机械原理 Mechanical Principles

　　Close up we see pistons move up and down or side to side. Pendulums sway, the small parts of machinery move. Gears drive larger wheels. Gears within gears spin. Shafts turn some mechanism that is out of sight. Screws revolve and move other gears; a bit rotates. More subtle mechanisms move other mechanical parts for unknown purposes. Weights rise and fall. The movements, undersc...