72 



THE POPULAR SCIENCE MONTHLY. 



the contrary of what Blackwall assigned, was the reason adherence 

 failed. Adherence likewise failed when the opposite side of the glass 

 was moistened with ether, in consequence of the condensation of vapor 

 occasioned by the evaporation of the ether. 



Adherence also fails completely when the glass is covered with a 

 thin wash of oil. And a fly which has been put upon a glass so 

 covered, and is then transferred to a clean glass, will not be able to 

 adhere to that till after some interval. An extremely thin coating of 

 oil is enough to bring about a failure to adhere ; even the rubbing of 

 the finger on the glass is sufficient. The failure in this case is caused 

 by the running together of the little drops of liquid on the hairs, by 

 which the adhering surface is much reduced below the total surface 

 presented by the little drops acting separately. Each foot then acts 

 as a single hair, the diameter of which is equivalent to its own ; and, 

 even if its diameter were equivalent to a millimetre, the six feet bear- 

 ing together upon the glass would not be competent to sustain the 

 fly. For, according to the experiment with the horse-hair, a diame- 



Fia. 3.— Foot of Poltdrosus sericeus.— 1. Palvilli, with hairs and hooks. 2. Three hairs, con- 

 siderably magnified. 3. A hair more considerably magnified. 



ter of 0*12 of a millimetre will bear 0*00085 of a gramme ; conse- 

 quently, a diameter of a millimetre will bear 0*007 of a gramme, and 

 the six feet together 0*042 of a gramme. 



It is very difficult, if not impossible, for a fly to walk on a vertical 

 polished surface when it is thinly covered with dust. When, after it 



