PERCUSSION.
When a blow is arrested by an object, the motion is converted into heat. The ancient flint-lock gun and the percussion-cap fire-arm both illustrate this fact. In the former, the descending flint struck out the spark, and in the latter the cap is exploded by the arrested hammer. The stroke of a cannon ball is attended with a flash. If the world were suddenly stopped in its course, heat enough would be generated to set it on fire. Nitro-glycerine and dynamite are exploded by percussion. Familiar illustrations of this scientific truth meet us in everyday life. It has even passed into a proverb with a moral application, that “hard cracks make the sparks fly.” A novel effect of percussion may have been noticed when a fall upon the ice has resulted in a mechanical disturbance of the optic nerve which revealed whole constellations of stars never yet catalogued.