PRESIDENTIAL ALDRESS. 23 



Freezing point ... - 2*335 C. 



Total solids ... ... 43 '8 gm. per kilo, sea water. 



Halogens as chlorides 24*29 gm. per kilo, sea water. 

 Sulphates as S0 4 ... 3*59 gm. per kilo, sea water. 



The ratio of halogens to sulphates which is close to that found in 

 all samples of sea water is evidence that this water is sea water 

 concentrated by evaporation." 



TIDES. 



On the open coast the tides are regular, and of consider- 

 able amplitude. A stream of flood tide runs northwards, 

 and the ebb southwards. At the entrance to the harbour 

 it is high water at new and full moon at 8*15 when the 

 spring tides rise about six feet. Further up the harbour 

 at Circular Quay, the tides are twenty-five minutes later, 

 and about ten inches lower. The highest tide recorded in 

 the harbour was that of January 5th, 1912, which amounted 

 to six feet nine and a quarter inches. 



A curious phenomenon is the difference between the night 

 and the day tides in summer and winter. In the winter 

 the night tides are highest but in summer those of the day. 

 After the equinoxes the difference gradually increases till, 

 in July and January, it may amount to as much as two 

 feet. The sedentary intertidal organisms are exposed to 

 both extremes of air temperature, cold in winter and heat 

 in summer. The on and off shore winds may magnify or 

 diminish any particular tide. 



Comparison of Local and Foreign Fauna. 



Both in species and in individuals, the local fauna is 

 extremely rich, probably more so than in any marine area 

 in the temperate zone of the northern hemisphere. Rocks 

 are turfed with ascidians for yards without exposing an 

 inch of stone. In other places oysters sheath the rocks 

 with a continuous crust. There is a crab that marches in 

 regiments. Even above ordinary tide level a small gre- 



