Deep-sea Temperature. 3 1 



of vast bodies of movincr water thousands of miles in lenq^th, 

 hundreds of miles in breadth, and hundreds of fathoms in depth, 

 now flowing side by side, now passing one above the other. It 

 was shown, for the first time, that a polar current may prolong 

 its course along the bottom of the sea into tropical latitudes, as 

 it was known before that warm currents of equatorial origin 

 traverse the temperate zone and extend their life-giving influence 

 into the polar regions. 



Apart from the obvious difficulties that stand in the way 

 of carrying on a series of scientific observations on the high" 

 seas with delicate instruments, the chief desideratum in 

 these earlier experiments was felt to be in the possession of a 

 thermometer constructed to register with sufficient accuracy the 

 temperature of the water at great depths, and to minimise the 

 effects of the enormous pressure, amounting to about a ton on 

 the square inch for every thousand fathoms. This pressure, 

 exercised on the bulb of an ordinary thermometer, if it did not 

 effect the total destruction of the instrument, always caused the 

 latter to record a higher temperature than that of the water at 

 the depth to which it had been lowered, the difference increasing 

 with the depth to as much as 5° C. for 2000 fathoms, and often 

 more. 



The want of an efficient deep-sea thermometer, which more 

 or less vitiates all the observations of earlier explorers, was 

 especially experienced during the cruise of H.M.S. "Light- 

 ning" in 1868, the first expedition fitted out for sounding and 

 dredging purposes by the joint co-operation of the Royal Society 

 and the Admiralty of England. Of the thermometers used on 

 this occasion, several returned to the surface broken by the 

 pressure to which they had been exposed, and the indications 

 given by the rest varied so much as to render the discovery of 

 a thermometer free from this error a matter of paramount 

 importance for the success of future deep-sea exploration. By 



