SURVEYING THE ROUTE. 33 



peaks or banks, the result chiefly of volcanic action, and of 

 comparatively small area, are not easily found by the ordinary 

 method of sounding at positions a few miles apart. When it ig 

 possible to have a vessel sounding ahead of the cable steamer 

 during the laying of a cable, the chances are that banks will be 

 detected in time to alter the course, but while this was fre 

 quently done in the early days of cable laying, when such 

 enterprises received the assistance of the Government, the 

 practice fell into disuse as the art became better understood. 

 To take the place of this a system of sounding is organised on 

 the cable ship while cable is being laid, and for this purpose it 

 is not necessary to find the actual depth, but to ascertain if 

 any rise in the ocean bed of importance occurs along the route. 

 Various forms of sounding apparatus, applicable while the 

 ship is in motion, are used for this purpose. Prof. Lambert, M. A., 

 of the Eoyal Naval College, Greenwich, enumerated the most 

 important of these in his interesting lecture at the Eoyal 

 United Service Institution on June 3rd, 1891, entitled 

 "Sounding Machines for the Prevention of Strandings, with 

 Special Reference to James' Submarine Sentry." Reprints of 

 this lecture, to which the reader is referred, can be obtained at 

 the office of the Submarine Sentry, 18, Billiter-street, London, 

 E.G. First and foremost comes the compressed air depth gauge 

 and depth recorder of Lord Kelvin, the former instruments 

 being usually known as "Thomson's Tubes." A thin glass 

 tube about 2ft. long, open only at the bottom, is let down 

 enclosed in a brass case with sinker. Water rises in the tube 

 proportionally to the depth and pressure, compressing the air 

 within the tube, and in doing so washes away a red chemical 

 preparation with which the inside of the tube is coated, thus 

 leaving a record of the greatest depth to which the instrument 

 has been lowered. After hauling in, the indication is reduced 

 to fathoms depth by reference to a table, which also supplies 

 the necessary correction for barometric pressure. Other forms 

 are Cooper and Wigzell's piston type of pressure instrument ; 

 Basnett's compression gauge, which retains the water column ; 

 Burt's "Bag and Nipper"; and the Submarine Sentry, 

 invented by Mr. Samuel James, C.E. The advantage of the 

 latter apparatus is that it can be towed by the ship at a fixed 

 depth within a considerable range of speed, and automatically 



