206 THE PHYSICAL GEOGRAPHY OF THE SEA. 
2 m-. 21 s. as the average time of descent from 400 to 500 fathoms. 
3 m. 26 s. " " 1000 to 1100 " 
4 m. 29 s. " " «' 1800 to 1900 " 
435. Now, by aid of the law here indicated, we could tell very 
nearly when the ball ceased to carry the line out, and when, of 
course, it began to go out in obedience to the current and drift 
alone; for currents would sweep the hne out at a uniform rate, 
while the cannon ball would drag it out at a decreasing rate. 
436. The development of this law certainly was an achieve- 
ment, for it enabled us to show that the depth of the sea at the 
places named 428) was not as great as reports made it. These 
researches were interesting; the problem in hand was important, 
and it deserved every effort that ingenuity could suggest for re- 
ducing it to a satisfactory solution. 
437. As yet, no specimens of the bottom had been brought up. 
The Hne was too small, the shot was too heavy, and it could not 
be weighed. In this state of the case. Passed Midshipman J. M. 
Brooke, United States Navy, who, at the time, was associated 
with me on duty at the Observatory, proposed a contrivance by 
which the shot, on striking the bottom, would detach itself from 
the line, and send up a specimen of the bottom. This beautiful 
contrivance, called Brooke's Deep-sea Sounding Apparatus, is 
represented in Plates 11. and III. opposite. 
A is a cannon ball, having a hole through it for the rod B. 
Plate II. represents the rod, B ; the slings, D D, with the shot 
slung, and in the act of being lowered down. Plate III. repre- 
sents the apparatus in the act of striking the bottom, and shows 
how the shot is detached, and how specimens of the bottom are 
brought up, by adhering to a little soap or tallow,* called " arm- 
ing," in the cup, C, at the lower end of the rod, B. With this con- 
trivance specimens of the bottom have been brought up from the 
depth of more than two miles. 
438. The greatest depths at which the bottom of the sea has 
been reached with the plummet are in the North Atlantic Ocean, 
and the places where it has been fathomed do not show it to be 
deeper than twenty-five thousand feet. 
The deepest place in this ocean (Plate XI.) is probably between 
the parallels of 35° and 40° north latitude, and immediately to the 
* A Stellwagen cup is found to answer better. 
