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 ‘ 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- 
