Plate 147 shows us the oldest known arrangement of a crown-wheel escapement in a clock. R is the crown wheel or balance wheel acting upon the pallets P and P’, which form part of the verge V.  This verge is suspended as lightly as possible upon a pliable cord C and carries at its upper end two arms, B and B, called adjusters, forming the balance.  Two small weights D D, adapted to movement along the rules or adjusters serve to regulate the duration of a vibration.  In Fig. 148 we have the arrangement adopted in small timepieces and watches:  B represents the regulator in the form of a circular balance, but not yet furnished with a spiral regulating spring; c is the last wheel of the train and called the fourth wheel, it being that number distant from the great wheel.  As will be seen, the verge provided with its pallets is vertically placed, as in the preceding plate.

[Illustration:  Fig. 149]

Here it will quickly be seen that regarded from the standpoint of regularity of motion, this arrangement can be productive of but meager results.  Subjected as it is to the influence of the slightest variation in the motive power and of the least jar or shaking, a balance wheel escapement improvided with a regulator containing within itself a regulating force, could not possibly give forth anything else than an unsteady movement.  However, mechanical clocks fitted with this escapement offer indisputable advantages over the ancient clepsydra; in spite of their imperfections they rendered important services, especially after the striking movement had been added.  For more than three centuries both this crude escapement and the cruder regulator were suffered to continue in this state without a thought of improvement; even in 1600, when Galileo discovered the law governing the oscillation of the pendulum, they did not suspect how important this discovery was for the science of time measurement.

GALILEO’S EXPERIMENTS.

[Illustration:  Fig. 150]

Galileo, himself, in spite of his genius for investigation, was so engrossed in his researches that he could not seem to disengage the simple pendulum from the compound pendulums to which he devoted his attention; besides, he attributed to the oscillation an absolute generality of isochronism, which they did not possess; nor did he know how to apply his famous discovery to the measurement of time.  In fact, it was not till after more than half a century had elapsed, in 1657, to be exact, that the celebrated Dutch mathematician and astronomer, Huygens, published his memoirs in which he made known to the world the degree of perfection which would accrue to clocks if the pendulum were adopted to regulate their movement.

[Illustration:  Fig. 151]