6o4 POPULAR SCIENCE MONTHLY. 



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 1G80 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 Avaiting 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 repeat- 

 ing 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 in- 

 genious machine of these aeronauts 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 de- 

 scensional 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 over- 

 comes 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 



