212 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1936 



essentially a piece of steel, 2 inclies by 1 inch by one-fourth inch, slid- 

 ing: in an appropriate keyway and containing a projection which 

 catches the end of the firing pin when "cocked." A slight downward 

 movement of the gun, on reaching the bottom, forces the trigger over 

 and disengages the firing pin, which is pushed forward by a stiff coiled 

 spring. The firing pin is streamlined at its forward end and is 

 grooved longitudinally to facilitate the movement of water out of the 

 space progressively occupied by the pin as it advances. This elimi- 

 nates a cushioning of the blow by the water. The forward end of the 

 pin contains a conical tip of appropriate size and shape to enter the 

 hole in the base of the cartridge and explode the primer through the 

 copper primer disk. A safety pin of hardened steel is so situated that 

 it holds the firing pin back, in the cocked position, even when the 

 trigger is disengaged, and even if the gun should be forced down and 

 shear this safety pin off on the outside, that w^hich remains would 

 prevent an accidental discharge. As this safety pin is i)ut in place 

 before the cartridge is attached, a premature discharge, even under 

 most extreme conditions, is almost impossible. After the apparatus is 

 over the side of the ship, and just before it is lowered, this pin is 

 withdrawn (by means of a lanyard, if desired) and the apparatus is 

 thus "armed." Should it be necessary to return the apparatus to the 

 deck, before firing, this pin can be inserted again before the apparatus 

 is hoisted over the side. 



WATER-EXIT PORT 



Early designs contained ample openings in the walls of the bit tube 

 at its top, and, should the bit be forced slowly into mud, the displaced 

 water would flow out through these. But because of the high velocity 

 of the bit, at the time of firing, the water within it acted as a solid 

 body and did not yield space for the mud to enter. The ideal condition 

 would be a bit tube completely open at both ends, which would then 

 pass through the water and mud, leaving them both stationary. But a 

 perfectly open top is not mechanically attainable because of the neces- 

 sity of keeping the violent blow of the explosion accurately centered 

 along the axis of the bit. Furthermore, this powerful blow must be 

 mechanically carried to the walls of the bit tube. This necessitates 

 rugged construction between the center axis and the outer walls. After 

 much experimentation the open-tube ideal was very closely ap- 

 proached, by taking advantage of aerodynamic and wind-tunnel data 

 and modifying the best curves in accord with the greater density of the 

 water medium. The exit port somewhat resembles a nozzle in reverse. 

 The inner walls slope outward along an ideal curve, and the center 

 projection is so shaped that the cross-sectional area (hence, volume) 

 available to the water is the same at any plane normal to the axis. 



