52 THE ATLANTIC. [cHapP. I. 
shut they point downward at about an equal angle. The beam 
connecting them carries a metal plate, E, of about thirty square 
inches area, secured by a pin, F, round which it moves freely 
through two right angles, when pushed upward; but when 
pulled downward it is arrested in a position at right angles to 
the beam by a tongue, G, resting upon a spring, H, the strength 
of which is such that, before giving way, the rush of the water 
past the plate will shut the stop-cocks. The plate then passes 
the spring, and any reversal of the motion of the apparatus will 
only reset the plate, and, on heaving up, shut the stop-cocks 
closer. As the water is thus hermetically inclosed, it is neces- 
sary to provide for its expansion on coming to the surface. 
This is secured by means of a safety-valve, K, the tube L of 
which penetrates well into the interior; so that, supposing the 
water to be overcharged with gas, it would, on coming to the 
surface, only suffer water to escape, the gas remaining at the 
top of the instrument. Brass funnels, M, M, are fitted at top 
and bottom, so as to give a greater draw through. They un- 
ship, and can be replaced by nozzles, N, screwing down upon 
washers air-tight, to the top of one of which can be affixed a 
gas-collecting apparatus, such as Bunsen used for boiling the 
gases out of water. The brass water-bottle thus replaces the 
flask ordinarily used, and renders transvasing unnecessary. In 
order that the instrument may answer this purpose, the parts 
must be fitted with the accuracy of an air-pump. Unfortu- 
nately, those in use on board the Challenger had to be finished 
so hastily that this condition could be only imperfectly com- 
plied with. 
The instrument arranged for sinking is represented in Fig. 
12,1; in Fig. 12, II, in section, when the line is checked on 
commencing to haul in; and in Fig. 12, ITI, in perspective, after 
it has been brought up, and with one of the funnels replaced 
by a nozzle. Care is necessary, in using the instrument, to see 
that the stop-cocks work easily, but not so easily as to make 
