Signals of this kind, are not much used for general purposes. Their employment is almost always to convey one or a few preconcerted messages. For signals of this class, one thing or indication is to be used, and it is not to be considered as varied, through it may seem to vary, in any signal. To mark the close of each complete signal, there must of course be a pause of time, or a pause-signal.
A good illustration of signals of this kind, is found in the striking of a clock, when twelve different hours are indicated by the same and single sound, repeated the proper number of times to suit each hour. If in the striking of any hour, this sound varies so as to make two or three different notes even, this difference of sound effects, in no way, the meaning of the signal.
Signals of this kind, may be used in the field, as where one rocket is thrown up to indicate any one message, two rockets a second message, three rockets a third message; and thus on to any given number.
Or a light may be shown a certain number of times, as a candle shown at a window, and then removed to stand for "one" or the first message; shown twice for "two," or the second message; three times for "three," or the third message; and thus on. Or, in a field or in a boat, a lantern may be kept lighted in a pail, and hoisted out of the pail and returned to it to make each flash. Or a lantern may be shown from behind a fence, or any kind of screen. In these illustrations, the appearance of the lights is the signal. Each complete signal may be shown by a wave of the light, or any other sign, as the pause-signal. Or guns may be fired the required number of times for any signal.
Now, in any of these signals, which are to depend upon the number of times a light is shown, or the number of times a gun is fired, no difference is, of course, made, if the light changes, or if there is difference of sound in different reports of the gun; for it is remembered only one element is used, and that the signal depends solely upon the number of repetitions of that element; for instance a white light shown twice, would stand for message number "two." A white light shown, and then a red light, making two in all, would also stand for "two." So the preconcert being that only one element is to be used in a set of signals, they may be made to seem much varied. Signals of one element, when used in the field are generally for instances as this: to fire two guns to indicate a completion of a military movement: to throw up three rockets, or one rocket; to announce that a portion of the army is to move. Of course several such messages can be arranged in one code.