EQUIPMENT. 



21 



Fig. 16. — Hilgard Salino- 

 meter. (Sigsbee. ) 



an ordinary aerometer. The specific gravity could also be de- 

 termined, as has been suggested by Professor Wolcott Gibbs, by 

 observing the index of refraction of the dif- 

 ferent samples of sea-water. 



Some of the most interesting problems 

 of biology are dependent for their solution 

 upon an accurate knowledge of the physics 

 of the salt water at the great depths from 

 which animal life has been brought up. 

 In order to analyze with accuracy the water 

 brought from the bottom, the cup in which 

 it is conveyed to the surface must be so 

 hermetically closed as not to allow any ad- 

 mixture of water from intermediate depths, 

 or any escape of the gases during the up- 

 ward passage of the water-cup. The water- 

 cups thus far employed for this purpose are 

 rather rude instruments, and do not guard 

 positively against these sources of error. Those in use on the 

 " Blake " seem to be the simplest, and superior in efficiency and 

 accuracy to any cups employed in the deep-sea explorations 

 preceding ours ; when once closed there is no danger of their 

 opening again from the pitching of the vessel or the stopping of 

 the upward motion. The propeller, which screws down the 

 valves, from the time the regular upward movement of the 

 bottle begins is, as it were, thrown out of gear, and cannot 

 undo the work it has once performed. As the cup descends, 

 the resistance of the water raises the valves, and also screws 

 up the propeller until the lower thread in the hub clears the 

 upper thread on the shaft, when the propeller uncouples and 

 revolves freely on it ; the blades of the propeller are bent on 

 their upper edge. It has been found, experimentally, that 

 with the blades thus bent, by rising and falling equal distances 

 through the water the propeller will screw up instead of down. 

 Without this bending it is evident that the propeller would 

 gradually screw down by a rising and falling motion. At any 

 stoppage each cup has within its cylinder a specimen of the 

 water from the place where it stops. (Figs. 17, 18.) 



