Man has sought in all times and at all places to find means of leaving the earth's surface, in imitation of the birds, and rising into the air. Ancient legendary lore furnishes many stories, like those of Dædalus and his son Icarus, of attempts of this sort. In the fourth century B. C., Archytas of Tarentum, a learned Pythagorean, who has been credited with the invention of the screw, the pulley, and the kite, according to Aulus Gellius, constructed a wooden dove which could rise and sustain itself in the air by some mechanism the arrangement of which is not known. Credible accounts exist of an English Benedictine monk, Oliver of Malmesbury, in the eleventh century, having tried to fly by precipitating himself from the height of a tower, with the assistance of wings attached to his arms and his feet. It is said that, after having gone along a little way, he fell and broke his legs. He attributed his accident to failure to provide his apparatus with a tail, which would have helped preserve his equilibrium and made the descent a gentler one. In the sixteenth century, Leonardo da Vinci first demonstrated that a bird, which is heavier than the air, sustains itself, advances in the air, "by rendering the fluid denser where it passes than where it does not pass." In order to fly it has to fix its point of support on the air; its wing in the descending stroke exerts a pressure from above down, the reaction of which from below up forces the center of gravity of its body to ascend at each instant to the height at which the bird wishes to maintain it. Some sketches that have come down to us prove that Leonardo occupied himself, like Oliver of Malmesbury, with giving man power to fly by the aid of wings suitably fixed to his body. We owe to Leonardo also the invention of the parachute, which he described in the following terms: "If a man had a pavilion, each side of which was fifteen braces wide and twelve braces high, he might cast himself from any height whatever, without fear of danger." It may be said, too, of Leonardo da Vinci, that he was the first to suggest the idea of the screw propeller. "If," he said, "this instrument in the form of a screw is properly made--that is, made of linen cloth, the interstices of which have been filled with starch--and if we turn it rapidly, such a screw will make a bearing nut for itself through the air and rise. This can be proved by moving a broad, thin rule rapidly through the air, when it will be found that the arm is forced to follow in the direction of the edge of the board. The frame for the cloth of which I have been speaking should be made of long, stout reeds. A model of it might be made in paper, with, for its axis, a thin strip of iron which we twist forcibly. When the strip is left free it will turn the screw."