Sound / by John Tyndall.

  • Tyndall, John, 1820-1893.
Date:
1875
Catalogue details

Licence: Public Domain Mark

Credit: Sound / by John Tyndall. Source: Wellcome Collection.

    135/464 (page 97)
    its length from its lower end a, and taking hold of a b at its centre, pluck it aside; the length b c above my hand instantly divides into two vibrating segments. With- drawing the hands wholly, you see the entire tube divided into three ventral segments, separated from each other by two motionless nodes b and b' (4). I pass on to the point b (5), which marks off one-fourth of the length of the tube, encircle it, and pluck the shorter segment aside. The longer segment above my hand divides itself imme- diately into three vibrating parts. So that, on withdraw- ing the hand, the whole tube appears before you divided into four ventral segments, separated from each other by three nodes b b' b (6). In precisely the same way the tube may be divided into five vibrating segments with four nodes. This sudden division of the long upper segment of the tube, without any apparent cause, is very surprising; but if you grant me your attention for a moment, you will find that these experiments are essentially similar to those which illustrated the coalescence of direct and reflected undulations. Reverting for a moment to the latter, you observed that the to-and-fro motion of the hand through the space of a single inch was sufficient to make the middle points of the ventral segments vibrate through a foot or eighteen inches. By being properly timed the impulses accumulated, until the amplitude of the vibra- ting segments exceeded immensely that of the hand which produced them. The hand, in fact, constituted a nodal point, so small was its comparative motion. Indeed, it is usual, and correct, to regard the ends of the tube also as nodal points. Consider now the case represented in (1) fig. 39, where the tube was encircled at its middle, the lower segment a b being thrown into the vibration corresponding to its length and tension. The circle formed by the finger and H