CHAP, vii.] DEEP-SEA TEMPERATURES. 299 



lashed to the sounding-line at a little distance from 

 one another, a few feet above the attaching ring of 

 a ' detaching ' sounding instrument. The lead is 

 run down rapidly, and, after the weight has been 

 disengaged by contact with the ground, an interval 

 of five or ten minutes is allowed to elapse before 

 hauling in. The shorter of these periods seems to 

 be quite sufficient to insure the instrument acquiring 

 the true temperature. In taking serial temperature 

 soundings that is to say, in determining the tem- 

 perature at certain intervals of depth in deep water 

 the thermometers are attached above an ordinary 

 deep-sea lead, the required quantity of line for each 

 observation of the series run out, and the ther- 

 mometers and lead are hove in each time. This is 

 a very tedious process ; one serial sounding in the 

 Bay of Biscay, where the depth was 850 fathoms 

 and the temperature was taken at every fifty 

 fathoms, occupied a whole day. 



I ought to mention that in taking the bottom 

 temperature with the Six's thermometer the instru- 

 ment simply indicates the lowest temperature to 

 which it has been subjected ; so that if the bottom 

 water were warmer than any other stratum through 

 which the thermometer had passed, the observation 

 would be erroneous. This is only to be tested by 

 serial soundings, but in every locality where the 

 temperature was observed during the f Porcupine ' 

 expeditions the temperature gradually sank, some- 

 times very steadily, sometimes irregularly, from the 

 surface to the bottom, the bottom water having been 

 constantly the coldest. It is probable that under 

 certain conditions in the Polar seas, where the sur- 



