46 THE LAST CRUISE OF THE CARNEGIE 



present. After treating with the same chemicals a second solu- 

 tion (whose composition is known) we have only to match the 

 intensity of one color against the other to obtain a value for the 

 unknown sample. The presence of as little as one part of phos- 

 phate per billion parts of water can be detected in this way. 



When the reports of the oceanographer, the chemist, the 

 biologist are correlated, we have a good picture of the life of 

 plankton. We can see what limits of temperature and salinity 

 they tolerate; what substances they need for food; and what in- 

 fluence variations in sunlight, oxygen, and acidity have on their 

 growth. 



The usual equipment of a chemical laboratory is more familiar 

 and will be passed by. But there are, besides this, microscopes, 

 dissecting instruments, and preservatives for the use of the biolo- 

 gist. 



Over in the corner of the room is a self-recording sea-water 

 thermograph. This device keeps a continuous record of the 

 changes in surface-temperature as we sail down the latitudes. 

 A large bulb of mercury is mounted on the outside of the vessel's 

 hull. It communicates with the recorder through a capillary 

 tube. Any changes in the volume of the mercury in the system, 

 due to changes in sea-temperature, are transmitted through a 

 hollow coil-spring to a recording pen. 



A short walk forward, a few steps up, and we are on the "bridge." 

 From here we can look upward at the lofty rigging, more be- 

 wildering in detail than many of our instruments. Or, we may 

 look toward the forecastle-head and see, coiled on the deck, the 

 two great hawsers which serve us for anchor-chains. But a weird 

 object, suggesting an automaton in a brass helmet, stands at the 

 center of the bridge, challenging attention. This is the "marine 

 collimating-compass." It gives us the magnetic declination, or 

 "compass-variation" as sailors call it. 



The principles on which it operates are simple enough. We 

 wish to find the angular difference between true geographic north 

 and the magnetic north as indicated by the compass. We can 

 use the Sun as our point of reference, since we know its true 

 bearing from the ship by using the Nautical Almanac. In the 



