The last wheel of the watch train is what is known as an escape wheel. It is formed with teeth of an odd shape, such as shown at A in Figure 27. These teeth are engaged by a pair of pallets B and C, carried by a three-armed lever D. The pallets are usually bits of sapphire or similar hard stone to prevent wear. The third of the lever is slotted at its extremity to engage a sapphire pin E, carried by a disk F, which is mounted on the staff of the balance wheel. The escape wheel A revolves in the direction of the arrow, being impelled by the mainspring acting through the train of gears. One of the teeth of this wheel engages the pallet B, causing the lever D to swing on its axis and push the sapphire pin E toward the left, thereby giving the disk F an impulse in the same direction. Here a delicate coil spring, known as the hairspring, comes into play. Without the hairspring the parts would stand still, the escape wheel being blocked by the pallet B. The hairspring is attached at one end to the shaft or staff of the disk F and the other to the frame of the watch. It tries to hold the disk F in a fixed position, but is disturbed by the action of the escape wheel and is constantly oscillating the disk in its effort to bring it back to its normal position. When the disk swings over to the left the pallet B is clear of the teeth of the escape wheel. This releases the escape wheel and it springs forward in the direction of the arrow, but before it can move through an interval of one tooth it is arrested by the second pallet C, which has been projected into its path by the swing of the lever D. The lever swings back until the pallet C clears the escape wheel and the pallet B engages the next tooth. And so the action continues, the lever swinging back and forth and at each complete oscillation releasing one tooth of the escape wheel. The hairspring takes up the shock of this intermittent motion and a balance wheel carried by the staff to which F is fastened steadies the oscillatory motion of the lever D. A watch is full of microscopic parts. In a small timepiece there are machine-made screws so small that without the aid of a magnifying glass one cannot see the screw threads cut upon them. But the most marvelous part of the whole watch is the delicate hairspring and the means of adjusting its tension and compensating for its expansion and contraction with changes of temperature. INANIMATE MATTER IN CONTINUAL MOTION