Observatory for public trial. But since then many improvements have been made in it, so that makers are now enabled to produce a pocket watch, with the short angle lever escapement, which marks time at a steady rate of within four or five seconds weekly,--a rate which approaches so near to the time-keeping of a pocket chronometer, that unless the minutest exactness for some specific purpose is required, the last-named watch is all that can be wished for. +----------------------------------------------------------------------+ | [Illustration: Lever Escapement.] | | | | [Illustration: Chronometer Escapement.] | +----------------------------------------------------------------------+ About the year 1780 was invented the escapement which is now denominated the Detached or _Chronometer Escapement_ (see opposite page), the principles of which are the nearest approach to perfection, the impulse to the balance being given at the centre of vibration. A is the escape-wheel, B the escape-wheel teeth, C the roller let on the verge, or axis of the balance. This roller is a circle of polished steel, with a notch cut out of it, into one side of which, D, a flat polished piece of ruby is inserted for the acting part. Below this steel roller, carried on the same verge, is a smaller roller of steel (E), denominated the discharging pallet, having a sapphire fixed on its outer edge. F is a slender spring, which is screwed at I to the stouter one, having its fixture at the stud L, and polished away very thin at K, in order that it may bend readily, so as to cause very little resistance to the balance while forcing it on one side. G is a projecting piece, carrying an upright pin made of ruby, against which the wheel tooth B rests; at B is a small screw against which the spring L K G strikes, and thus prevents it from springing too far back. The action of these parts is as follows:--When at rest the circular edge of C is just clear of the two teeth of the wheel B, which cannot be set in motion while E and G remain quiescent; G rests against the screw at B, and the tooth resting against the locking pallet G, the escapement-wheel cannot turn. To set the chronometer going it is necessary to give it a rotary motion, which sets the balance in action. This causes the lower piece on the verge (called the lifting piece or discharging pallet) to strike against the end of the spring F, which, from its over-lapping the curved end of the prolonged spring K G, pushes it back, and thus releases the pin or locking stone G from before the tooth of the wheel: that is, it unlocks the escapement-wheel, which is immediately set in motion by the force of the mainspring. The same vibration given to balance and verge brings the ruby pallet D round before the tooth B, which strikes against it and carries it round.