4 
animal. In depths less than 750 fathoms it was found useful to attach 
two horizontal arms to the frame of the dredge, to which " swabs '' were 
secured, in the hempen tangles of which large numbers of animals were 
caught, so that the bottom of the sea was swept as well as scraped. 
The dredge rope was the best 2 inch Chatham rope, of which three 
miles were provided in coils upon the bulwarks. The dredge and 
its contents were "hove in" by a small double -cylinder donkey engine, 
at the rate of 1,000 fathoms per hour. By these means several cwt. of 
material were brought up at one time from the sea bottom at enormous 
depths, the utmost quantity obtainable by a sounding machine being 
about 21bs. The animals were separated from the sand or mud by a series 
of graduated sieves, on which the mud— which from depths over 500 
fathoms closely resembled china clay — was washed in a tub of water, and 
the animals were deposited upon the sieves, according to their size. 
The operation of sounding, or ascertaining the exact depth, was per- 
formed by a weight attached to the end of a small but very strong line, 
which was marked at intervals ; when the weight touched the bottom the 
distance of line run off was read. Many precautions had to be observed 
to obtain a correct depth, one of the most important being to use a suffi- 
ciently heavy weight in deep water, so that the line might continue to run 
out rapidly during the whole operation, and not run the risk of being 
deflected sideways by submarine currants. To avoid the risk of breaking the 
line and consequent loss of it and of the instruments attached, in drawing up 
the weight, an apparatus known as the " Hydra Machine" was employed, 
which caused the weight (frequently 3 cwt.) to detach itself on striking the 
sea bottom. Both the sounding line and the dredge rope were carried 
over pullies fastened to a derrick at the stern of the vessel, and these 
pullies were attached to an "accumulator," or set of india-rubber springs, 
by which the effect of any sudden jerk from the rise or fall of the vessel 
in the sea was much diminished, and the rope was consequently not 
strained or broken. 
The temperature determinations were made by a modified form of Six's 
self-registering thermometer, which was attached to the sounding line, 
^ immediately above the weight. Two or more instruments were employed 
for every observation. The modification was one, whose object was to 
counteract the error, caused by the effect of pressure upon thermometric 
bulbs in forcing the mercury too far up the column, thus causing the 
thermometer to register an apparently higher degree of temperature than 
really existed. This pressure amouuted to one ton per square inch 
