The general features of the acetylene fog-gun may be observed from the illustration (facing p. 64). The acetylene, dissolved in acetone, is contained under pressure in a cylinder, and thence passes through a reducing valve to an annular space, where it is ignited by an electric spark. A trumpet is attached to the firing chamber, so that the sound becomes intensified. If desired, the explosion can be effected at the burner, so that, in addition to a sound-signal, a flashing light is given. The applications vary according to the circumstances. Suppose there is an unlighted bell-buoy at the bar of a port. Here the procedure is to install a gun and light combined, so that the flash of the explosion may give visual and the report audible warning. Or, should there be a lighted buoy already in position, its effectiveness may be enhanced by adding the gun, the detonation alone being employed for warning purposes. The size of the cylinder containing the dissolved acetylene may be varied, so that renewal need only be carried out once in one, two, or more months, according to conditions. If the increasing traffic around a certain rock demand that the latter should be marked, a combined sound and light apparatus can be installed. It may be that the head of a pier which is accessible only at certain times, or a beacon which can be reached only at rare intervals, may require improved facilities. In this case the gun can be set up and a cable laid to a convenient spot which may be approached at all times by an attendant. Then the latter, by the movement of a switch, can bring the gun instantly into action upon the alarm of fog, and it will keep firing at the set intervals until, the fog lifting, the gun is switched off. In some cases, where the apparatus is set upon a lonely rock, a submarine cable may be laid between the marked point and the control-station. The cable is not a very costly addition. There are many lights where wages have to be paid merely for a man to bring the fog-signalling bell machinery into action. In such cases a fog-gun can be installed and the annual cost of maintenance decreased enormously, thereby enabling the outlay on the gun to be recouped within a very short time; while the light may be improved by using the flashes, so that the warning can be rendered more distinctive.