260 LETTERS ON NATURAL MAGIC. 



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 pro- 



gressive motion is effected by fixing the claws 

 shown in the figure into the irregularities of the 

 surface upon which the fly moves, whether it is 

 glass, porcelain, or any other substance. 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. 



This conclusion Sir Everard has been led to 



