SECT. 2] 



UNDERWATER PHOTOGRAPHY 



475 



Fig. 1. Photograph of oceanographic camera interior showing "in-line" construction in a 

 cylindrical steel housing. A "data" chamber at the rear of the camera gives a photo- 

 graphic record of j^ressure (depth), time (24-h clock), positional data, and notes. 



wrongly or accidentally interchanged. One examjjle of a data-chamber record 

 is shown in Fig. 1. 



B. Sonar 



A sonar "pinger" on a camera, operated by internal batteries, has proved 

 to be very useful for signalling information to the surface. The sonar can be 

 used as a "go" or "no-go" device operated by a bottom switch. Also, it can be 

 used to measure the camera-bottom surface by measuring the time interval 

 betw^een the direct ping and the reflected ping, since the velocity of sound in 

 sea-water is known (about 5000 ft/sec). A 5-ft sonar-bottom distance corre- 

 sponds to a 2-msec interval, which can be conveniently measured on a cathode- 

 ray oscillograph or a standard graphical recorder such as is commonly used as 

 a sonar receiver on oceanographic shijjs. 



3. Cables and Winches for Cameras 



Many of the practical problems of johotography in the sea are caused by 

 lowering methods. Most oceanographic ships are equipped with winches which 

 do many duties besides camera lowering, such as the lowering of Nansen 



