I now stop the motion, and by a sudden jerk raise a hump upon the tube, which runs along it as a pulse toward its fixed end; here the hump reverses itself, and runs back to my hand. At the fixed end of the tube, in obedience to the law of reflection, the pulse reversed both its position and the direction of its motion. Supposing _c_, Fig. 36, to be the fixed end of the tube, and _a_ the end held in the hand: if the pulse on reaching _c_ have the position shown in (1), after reflection it will have the position shown in (2). The arrows mark the direction of progression. The time required for the pulse to pass from the hand to the fixed end and back is exactly that required to accomplish one complete vibration of the tube as a whole. It is indeed the addition of such impulses which causes the tube to continue to vibrate as a whole. [Illustration: FIG. 37.] If, instead of a single jerk, a succession of jerks be imparted, thereby sending a series of pulses along the tube, every one of them will be reflected above, and we have now to inquire how the direct and reflected pulses behave toward each other. Let the time required by the pulse to pass from my hand to the fixed end be one second; at the end of half a second it occupies the position _a b_ (1), Fig. 37, its foremost point having reached the middle of the tube. At the end of a whole second it would have the position _b c_ (2), its foremost point having reached the fixed end _c_ of the tube. At the moment when reflection begins at _c_, let another jerk be imparted at _a_. The reflected pulse from _c_ moving with the same velocity as this direct one from _a_, the foremost points of both will arrive at the centre _b_ (3) at the same moment. What must occur? The hump _a b_ wishes to move on to _c_, and to do so must move the point _b_ to the right. The hump _c b_ wishes to move toward _a_, and to do so must move the point _b_ to the left. The point _b_, urged by equal forces in two opposite directions at the same time, will not move in either direction. Under these circumstances, the two halves, _a b_, _b c_ of the tube will oscillate as if they were independent of each other (4). Thus by the combination of two _progressive pulses_, the one direct and the other reflected, we produce two _stationary pulses_ on the tube _a c_. The vibrating parts _a b_ and _b c_ are called _ventral segments_; the point of no vibration _b_ is called a _node_. The term “pulse” is here used advisedly, instead of the more usual term _wave_. For a wave embraces two of these pulses. It embraces both the hump and the depression which follows the hump. The length of a wave, therefore, is twice that of a ventral segment. [Illustration: FIG. 38.]