34 THE DEPTHS OF THE SEA. [chap. i. 



feebly, and the spines and pedicellarire moved on the 

 shells of the urchins, hut all the animals had evi- 

 dently received from some cause their death-shock. 

 Dr. Perceval Wright mentions 1 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 io 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.) 



