CHAP. 1. ] INTRODUCTION. SH 
the northern extension of the gulf stream, the whole 
column of water from the surface to the bottom is 
reduced to the lowest temperature which it will bear 
without freezing, and is thus an ample source of the 
coldest water of the highest specific gravity. 
The proof that the flow of the cold indraught is 
almost secular in its slowness, is that over a large 
portion of the ocean where the low bottom tempera- 
ture is known to prevail, the sea-bed is covered with 
a light fleecy deposit of microscopic organisms of 
great delicacy, into which the sounding-lead has in 
some instances sunk several feet, and which must 
inevitably be drifted away by a current of appreciable 
velocity. In all places where any perceptible current 
exists, the bottom consists of sand or mud or gravel 
and rolled pebbles. In some cases also, sounding in 
the deep water of the mid-Atlantic, the line, after 
running out greatly in excess of the depth, has been 
found to have coiled itself in a tangled mass right 
over the lead—a proof of almost absolute stillness. 
In some places, owing to the conformation of the 
neighbouring land or of the sea-bottom, warm and 
cold currents are circumscribed and localized, and this 
sometimes gives us the singular phenomenon of a 
patch or stripe of warm and a patch of cold sea meet- 
ing in an invisible but very definite line. There is 
a curious instance of this in the ‘cold wall’ which 
defines the western border of the gulf stream along 
the coast of Massachusetts, and another scarcely less 
marked which we discovered during the trial cruise 
of the ‘ Lightning’ has been fully described by Dr. 
Carpenter in his report of that cruise, and will be 
referred to hereafter. 
