A very good Metronome may be made by fastening a bullet to the end of a piece of tape, and swinging it backwards and forwards, regulating the tape according to the time required. Such a Metronome is very portable, and extremely useful where the conveyance of the clockwork instrument would be troublesome. Moreover, its beats can be seen by a great number of persons. I have often used it myself.
Such a Metronome is used in the army, in order to regulate the pace of the soldier’s step, it being of the last importance that the pace should always be the same. Otherwise it would be impossible to calculate the time which ought to be consumed in marching a certain distance, and the military calculations on which depends the success or failure of a campaign would be wholly upset, half an hour too soon or too late meaning failure.
The Escapement.
As we are on the subject of the pendulum and Escapement, we will say a few words about the latter piece of mechanism. It is here given on a larger scale than in the previous illustration, so that its action may be more easily understood. Whether in watch or clock, the Escapement is exactly the same in principle.
First there is the escapement wheel, the circumference of which is furnished with a number of very deep cogs, varying as to the work which they have to do. Then there comes the escapement itself, which swings on its pivot, and is regulated in its oscillations by the pendulum. As it swings backwards and forwards, it is evident that only one tooth of the wheel can “escape,” and only that in one direction.
We can reverse a steam-engine, but the man has yet to be found who can reverse a clock, i.e. enable it to continue going in the opposite direction. The only mode would be to enable one set of cogs to flatten themselves, so as to pass the escapement, and a second set to start up in exactly the opposite direction. Or perhaps there might be two parallel escapement wheels, capable of being connected or disconnected with the clock at pleasure. As, however, a reverse movement is quite needless, no such invention seems to have been made.
On the left hand is seen an example of the same principle as shown in Nature. It represents a larva or grub of the Burying-beetle. It has no legs available for locomotion, and yet it can get along with tolerable speed.
Many years ago, when living in Wiltshire, I was much struck with this fact. There had been an epidemic among sheep, which killed them off so fast that the farmers would at last not even bury them, but took off the skins, and left the bodies to moulder as they best might.
It was very unpleasant for the farmers, but just the contrary for the Burying-beetles, which simply swarmed in the deserted carcasses. If one of them were tapped with a stick, hundreds of these larvæ came scuttling out, displaying an activity which was really remarkable in creatures practically legless.