happened during that interval of time. It is evident that, by drawing the vertical lines closer to each other, much shorter intervals, even to the one-thousandth of a second, may be measured with accuracy, provided the cylinder moves with uniform velocity. It is not easy to secure the latter condition. You see when I start this cylinder it moves slowly at first, and gathers speed as it goes on, and even when it
Fig. 26.—Tracings of the vibrations of a tuning-fork, ten vibrations per second. a b, cylinder moving rapidly; c d, cylinder moving slowly.
attains full speed I have no guarantee that it is then travelling at a uniform rate. It may make short spurts, or, as the spring becomes unwound, it may by and by move more slowly. The method by the cylinder, therefore, is not sufficiently accurate.
Thomas Young was also the first to devise the method of inscribing on a rotating cylinder the vibrations of a rod bearing a very light
