Sir John Herschel was the first to propose to divide a stream of sound into two branches, of different lengths, causing the branches afterward to reunite, and interfere with each other. This idea has been recently followed out with success by M. Quincke; and it has been still further improved upon by M. König. The principle of these experiments will be at once evident from Fig. 152. The tube _o f_ divides into two branches at _f_, the one branch being carried round _n_, and the other round _m_. The two branches are caused to reunite at _g_, and to end in a common canal, _g p_. The portion _b n_ of the tube which slides over _a b_ can be drawn out as shown in the figure, and thus the sound-waves can be caused to pass over different distances in the two branches. Placing a vibrating tuning-fork at _o_, and the ear at _p_, when the two branches are of the same length, the waves through both reach the ear together, and the sound of the fork is heard. Drawing _n b_ out, a point is at length obtained where the sound of the fork is extinguished. This occurs when the distance _a b_ is one-fourth of a wave-length; or, in other words, when the whole right-hand branch is half a wave-length longer than the left-hand one. Drawing _b n_ still further out, the sound is again heard; and when twice the distance _a b_ amounts to a whole wave-length, it reaches a maximum. Thus, according as the difference of both branches amounts to half a wave-length, or to a whole wave-length, we have reinforcement or destruction of the two series of sonorous waves. In practice, the tube _o f_ ought to be prolonged until the direct sound of the fork is unheard, the attention of the ear being then wholly concentrated on the sounds that reach it through the tube. It is quite plain that the wave-length of any simple tone may be readily found by this instrument. It is only necessary to ascertain the difference of path which produces complete interference. Twice this difference is the wave-length; and if the rate of vibration be at the same time known, we can immediately calculate the velocity of sound in air.