18 EVENINGS AT THE MICKOSCOPE. 



falling on the housetop, has a tendency to flow down- 

 wards, from gravitation ; and as the slope of the roof is 

 in that direction, the current passing over each tile is 

 deposited from its bottom-edge on the middle of the 

 next, whence it still flows down to the free edge, 

 and so in succession. So the motion of the bird through 

 the air, and of the fish through the water, produces the 

 very same effect as if these fluids were in motion, and the 

 animals were still ; and therefore the bodies of the latter 

 are, as it were, tiled with feathers or scales, the free edges 

 of which, looking in the opposite direction to the coming 

 of the current (that is, the same direction as its flow), de- 

 posit the successive particles of the moving fluid in the 

 midst of the successive feathers or scales. Thus two 

 results ensue, both essential to the comfort of the animal : 

 first, the air or water does not run upward between the 

 feathers or scales to the skin ; and secondly, the surface 

 presents no impediment to free motion. This latter ad- 

 vantage will be appreciated, if you take hold of a dead bird 

 by the legs, and push it rapidly through the air tail- 

 foremost: the feathers will instantly rise and ruffle up, 

 presenting a powerful resistance to movement in that 

 direction. 



These scales of the Perch have their hinder, or free 

 edge, set with fine crystalline points, arranged in succes- 

 sive rows, and overlapping. Their front side is cut with a 

 scolloped pattern, the extremities of undulations of the 

 surface that radiate from a common point behind the 

 centre. These undulations are separated by narrow fur- 

 rows, across which, contrary to the ordinary rule, the 

 close-set lines that follow the sinuosities of the outline are 

 not visible. Under the microscope they look as if they 

 had been split in these radiating lines, after the whole 

 number of layers had been completed, and the fissures had 

 then been filled with new transparent substance. 



The middle scale is, as I have said, from the lateral 



