But as for practical convenience we are obliged to make it a circle, it follows, as we saw, that for every tenth of an inch of increase of swing of an ordinary seconds pendulum about a second a day of error is introduced. To remove this difficulty a gravity escapement was invented by Mudge in the eighteenth century, improved by Bloxam, a barrister, and perfected by the late Lord Grimthorpe. The idea was to make the scape wheel, instead of directly driving the pendulum, lift a weight, which, being subsequently released, drove the pendulum. The consequence was, that inequalities in wind pressure, which affected the driving force of the scape wheel, would not act on the pendulum, which would be always driven by the uniform fall of a fixed and definite weight. A movement of this kind has been fixed in the great clock at Westminster, and has gone admirably. A description of its details will be found in the _Encyclopædia Britannica_, written by Lord Grimthorpe himself. All sorts of eccentric clocks and watches have been proposed. For instance, it seems wonderful to see a pair of hands fitted to the centre of a transparent sheet of glass go round and keep time with apparently nothing to drive them. But the mystery is simple. The seeming sheet of glass is not one sheet, but four. The two centre sheets move round invisibly, carrying the hour hand and minute hand with them, being urged by little rollers below on which they rest. When you touch the glass the outside sheets appear at rest, and you do not suspect that it is other than a single sheet. But beware of dust, for if dust gets on the inner plate you detect the trick. In this way a mechanical hand was made that wrote down answers to questions. This plan can be applied to all sorts of tricks. Sir William Congreve, an ingenious inventor, proposed to make a clock that measured time by letting a ball roll down an incline. When it got to the bottom it hit a lever, which released a spring and tipped the plane up again, so that the ball now ran down the other way. It is a poor time-keeper, and the idea was not original, for a ball had been previously designed for the same purpose. Sometimes clocks are constructed by attaching pendulums to bronze figures, which have so small a movement that the eye is unable to detect it. The figure appears to be at rest, but is in reality slowly rocking to and fro. It is necessary to make the movement as small as about one four hundredth of an inch in half a second, if the movement is to escape human observation. For a movement of one two hundredth of an inch per second is about the largest that will certainly remain unperceived.