32 THE WATER 



the sea differs from that of the land in that it is not 

 opaque, but to a certain extent transparent, at least 

 to certain rays of light. Buchan (" Challenger " Report) 

 found evidence that the effect of the sun's rays was 

 appreciable to a depth of about 500 feet from the 

 surface in deep water ; more recent experiments have 

 traced it to still farther depths. Radiation must go on 

 to a certain depth, resulting in larger or smaller varia- 

 tions of temperature. Again, the surface of the sea is in 

 contact with the air ; the latter is constantly changing 

 its temperature, and these changes must be com- 

 municated to the sea water by conduction. The 

 combined results of these factors are given by Buchan, 

 who found that on the average of all the " Challenger " 

 observations a diurnal range of the temperature in the 

 air is three to four times that of the surface waters of 

 the sea over which it lies. 



2. The variations of temperature, taken along with 

 those of salinity, determine variations in specific gravity 

 in the manner already described, and they also furnish 

 valuable additional means of tracing the major move- 

 ments in the depth. 



With regard to the actual methods of investigation, 

 it may be definitely laid down that observations of 

 the temperature and the currents must be made at 

 sea ; but determinations of the density and chemical 

 composition, with sufficient accuracy for modern re- 

 quirements, can only be carried out on shore. Analysis 

 of the dissolved gases is a possible exception to this 

 rule, and reference should be made to the various 

 papers which have been published on the subject. 



Samples of water must therefore be collected for 



