262 ROUND THE YEAR 



of Quadrupeds are possibly examples of the same 

 expedient. In certain Birds, Herons for example, there 

 are patches of feathers which crumble to powder (powder- 

 down feathers). I cannot venture upon any explanation 

 of this curious structure ; the powder is often greasy. 



Now let us take a single quill, and examine its parts. 

 There is the barrel, a hollow cylinder, often trans- 

 parent, the shaft, filled with a white pith, and grooved 

 along its inner side, and the vane. The barrel has 

 usually a small hole at its attached end, and a scar 

 upon its inner side, where it joins the shaft. Between 

 these points there can often be seen a chain of dried up 

 husks, often of oval or conical shape. They are easily 

 seen in a goose-quill, especially if one side of the 

 barrel is cut away to expose them. Notice the 

 curvature of the whole quill along its length, and also 

 its more marked curvature from side to side. The 

 concave side is turned towards the inner side of the 

 wing or the under side of the tail ; it is usually paler 

 in colour than the other, and marked by the groove 

 along the shaft. 



The barrel of a feather is very light, being filled 

 only with air, but it is very strong. I lately took the 

 barrel of a goose-quill, laid it horizontally on supports 

 2 \ inches apart, hung a scale-pan by means of a hook 

 to its middle point and gradually added weights. 

 When the load amounted to /Ibs. the quill began 

 visibly to yield, and at 7|lbs. it collapsed. 



I have already attempted to explain the mechanical 

 principle which renders the hollow cylinder so -strong 

 in proportion to its weight. 1 



1 See page 1 54. 



