102 A TEXTBOOK OF OCEANOGRAPHY 



acid per litre, and in autumn considerably more, up to 1*46 

 milligrams per litre. 



The same reasoning applies to silica, which is present in 

 North European waters in from 0^65 to 1*45 milligrams per 

 litre. There is, a yearly periodicity, the minimum being in 

 May and the maximum in February. This appears to be 

 connected with the enormous growth of diatoms, silica- 

 utilising organisms, which takes place between February 

 and May. 



SALINITY. 



An important subject for the oceanographer's consideration 

 is the distribution, both horizontal and vertical, of the salinity 

 of the waters of the sea, and for the vertical distribution it is 

 necessary carefully to collect samples of water from varying 

 depths. For this purpose many kinds of " water-bottles " 

 have been devised, one of which, that of Pettersson-Nansen, 

 is described here. The bottle is really a device for securing a 

 sample of sea-water from any required depth, the sample being 

 insulated in such a manner as to prevent any change in the 

 temperature of the water during the time the bottle is being 

 hauled inboard. 



The bottle, which consists essentially of three concentric 

 cylinders of a non-conductive medium (vulcanite), is lowered 

 to the required depth when open, and closed by means of a 

 messenger sent down the wire. In the upper lids of the bottle 

 a deep-sea thermometer is fixed. Water is thus collected from 

 the depth to which the bottle has been lowered, and this 

 instrument is found satisfactory up to depths of 500 to 800 

 metres (273 to 382 fathoms). The temperature is checked by 

 means of a delicate deep-sea thermometer of the Richter type, 

 which is attached to one arm of the Pettersson-Nansen 

 apparatus outside the bottle, and which is inverted by the 

 same messenger which closes the bottle (not shown in the 

 figure). If the bottle be used for depths of 400 fathoms and 

 over, then, although the isolation of the enclosed water sample 





