Page 415 equipment and instruments 476 



the lowering line is attached. The air is exhausted from the sphere to form a partial 

 vacuum within the cavity, and when the instrument is lowered, water is allowed to flow 

 into the tube. The sphere is provided with a valve which opens when the cutting edge 

 of the tube strikes bottom, allowing the water in the tube to flow into the sphere. The 

 enormous hydrostatic pressure created forces the tube into the bottom. This type of 

 coring instrument has been successfully used to obtain bottom samples in moderate 

 depths of water, but there is no report yet of its having been tested at great depths. 



4764. Plggot Gun 



The most successful type of coring instrument for deep-sea samples is the Piggot 

 gun. This instrument uses an explosive charge to drive a metal bit into the bottom at 

 the instant the sharp end of the bit strikes the bottom. The quantity of explosive may 

 be varied for different depths and different types of bottom, to make the full length of 

 the tube penetrate into the bottom, provided it is neither hard nor rocky. It has been 

 used at a maximum depth of 2,800 fathoms in the Bartlett Deep, but its limit is apparent- 

 ly only the greatest depth of the ocean. 



In construction, the Piggot gun is simple but ingenious. It consists of five principal 

 parts: a weight or gun, a cartridge, firing mechanism, water-exit port, and a tube or bit. 



The gun is made of cold-rolled steel, 10 inches in diameter and 20 inches long, with a 1-inch eyebolt on top, to which the lowering 

 line is attached. The gun has an opening in the lower end forming a straight smooth bore, slightly larger than 2 inches in diameter 

 to furnish a snug fit (clearance 0.0002 to 0.f005 inch) for the 2-inch diameter cartridge and firing-mechanism housing. It is a muzzle- 

 loading gun and the bore is only deep enough to accommodate the combined lengths of the cartridges and firing-mechanism housing. 

 The lower end of the gun is tapered at 45° to reduce the diameter to 4 inches, at the muzzle. One inch above the end of the muzzle four 

 holes are drilled radially, into one of which a }i-lnch brass shear pin is fitted to hold the mechanism in the bore until it is sheared by the 

 force of the explosion. 



The cartridge is made in three sections, a midsection tliat contains the explosive charge and a section on each end to seal it water- 

 tight. There are three parts to the explosive charge, one of a variable number of pellets of howitzer powder depending on the force 

 required, 2 grams of fine-grained powder to ensure ignition and build up a pressure quickly, and a 30-30 riP.e primer centered in the 

 lower end of the cartridge which the firing pin strikes to set off the charge. The upper section of the cartridge contains a rupture disk 

 that seals it against the pressure of the water and ruptures when the explosive charge is ignited. A thin copper disk seals the lower end 

 of the cartridge and covers the primer, but is sufl5ciently dented, when struck by the firing pin, to set oS the primer charge. 



The firing mechanism consists of a trigger, a firing pin, and a stiff coil spring housed in a brass easing which is attached to the lower 

 end of the cartridge by a baj-onet socket. The trigger is a flat piece of metal that fits into an appropriate horizontal keyway, so located 

 that it will be at the muzzle of the gun when assembled. The outer end of the trigger protrudes beyond the bore of the gun and is 

 beveled, while the inner end is notched to receive the end of the firing pin when cocked. The downward momentum of the gun, after 

 the bit has struck bottom, slides the trigger across in the keyway, releasing the firing pin. The coiled spring then forces the firing pin 

 against the thin copper disk, setting off the primer charge. The trigger is provided with a safety pin which is withdrawn after the 

 instrument is over the side and ready to be lowered. 



Between the firing mechanism and the bit, a water-exit port of special design is screwed which allows the water to flow rapidly out 

 of the bit. The high velocity with which the bit is discharged from the gun requires a streamlined exit port of diameter larger than the 

 bit, which also serves to engage a collar stirrup, that fits around the bit, providing a means for extracting the bit from the bottom. 



The bit in use in 1940 was a tube 10 feet long and 214 inches in diameter, made of alloy steel to give it great strength. The inside 

 of the bit contains four longitudinal lands and the grooves between the lands lead into four outside openings which admit water, which 

 flows to the lower end of the bit to fill the cavity when the bit is withdrawn from the bottom. A brass tube fits inside the bit 

 against the lands. The bottom sample is taken in this brass tube, both tube and sample being removed from the bit after return to 

 the surface. The bottom end of the bit contains a cutting edge attachment, made of hardened tool steel, that fits loosely between the 

 sample tube and bit. This attachment is held in place by two small screws, through slots that provide for a slight vertical movement . 

 When the bit penetrates the bottom the attachment is jammed against it, but when the bit is withdrawn, the attachment opens slightly 

 to allow water to flow out of the grooves. 



The assembled Piggot gun is suspended by a cable attached to the top of the gun. The firing mechanism is attached to the lower 

 end of the cartridge and they are both held in the bore of the gun by means of a brass shear pin in one of the four holes provided in the 

 muzzle. The water-exit port, with bit attached, is screwed into the lower end of the firing mechanism. The stirrup is placed on the 

 bit and adjusted so that it hangs near the lower end by a lanyard attached to the sounding cable. When the bit strikes bottom, the 

 weight of the gun operates the mechanism that actuates the trigger to release the firing pin. The exploded charge blows the assem- 

 bled parts into the bottom, separated from the gun but still attached to the sounding cable by means of the stirrup and lanyard. When 

 the instrument is raised, the collar of the stirrup slides up the bit until it comes in contact with the enlarged diameter of the water- 

 exit port and the bit is withdrawn from the bottom. 



