SEA-FISHERIES LABORATORY 281 



points midway between tliese stations have been observed 

 in the cruises of the last three or four years, and it has 

 been noticed that the variation is fairly regular, and can 

 usually be predicted. 



(3). With very few exceptions hydr-ographic 

 soundings have been made on the periodic cruises at 

 Stations 5, 6, and 7. The object of this practice has not 

 been to investigate temperature or salinity stratification ; 

 for it was apparent, when these investigations had been 

 in progress for a short time, that the Irish Sea, North of 

 Holyhead, was an homothermic and homosaline water 

 mass ; and that the differences in temperature and salinity 

 between surface and deep layers was only significant in 

 the shallow water near the coast of Lancashire. It was 

 seen that these differences were relatively large only in 

 calm weather in the summer, when relatively warm and 

 light water was carried by the tide from the heated sand- 

 banks along the land, and overlay water w T hich was colder 

 and denser. Soundings are, therefore, made on the 

 Holyhead to Calf of Man line, in order to provide a check 

 against any " accidental " conditions, or undetected errors 

 in the observations. The mean temperatures at the sur- 

 face at each station, for each sample month throughout 

 the period 1907-1912 have, however, been calculated 

 (Table VIII), and also the mean differences between this 

 temperature and that of the bottom water. The difference 

 is, of course, negative in all cases, and is maximal at 

 Station 5 in August, when the mean bottom temperature 

 is 0'4° C. lower than that at the surface. 



The difference between the temperature at the 

 surface at these stations is, therefore, very small, and 

 indicates nothing more than the small effect (on this 

 line) due to the greater proximity to the land of the 

 Stations 5 and 7. The differences between surface and 



