312 PHYSICAL GEOGRAPHY OF THE SEA, AND ITS METEOROLOGY. 



Thus the mean of oiir experiments gave us, for the sinker and 

 twine used, 



2 m. 21 s. as the average time of descent from 400 to 500 fathoms. 



„ 1000 to 1100 „ „ 1000 to 1100 



4 m. 29 s. „ „ „ 1800 to 1900 



572. The law of the jdummefs descent. — 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 line out at a uniform rate, while the cannon- 

 ball would drag it out at a decreasing rate. The development 

 of this law was certainly an achievement, for it enabled us to 

 show that the depth of the sea at the places named (§ 567) w*as 

 not as great as reports made it. These researches were interest- 

 ing : the problem in hand was important, and it deserved ever}'- 

 effort that ingenuity could suggest for reducing it to a satis- 

 factory solution. 



573. Brooke's sounding ajpparatus. — As yet no specimens of the 

 bottom had been brought up. The line was too small, the shot 

 was too heavy, and it could not be weighed ; and if we could 

 reach the bottom, why should w^e not know its character ? 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 b}^ 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 

 on p. 313. A is a cannon-ball, having a hole through it for the 

 rod B. Figure 1 represents the rod B, and the slings D D, with 

 the shot slung, ready for sounding. Figure 2 represents the 

 apparatus in the act of striking the bottom ; it shows how the 

 shot is detached, and how specimens of the bottom are brought 

 up, by adhering to a little soap or tallow,* called " arming," 

 in the cup C, at the lower end of the rod B. AVith this con- 

 trivance specimens of the bottom have been brought up from the 

 depth of nearly four miles. 



* The barrel of a common quill attached to the \vA has been fouud to 

 answer better. 



