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Page:Popular Science Monthly Volume 56.djvu/620

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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."

In 1680 Borelli published some studies of a remarkably correct character on the flight of birds. According to his view, the wing acts upon the air in the phase of beating down, in the manner of an inclined plane, so as, by virtue of the resistance opposed by the air, to push the body of the animal upward at first and then onward. The action of the ascending wing was compared to that of a kite, and it would consequently continue to sustain the body of the bird while waiting the following stroke. But Borelli never thought of turning his observations to advantage, so as to supply man with the means of flying. Attention was much engaged in 1742 with the attempt of the Marquis de Bacqueville, substantially repeating that of Oliver of Malmesbury, which was terminated by a similar accident. Mention should also be made of Paucton, who in 1768 drafted a plan for a screw machine. In 1784 Launoy and Bienvenu exhibited and operated, before the Academy of Sciences in Paris, a screw which was moved by a strong spring. Before this, however, Joseph and Stephen Montgolfier had filled the world with the noise of their discovery of the air balloon, and the ingenious machine of these aëronauts failed to receive the attention it deserved.

It has been known since the days of Archimedes that every body partly or wholly submerged in a liquid in equilibrium suffers a vertical push upward from the fluid equal to the weight of liquid it displaces.

Let us consider the case of a body entirely plunged in a liquid—water, for example. If its weight exceeds the thrust it suffers it will fall to the bottom of the water under the action of a descensional force equal, at each instant, to the difference between the weight of the body, which is invariable, and the thrust, which is invariable also, and thus constant in direction and also in amount. If the weight of the body is less than the thrust, the latter overcomes it, and, contrary to the usual laws of weight, the body will rise under the action of an ascensional force, which will evidently be likewise constant in amount as well as in direction. A cork held down at the bottom of a vessel of water and then left to itself will supply an example of this ascensional movement.

A third case may be presented—that in which the weight of the body is equal to the thrust of the water. Weight and thrust are then in mutual equilibrium. No force invites the body either to descend or to rise, and it remains balanced in the midst of the