PHYSICAL OCEANOGRAPHY 



sending down 

 instruments. 



many important problems we have not yet sufficiently numer- 

 ous observations. In a rapid sketch like this, only some of 

 the principal facts can be dealt with ; we shall first examine 

 the methods employed in physical oceanography, and then 

 endeavour to draw some conclusions from the observations 

 available. 



In the first place, one must have a line with which to send Lines for 

 down the instruments and draw them up again. Formerly 

 hemp lines were used, but they have now been superseded by 

 wire ; steel piano-wire is used for sounding, and wire rope 

 for thermometers, water-bottles, etc. For general use the wire 

 need not be more than 2 to 3 mm. in 

 diameter, and it will, nevertheless, bear 

 the weight of several hundred kilograms 

 without breaking. The old hemp line 

 was marked at regular intervals for the 

 determination of the depth, but this can- 

 not well be done with the wire, which is 

 run out over the metre- or fathom-wheel Metre-wheel. 

 (see Fig. 151), and this is both a con- 

 venient and accurate method. The 

 wheel communicates with a clock-work 

 arrangement with dials and hands, by 



means of which the length of wire run 



Fig. 151. — Metre-Wheel. 



out can always be read off correct to 

 within a metre. When, however, an 

 observation is to be taken at a depth of 

 1000 metres, it is not enough to run out 

 1000 metres of line. The line must be 

 "up and down," and this is not always 

 easily managed, especially in a wind or strong current, when 

 the ship is drifting. Some manoeuvring is then required, and 

 the apparatus must either in itself be sufficiently heavy to 

 straighten the line, or an extra weight must be added. Many Several 

 of the instruments are so constructed that they may be attached lIsed's'imuK 

 to the side of the line as well as at the end, and thus several taneousiy. 

 instruments may be used simultaneously. They are fastened at 

 certain intervals on the line as it is being paid out, and a 

 number of observations are made at the same time at different 

 depths. By this method a comprehensive series of observations 

 from the surface down to two or three thousand metres may be 

 taken in a couple of hours. This method was employed during 

 the "Challenger" Expedition. 



