EQUIPMENT. 21 
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 н 
of the salt water at the great depths from ғ. 
which animal life has been brought up. ^ 
In order to analyze with accuracy the water d 
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- = | 
eups thus far employed for this purpose are олы m 
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 eannot 
undo the work it has once performed. As the eup descends, 
the resistance of the water raises the valves, and also serews 
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.) 
