40 THE LAST CRUISE OF THE CARNEGIE 



These German-made thermometers are of two types. Some 

 are protected from the enormous pressures encountered in the 

 deeps, and give the true temperature. Others are unprotected, 

 and give a fictitious reading: the sum of the true temperature 

 and the effect of the pressure exerted mechanically on the naked 

 bulb by the weight of the water-column above it. The difference 

 between the readings of such a pair is then a measure of the pres- 

 sure. By rather complicated calculations we may then convert 

 this to meters of depth. 



The thermometers are sent down, inverted, in their frames on 

 the side of the Nansen bottles. They are given time to assume 

 the temperature of the surrounding water, and are then reversed 

 along with the bottle, when the messenger comes down the wire 

 from the surface. This reversal breaks the mercury-thread in 

 the tiny capillaries in such a way that the changes in temperature 

 and pressure encountered on the way back to the surface will 

 not be registered, and the observer on deck can get a true picture 

 of conditions at the desired depth. 



By the use of these readings and the salinity-values for each 

 sample, we are able to calculate"dynamic pressures" for each 

 water-level to the bottom. Plotting the figures on a chart we 

 can determine the speed and direction of the ocean-currents below 

 the ship — a subject of great importance to oceanography. These 

 charts are made in much the same way as weather-maps prepared 

 by the Weather Bureau — based as they are on pressure readings 

 taken at a multitude of stations, and from which winds can be 

 predicted. 



There are more direct means for measuring ocean-currents. 

 We may trace the course, speed, and direction of floating objects. 

 This is not satisfactory, for only the surface-current is represented, 

 and the effect of changing winds on the object may confuse the 

 true picture. A more useful method is to lower from an anchored 

 ship an instrument similar to an anemometer. We had insuf- 

 ficient power for hauling in a deep-sea anchor, and so we relied 

 entirely on the "dynamic-pressure" computations. 



The configuration of the ocean-floor is of great interest to 

 seismologists studying the movements of the Earth's crust. 



