[83] FISHERIES OF THE UNITED STATES. 595 



much within the margin of safety ; and on relowering, the propeller in 

 that case would again rise and uncouple. However, the bending of 

 the blades overcomes any bad effects from the motion of the vessel, and 

 the valves are free to open during the whole descent withouc regard to 

 the number of stoppages made. At any stoppage each cup has within 

 its cylinder a specimen of the water from the place where it stops. 



On hauling in, the propeller of each cup screws down, by the resist- 

 ance of the water, until the upper thread of the hub clears the lower 

 thread of the corresponding screw on the shaft, when the propeller 

 drops on the screw-follower, which until that time has been at rest, the 

 lugs of the propeller clutch into the slots of the follower, and the latter 

 is screwed down until it touches the upper valve, thus closing both 

 valves. It is evident that the follower can be got out of this second 

 position only by hand, for the lugs and slots being beveled the former 

 can clutch the follower only in one direction. The resistance of the 

 water, which would reverse the propeller at a stoppage on the ascent, 

 would also lift the propeller clear of the screw-cap. If each propeller 

 were regulated to close down the follower in passing through equal dis- 

 tances in water, each cup would be locked when the cup had passed 

 through that distance after the beginning of the ascent. The follower 

 is found screwed down so tight in coming out of the water that it can 

 be set no tighter without endangering the thread. This favorable re- 

 sult is doubtless due, to some extent, to the expansion of the several 

 metallic parts after leaving the frigid water of the lower depths. 



Water Specimen Cup, for obtaining a single specimen at each haul, used 

 by the United States Coast Survey before the invention of Sigsbee's 

 water specimen cup. Represented by a diagram (Plate 19, of Sigs- 

 bee's series). 



United States Coast and Geodetic Survey. 



The structure of this cup is clearly shown on the plate. It is fur- 

 nished with independent poppet valves, which are free to open and close 

 at all times. 



Hilgard's Ocean Salinometer, for determining the density of sea water by 



means of a glass float. 

 Prof. J. E. Hilgard, Superintendent United States Coast Survey. 



"This instrument consists of a single float, about 9 inches in length. 

 The scale extends from 1.020 to 1.031, in order to give sufficient range 

 for the effect of temperature. Each unit in the third place, or thou- 

 sandths of the density of fresh water, is represented by a length of 0.3 

 of an inch, which is subdivided into five parts, admitting of an accu- 

 rate reading of a unit in the fourth place of decimals by estimation. 

 The float is accompanied by a copper can, with a thermometer inserted 

 within the cavity, which is glazed in front. In use, the can is nearly 



