Whether the credit of practically applying the mathematical theory and properties of the pendulum was or was not due to Huygens the Dutchman, we have seen that Harris, a London clockmaker, put up the first pendulum clock in St Paul's Church, Covent Garden, in 1621. The great advance upon this discovery was that the pendulum bob must move not in a circle but a cycloid; and that back and front should be alike both in weight and shape to secure regular vibration. Cylindrical bobs are now in general use for large clocks. The old iron rod pendulums were soon discovered to be affected considerably by variations of heat and cold,--the difference between winter and summer being ascertained to amount to the loss of a minute a week. Harrison's gridiron pendulum was one of the chief endeavours to prevent such variation, followed after a long interval by other ingenious inventions, which gained temporary approval and gradually fell into disuse. Room should be provided by the architect of every clock-tower in the chamber below that containing the movement, to allow of the swing of a 15-foot pendulum. FALL OF THE WEIGHTS. We have seen that the position in which a clock is placed in regard to the dial or dials whose hands it is to drive is a matter requiring some attention. Properly the floor of the clock-chamber should be so planned that the clock might stand immediately behind, and level with the dials; for there is extra expense and inconvenience connected with any more distant situation of the works,--the fall of the weights being sometimes difficult in such case to be provided for. The weights should hang, wherever it is possible so to arrange, immediately from the barrel to which they are affixed, without the intervention of pulleys of any kind, and much expense may be saved by providing for the descent of the weights to a considerable depth below the clock-chamber. As an instance however of the extent to which such difficulties can be overcome, I may mention that the hands of my great clock at the International Exhibition were situated nearly 400 feet from the clockworks, while the weights were carried by iron wire ropes over pulleys below the floor to a distance of 200 feet from the movement, then over another pulley fixed at a height of 80 feet from the ground. The ESCAPEMENT is perhaps the most important part of a clock. CROWN-WHEEL ESCAPEMENT. +----------------------------------------------------------------------+ | [Illustration: Escapement.] | +----------------------------------------------------------------------+ This is the earliest known escapement, and is to be found, as we have said, in Henry de Wyck's clock, all the difference between his escapement and the above being that one of the weights in de Wyck's balance is now set in a vertical instead of a horizontal plane. The bent end or fork seen in the illustration connects the pendulum with that arm technically called the crutch.