20 THE WORLD OF THE SEA. 
passes over a pulley fixed at a higher elevation. By this the diver 
is let down or pulled up. A watchful assistant holds in his hand 
the little cord used as a signal. The diver is sunk by wearing 
shoes of lead, which aid him at the same time in keeping an 
upright position when at the bottom. It only takes about two 
minutes to draw the diver out of the water, and take off his 
helmet. Mr. Edwards descended with this apparatus twenty- 
eight or thirty feet, and his undertaking was crowned with com- 
plete success. In these submarine excursions, this clever naturalist 
could study on the spot in their most secluded retreats, and in 
seemingly inaccessible places, radiate animals, mollusks, crusta- 
ceans—especially their larvee and eggs, and by his descriptions has 
contributed largely to make known the developments, the functions, 
and the habits, of a number of the sea inhabitants, whose manner of 
life appeared to baffle all our efforts at their investigation. It has 
been proposed in all researches which require a long sojourn 
beneath the surface, to use the diving vessel of Lamiral and 
Payerne. This vessel is a reservoir of compressed air. It fur- 
nishes the means of respiration without communication with the 
surface ; it facilitates direct contact with submerged objects, and 
permits easy locomotion at the bottom of the sea. 
It is even possible to study the living beings which are shel- 
tered by the sea, by preserving them in convenient reservoirs. It 
is to M. Charles des Moulins of Bordeaux we are indebted for 
the possibility of carrying on this study at home (1830). 
When we place some mollusks, crustaceans, or fish, in a bowl of 
fresh water, the liquid loses its transparency at the end of a few 
days, and little by little becomes impure, so as to render it necessary 
to change this water from time to time, a process which not only 
disturbs the creatures, but causes them much suffering, and even 
sometimes is the means of their death ; in addition to which the fresh 
water has not always the same composition, the same temperature, 
nor the same quantity of air mixed with it as the old liquid had. 
M. Charles des Moulins proposed to put into the vase a certain 
number of aquatic plants, floating or submerged, such as duck-weed, 
for example, since these plants affect the water in the very opposite 
way to animals. We know that vegetables decompose carbonic 
