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VI. § 72.] DIFFERENCE OF PHASE. 143
Let the forks have been set sounding by precisely simultaneous blows. They will then commence swinging out qf their positions of rest in the same
direction at the same instant. The points in an associated wave-form which correspond to the initial position of a vibrating particle are those in which the wave-form cuts the initial line. Hence in laying down the tributary waves we must, in the present instance, begin them at one and the same point in the initial line, and take care that their convexities at that point are turned the same way, as at 0, Fig. 46.
In this case the two vibrations are said to start in the same phase.
If the two forks are set in vibration at different moments, they may not swing out of their equilibrium positions in the same direction together. Hence we no longer necessarily have a point where both sets of associated waves cut the initial line. The result is