36 ECHO-SOUNDING AND HYDROGRAPHICAL STUDIES 



placements still occur, and are liable to alter the depth levels at any time, 

 is shown by the fact that we trawled fresh fragments of rock at the very 

 lowest depths. Another interesting fact is that the centres of earthquakes 

 have been recorded at the same place as many as five times in 10 years. 

 It is precisely at these brittle spots in the earth's crust that the character 

 of the Philippine Trench suddenly changes. 



Just as we must have a knowledge of the depths in order to work the 

 gear, so too we must know the physical and chemical nature of the water 

 if we are to judge the biological potentialities. Consequently, we carried 

 out continuous hydrographical studies in close association with the biolo- 

 gical work. The choice of hydrographical stations was dictated by the 

 desire to obtain the utmost possible information about the sea area con- 

 cerned; but, the PhiHppine Trench apart, we established, as a general 

 rule, only one deep station in each area, owing to shortage of time. 



Some information about water at a depth is given in ancient literature; 

 Aristotle, for example, deduced that deep water is cold from the fact that 

 a plummet is cold when it comes up. Thermometer readings are of far 

 more recent date, and the technique remained imperfect down to the 

 adoption of the reversing thermometer in the present century. The first 

 was designed in 1874 by Negretti and Zambra of London. The instru- 

 ments now made, in several countries, work to an accuracy of 0.01° C. 

 The distinguishing feature of a reversing thermometer is a constriction 

 in the bore and a back-twist between this and the bulb. When the ther- 

 mometer has adjusted itself to the underwater temperature, it is reversed. 

 The mercury column then breaks at the constriction and runs down into 

 the opposite end of the bore, where the temperature is read off a scale 

 which is now upright. An auxiliary thermometer mounted alongside the 

 main instrument gives the temperature on deck, and it is then possible 

 to reckon the extent by which the temperature recording of the main 

 thermometer has changed since its reversal under water. The two ther- 

 mometers are enclosed together in a stout glass sheath, factory-tested for 

 a pressure of 500 atmospheres, which is the pressure at a depth of 5,000 

 metres. The object of the sheath is to prevent the thermometer bulb from 

 being compressed and squeezing the mercury too far up the bore. A pro- 

 tected thermometer mounted alongside an unprotected one is the means 

 of checking the depth of the instruments. 



The thermometers are mounted in pairs on a reversing water bottle, 

 which when lowered is open at both ends. By means of a messenger weight 

 which is slid down the wire the bottle is made to reverse, and the same ope- 

 ration closes the end plates on the sample of water inside. Below the water 



