Manchester Memoirs, Vol. Iviii. (1914), No. S. 15 



appearance in Lough Neagh and Rostherne Mere may be 

 partly explained by the differences in the annual range of 

 temperature, more especially to the difference in the 

 maximum temperature reached in both sets of lakes. 

 The somewhat irregular nature of the curve in both the 

 summers concerned is possibly to be explained by some 

 such cause as the presence or absence of recently-fallen 

 rain or excessive inflow water at the times of our visits. 



Chemical Composition of the Water. 



One of the most noteworthy features of the fauna of 

 Rostherne is the presence in its waters of the Smelt, 

 Osmerus eperlanus, a Salmonoid fish usually associated 

 with estuaries. We have no direct knowledge as to how 

 it got there, though it is possible that it was introduced 

 into the mere at some time or other and has become 

 acclimatised. There is no inherent difficulty in this 

 possibility, as we know that in many of the fresh-water 

 lakes of Sweden it is resident all the year round. Its 

 occurrence in Rostherne, however, has given rise to 

 interesting, if groundless, speculation, and two main 

 opinions have been held to account for its presence there. 

 Firstly, Rostherne has been supposed to be in some way 

 or another influenced by the tide, and secondly, by reason 

 of its origin as a subsidence or dissolution basin on strata 

 overlying deposits of rock salt, it has been alleged that, 

 at the bottom of the mere, the water is salty, due to the 

 solution of rock salt from the underlying strata, and thus 

 affords a suitable home for Smelt. No evidence has, so 

 far as we are aware, been brought forward to support 

 either of these opinions. They seem to have been manu- 

 factured to fit the case, and we can find absolutely no 

 evidence in support of the first of them. As for the 

 second, it seems almost obvious that, if the bottom layer 



