The following method of avoiding collision at sea depends on the use of synchronised signals, transmitted in different media. Such signals, travelling at different rates, enable the distance of their source to be inferred by observation of the gain in time of the faster upon the slower travelling signal. Thus, if signals be simultaneously emitted by wireless and by submarine bell (or Fessenden oscillator), the former being transmitted with practically infinite velocity, the latter arrive with a lag which is the time the submarine sound requires to traverse the intervening medium. The rate of propagation of sound in water being closely 4800 feet per second, the lag is 0·62 second for one-half sea-mile. In practice the signals may be so ordered as to dispense with the stopwatch or chronograph. This is accomplished by sending out the wireless ticks in groups of, say, 20 “dots” spaced to intervals of 0·6 second. The stroke of the bell precedes the first of these dots by one of these intervals. Thus, when the sailor is half mile from the source he hears the first wireless dot along with the bell stroke. If he is 1 mile distant the bell stroke comes in with the second dot, and so on. He has, in fact, only to count up the dots till he hears the bell, and the number of the dot coincident with the bell is the number of half sea-miles intervening between his ship and the source of the signals. It is possible to estimate the quarter mile by noting a want of coincidence between bell stroke and dot. This method of estimating distance is in actual operation in assisting mariners to navigate the approach to New York Harbour, the signals being emitted from the Fire Island Light Ship. It is of special value in coastal navigation.
A method of avoiding collision at sea
