28 THE WATER 



areas in which charts of all sorts remain blank to this 

 day, even as regards the depth ; and we know that 

 each point filled in by a new sounding is, beyond a 

 certain depth at least, practically filled in once for all. 



Thus there are, so far at least as physical oceanography 

 is concerned, two distinct kinds of work waiting to be 

 done the work at the surface and in moderate depths, 

 which can be undertaken more or less fully by practi- 

 cally every ship that puts to sea ; and the work in the 

 abysmal depths of the unfrequented parts of the great 

 oceans, which can be done by larger vessels, more 

 fully equipped and not restricted as to route or time. 



The following may be taken as the principal observa- 

 tions common to both types of work. It being 

 assumed that a vessel has arrived at a definite point 

 in the ocean, the latitude and longitude of which are 

 known, information is wanted on the following points : 



1. Depth. 



2. Temperature of water at surface, bottom, and 



intermediate depths. 



3. Composition of water. 



4. Specific gravity of water. 



5. Currents. 



6. Nature of bottom. 



The first and last of the above headings are dealt 

 with in another chapter, and we need only note here, 

 with regard to the first, that the collection of material 

 under the headings (2) and (3) can only be associated 

 with those methods of sounding in which the measure- 

 ment by vertical sounding-line is concerned ; the 

 methods depending on measurement of pressure, as in 

 Kelvin's sounder, are inadmissible. 



The object of making these observations, and of 

 collecting samples of water, being ultimately to ascer- 



