CHAP, i.] INTRODUCTION. 37 



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 



o O ' 



cold currents are circumscribed and localized, and tins 

 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. 



