By the use of the Vernier pendulum, when connected with an index, all chance of error in reading the clock will, it is conceived, be avoided. Having touched the trigger at the moment of observation, the observer has, as it were, registered the time, and he may examine the clock at his leisure, for it is manifest that a comparison of the index of the Vernier pendulum with that of the clock will at any time determine the moment of observation. It will also be seen that, should the observer omit to notice the first coincidence of the pendulums, no inconvenience except delay will arise, because the same coincidences will occur in a regular series as long as the pendulums continue in motion.
There are a few provisions necessary for extreme accuracy which, in this hasty sketch, it would be out of place to notice. I will just mention, however, that the apparatus contains within itself the means of measuring what may be called the mean error of the observer, or the average interval which, as regards the particular individual, elapses between the instant of observation and the release of the Vernier pendulum.
To subject the plan which I have here attempted hastily to describe to a rigid trial will require instruments of much greater accuracy than those which I can command, and if the Society possess a good clock not now in use, I shall feel extremely obliged if I can obtain the loan of it. An additional pendulum the requisite length, is not, I presume, to be found among the Society’s instruments.
I have the honour to be, Gentlemen,
Your obedient servant,
Rowland Hill.