NATURAL HISTORY. 123 



" His leaver brushed, his shoes and gown, 

 Away he trudges into town." SWIFT. 



in as regular a manner as a cutter cuts in felling a tree, bringing the bottom of the 

 wood to a point ; others roll the pieces along to the water, others dive and with 

 their feet scrape holes, in order to place them in, while others exert their efforts 

 to rear them in their proper places ; another party is employed in collecting twigs 

 to wattle the piles with ; a third collecting earth, stones, and clay ; a fourth is 

 busied in beating and tempering the mortar ; others in carrying it on their broad 

 tails to proper places, and with the same instrument run it between the piles, or 

 plaster the inside of their houses. Whilst at work, one of the party acts as an 

 overseer, and by striking his tail indicates which parts are weakest. A certain 

 number of smart strokes with the tail is a signal given by the overseers for 

 repairing to such or such places, either for mending any defects, or at the 

 approach of an enemy ; and the whole society attend. 



379. Wliy are the hind fed of the beaver far better adapted 

 for swimming than the front ones ? 



Because when making way through lakes and streams* it has 

 frequently to use the fore feet for purposes not connected with 

 swimming, but to assist in holding, directing, or propelling the 

 billets of wood, or such other matters as the animal may havo 

 gathered, either for building or for food. The beaver has the 

 swimming power more highly developed in its hind feet than 

 has any other quadruped. 



380. Why do beavers build their dams sometimes straight 

 across the stream, and at other times in an oblique direction? 



When the current is slow the dam is carried straight across ; but 

 if the current be rapid, the dam is carried in an oblique line. The 

 dam straight across is the shortest possible, and, therefore, costs 

 the least labour ; but it is the one upon which an equal volume 

 and velocity of water would act most powerfully ; therefore it is 

 used in those cases where the force of the current is least. The 

 oblique dam requires more labour in its construction, because it 

 must be longer for the same breadth of the river ; but as the action 

 of the water upon it diminishes in proportion to the obliquity with 

 which the current meets it, it resists better than the straight dam, 

 and its resistance increases with its obliquity ; therefore it is used 

 against the more rapid and powerful currents. 



