NATURAL THEOLOGY. 113 



and therefore the downward pressure is resisted by an 

 upward one precisely equal. But if the air on one side 

 of the hand was removed, the weight upon the other 

 side would remain unbalanced, and we should be sen- 

 sible of the force exerted. An experiment is easily 

 tried. If we put a piece of burning paper into a wine 

 glass and then suddenly cover it with the hand, the 

 hand will be holden fast to the glass. The reason is, 

 the air has been in some measure expelled by the fire, 

 so that there is now an unequal pressure upon the two 

 surfaces of the hand, the greater pressure being above, 

 which produces the effect we experience. If we could 

 press our foot upon the floor with sufficient force to 

 make the sole absolutely touch at every point, and per- 

 fectly expel every particle of the atmosphere, there 

 would be the same effect. A foot of the common 

 size would be holden down with a weight more than 

 equal to that of a barrel of flour ; it could no more be 

 lifted than if such a weight were placed upon it, not 

 till in some manner the sole was drawn up, beginning 

 at one edge and gradually letting in the air, like rais- 

 ing a plaster. This is found to be the construction of 

 the fly's foot. There is a skin or flap upon the sole 

 w T hich it can draw down so close upon the wall as to 

 squeeze out the air completely, and the consequence is 

 that the foot adheres with considerable force. 



It has also been found that some of the large am- 

 phibious animals which inhabit the polar regions have 

 the same formation of the foot, only upon a greater 

 scale. By this means they are able to climb the 

 floating masses of ice among which they live. 

 k2- 



