SHARP-NOSED EEL. 385 



I am, however, of opinion, that the passage of adult Eels to 

 the sea, or rather to the brackish water of the estuary, is an 

 exercise of choice, and not a matter of necessity ; and that the 

 parent Eels return up the river as well as the fry." 



" All authors agree that Eels are extremely averse to cold. 

 There are no Eels in the arctic regions, none in the rivers 

 of Siberia, the Wolga, nor even in some of the tributaries of 

 the Danube ; yet the rivers of the southern parts of Europe 

 produce four species. There is no doubt that fishes in general, 

 and Eels in particular, are able to appreciate even minute 

 alterations in the temperature of the water they inhabit. 

 The mixed water they seek to remain in during the colder 

 months of the year is of a higher temperature than the pure 

 fresh water of the river, or that of the sea. It is a well- 

 known law in chemistry, that when two fluids of different 

 densities come in contact, the temperature of the mixture 

 is elevated for a time in proportion to the difference in the 

 density of the two fluids, from the mutual penetration and 

 condensation. Such a mixture is constantly taking place at 

 the mouths of rivers that run into the sea, and the mixed 

 water maintains a temperature two degrees warmer than that 

 of the river or the sea. This elevation in the temperature of 

 the water of estuaries and the mouths of rivers is, I have no 

 doubt, one reason why they in general abound in young fish." 



In a tideway river the descent of the Eels towards the 

 brackish water takes place during the autumn, and various 

 devices are employed in different streams to intercept them 

 in their progress. The figure given on the next page repre- 

 sents the form of an apparatus used in various parts of the 

 Thames, called an Eelbuck, consisting of a framework of 

 wood supporting various wicker-baskets of a particular form. 

 The large open end of each basket is opposed to the stream, 

 and by the peculiar structure of the inside, any fish once 

 within the body of the basket cannot escape. 



VOL. n. 2 c 



