314 LETTERS ON NATURAL MAGIC. 



that purpose. In examining the foot of the fly with a 

 powerful microscope, it is found to consist of two conca- 

 vities, as shown in Fig. 59 and 60, the first of which is 

 copied from a drawing by G. Adams, published in 1746, 

 and the second by J. C. Keller, a painter at Nuremberg, 

 who drew it for a work published in 1766. The author 

 of this work maintains that these concavities are only 

 used when the fly moves horizontally, and that, when it 

 moves perpendicularly or on the ceiling, they are turned 

 up out of the way, and the progressive motion is effected 

 by fixing the claws shown in the figure into the irregu- 

 larities of the surface upon which the fly moves, whether 



Pig. GO. 



it is glass, porcelain, or any other subtance. Sir Everard 

 Home, however, supposes with great reason, that these 

 concave surfaces are (like the leathern suckers used by 

 children for lifting stones) employed to form a vacuum, 

 so that the foot adheres as it were by suction to the 

 ceiling, and enables the insect to support itself in an 

 inverted position. 



