34 THE DEPTHS OF THE SEA. (CHAP. T. 
feebly, and the spines and pedicellariz: moved on the 
shells of the urchins, but all the animals had evi- 
dently received from some cause their death-shock. 
Dr. Perceval Wright mentions’ that all the sharks 
brought up by the long lines from 500 fathoms in 
Setubal Bay are dead when they reach the surface. 
Various methods have been proposed to test the 
actual pressure at great depths, but as all the elements: 
in the calculation are well known, it is easier to work 
out the question in the study than in the field. A 
neat instrument was constructed for the American 
Coast Survey. A brass piston or plunger was fitted 
accurately into a cylindrical hole in the wall of a brass 
water-tight chamber. The chamber was completely 
filled with water, and a clasping index on the plunger 
marked to what extent the plunger had been driven 
into the water contained in the chamber by the 
extreme pressure. The required indication is no 
doubt given, but such an instrument is at the same 
time an extremely delicate thermoscope, and until 
lately there has been no perfect means of correcting 
for temperature. A more important application of 
the pressure-gauge is to check the accuracy of deep 
soundings. Probably the best arrangement which 
has been proposed for the purpose is a long capil- 
lary glass tube, calibrated and graduated to milli- 
metres, open at one end, and provided with a moveable 
index to show to what amount the air contained in 
the tube has been compressed by the entrance of the 
water. The principal objection to this device is the 
1 Notes on Deep Sea Dredging, by Edward Perceval Wright, M.D., 
F.L.S., Professor of Zoology, Trinity College, Dublin. (Annals and 
Magazine of Natural History, December 1868.) 
