Motion. A bicycle keeps upright only as long as it is moving. When standing still it has to be supported against something. It seems rather odd that the bicycle wasn't invented long before our day. Newton, the same great thinker who discovered the law of falling bodies, and separated rays of white light into the colors of the spectrum, also discovered certain laws of motion. Of all the laws of motion he placed this first : "A moving thing will move at a constant speed, and in the same straight line forever, unless some other force stops it." Every child who rolls a hoop uses that law. He sets the hoop moving in a straight line. The earth pulls the hoop over as soon as the force that set it in motion becomes weaker than the earth's pull. In the bicycle, the rider constantly applies new force to the wheels through the pedals, and so keeps the wheels rolling forward.