of the species. The salmon, too, affords a remarkable example of 

 the rapid growth of fish under certain circumstances. The salmon, 

 which at one and a half years old, weighs only from two to three 

 ounces, goes down to the sea for the first time, and returns in 

 about three months, weighing from two to five pounds, and 

 occasionally more. 



Their arrival in our rivers varies very greatly; which I 

 apprehend, depends much on the temperature of the water. This 

 is strikingly exemplified in the Oikel and the Shin, in Sutherland- 

 shire. The Shin joins the Oikel five miles from the entrance of 

 the latter into the sea ; it might therefore be presumed, that the 

 salmon which entered the Oikel in the spring, direct from the sea, 

 would continue a straight forward and upward course. The 

 chief body of them, however, turns up the Shin. The temperature 

 of the Shin is higher than that of the Oikel ; and they do not 

 journey up this latter river, till the advancing season has raised 

 its temperature. Precisely the same thing occurs in the Esk and 

 the Eden of Cumberland : though emptying themselves into the 

 same estuary, salmon enter the Eden many weeks earlier than 

 the Esk — the Eden is the warmer stream. For a similar reason, 

 salmon remain some time within tidal influence. The water 

 there is two degrees warmer than either salt or fresh water, 

 separate ; chemistry having proved that when two fluids of dif- 

 ferent densities come in contact, the temperature is elevated for a 

 time, in proportion to the difference in their densities ; and as this 

 mixture is constantly occurring at the mouths of those rivers 

 which run into the sea, salmon remain there, a shorter or longer 

 time, according to the warmth or coldness of the pure fresh 

 water. 



The salmon-trout and the sewin are such rare visitants to this 

 county, and their habits so closely resemble those of the salmon, 

 that they only need incidental mention as Herefordshire fishes ; 

 indeed, of my own knowledge, I cannot vouch for either of them 

 being entitled to the honor. 



The most universally distributed of the salmonidae, is the 

 common trout (S. fario). There is no purely fresh water fish 



