93 



or the softness of the ground, frequently obliges them to make more 

 violent efforts to escape from the hungry wolf than would be called for 

 in July. The increase of labour thus imposed would be considerably 

 augmented, if there was as great an increase in the length and weight of 

 their hair, as there is in the more domestic animals, the goat and sheep. 

 Providence has therefore devised a plan, by which the utmost possible 

 amount of warmth may be obtained at the smallest possible cost. 



This may be illustrated by the following table of the comparative 

 warmth (to speak popularly) of the following substances : — The experi- 

 ments were conducted by Rumford. A thermometer was placed in a glass 

 tube, and the bulb was closely invested by the substance to be examined. 

 The whole was first plunged into boiling water, and then removed into 

 melting ice. The time required for the thermometer to cool from 190 

 F., to 54.5, was then noted in seconds, which thus became a compara- 

 tive measure of the conducting power of the body. When air alone was 

 interposed, it required 576 seconds to cool down ; with 16 grains of sew- 

 ing silk, which is not cellular, it required 817; with fine lint, 1032; 

 cotton, 1046 ; wool, 11 18 ; raw silk, 1284. All these would mechanically 

 prevent a free convection of air by their long staple, &c. ; beaver s fur, which 

 is very cellular, and has not a long staple, required 1296 ; eider down, 

 which has a very long and entangled staple, 1305 ; and hai'e's fur, 

 which is comparatively short, but very cellular, required 1315 seconds 

 before the thermometer fell to 54. 



A singular appearance is met with in the shrew mole, and other 

 burrowing animals. When first the haft- leaves the skin it is small and 

 solid ; at a short distance from its exit, it becomes cellular, greatly 

 dilated and ribbon like ; the fibrous portion that occupied the centre is 

 almost wholly driven to the circumference, and we have increased bulk 

 without increased weight. In a little time the hair contracts again, and 

 shows a simple central canal ; it soon dilates again, each dilatation being 

 usually greater than the last ; after as many as eight, or even more 

 alternations, it ends by forming one large terminal head, of different 

 colour to the rest of the hair. In the mole, the alternations are not 

 quite so numerous, and the top joint is of the same colour as the rest. 

 The scales on these hairs are very prominent, and, unless examined care- 

 fully, appear to be nothing more than projections from the side. They 

 are not set uniformly in one direction, but in a sort of spiral manner, 

 the greatest projection changing sides, after every contraction of the hair. 

 On the terminal joint the scales are agglutinated together as they are 

 on other hairs, and do not show any projections. 



This formation receives its fullest development in the hairs of the 



