the requisite number of times to indicate the hundreds, the tens, and the units denoting the light, allowing, of course, the usual pauses and the same long intervals, the number of the light-house or light-ship may be known as quickly and as certainly by means of bells, or gongs, or other sounds, as by the occultations of its light. It may be worth examining what musical notes are heard at the greatest distances through fogs, and the sounds of what instruments penetrate farthest amidst the roar of winds and waves. The shrill whistle of the steam carriage should be tried against the deep tones of the organ and the loud noise of the trumpet. The most powerful sounds produced by air require but little physical force for their generation; and whenever the directions in which it is necessary to give warning are known, the sounds employed may be concentrated by reflectors, in the same manner as light. The depth of water at the entrance of harbours may easily be indicated in the day-time by a tide-telegraph governed by the same float which produces the occultations during the night. Its form may be as below, [Illustration: The tide-telegraph.] in which the arms projecting on the left side indicate the tens; those on the right side the units. The long arm for the fifth saves trouble in counting. These arms must be movable on centres within the mast, and must be governed by cams connected with the float, so as to indicate at any time the state of the tide. If it were found necessary to distinguish light-houses during the day, then signs expressing their permanent numbers might be painted upon them, or fixed to masts rising out of each. The right side of the telegraph might, if required, be used as a day telegraph for communicating with vessels.