230 |” THE ATLANTIC. [CHAP. IIL 
can be applied in general practice to its present purpose with- 
out great modification. The instrument is bulky, and, with 
the battery, and galvanometer, and thermometers, and freezing 
mixture, troublesome to work on board a. ship, and difficult to 
observe if there is any motion. The cable is bulky and tender, 
and would probably not stand the strain of being often used as 
a sounding-line. 
Throughout this section Mr. Buchanan has taken the specific 
gravity of the sea-water from the surface daily, and from the 
bottom and from intermediate depths as often as it was possi- 
ble to obtain samples. A description of the instruments used 
in procuring the water from the bottom and from intermediate 
depths, and for determining the specific gravity, has been al- 
ready given (p. 47 et seq.). 
I need only mention here that, on the voyage from Tener- 
iffe to Sombrero, Mr. Buchanan found the remarkable and un- 
expected result that the water has virtually the same specific 
gravity from the bottom to within 500 fathoms of the surface. 
From 500 fathoms the specific gravity rapidly rises, till it usu- 
ally attains its maximum at the surface. Some minor variations 
in the specitic gravity, both of the upper and of the lower layers, 
have manifested themselves from time to time, but to these we 
shall refer hereafter. 
In the table of specific gravities appended, one or two in- 
stances occur (marked with an asterisk on the table) in which 
the specific gravity is as great at the bottom as it is at the sur- 
face. In these cases we are forced to believe that by some mis- 
adventure the cylinder of the water-bottle became disengaged 
just as it reached the water. The lowering of the slip water- 
bottle requires some care until it is fairly beneath the surface, 
after which there is no chance of the cylinder falling until the 
instrument reaches the bottom. 
In this our first section across the Atlantic, the dredge was 
lowered thirteen times, and nine times brought up a sufficient 
