attention is first directed to fig1 of the drawings . there , a casing 10 is cemented in a well borehole . it passes through a strata or formation 11 which will be perforated . that is , the casing 10 will be perforated , the perforations extending into the formation 11 such that the formation will produce into the well . the casing 10 is customarily cemented in place and the perforations must therefore pass through the casing , the cement on the exterior to open passages into the formation 11 to produce the formation . as a preparatory step , a bridge plug 12 is positioned at a suitable depth below the formation 11 to close off the casing . a packer 13 is set above the formation 11 . the packer 13 will be subsequently used to register the equipment as will be described , and seals to enable isolation of the zone between the plug 12 and the packer 13 . in this area , the pressure is ultimately controlled prior to detonation of the shape charges . the numeral 14 identifies a tubing string . it supports a tcp gun assembly generally indicated at 15 . the tcp gun assembly 15 includes a firing mechanism identified in dotted line at 17 . it is normally triggered or fired by means of a sinker bar shown at dotted line 18 . alternatively an annulus pressure operated system such as that shown in u . s . pat . no . 4 , 509 , 604 could be used if desired . the present invention can be used as a backup firing system in either case . in a sinker bar activated primary firing system the sinker bar strikes the firing mechanism with sufficient force to intiate firing , the detonation being conducted to the various perforating shaped charges by means of a primer cord in dotted line 19 . the primer cord extends from the firing mechanism 17 down past the many perforating shaped charges . the number of shaped charges is varied , potentially several hundred shaped charges to perforate over a lengthy region . the bottom of the tcp assembly 15 supports the tubing pressure operated firing initiator 20 of the present invention . in the event of malfunction of the first detonator assembly 17 , the backup system 20 is then used . the remoteness of the backup system isolates it from damage that might arise on partial detonation . moreover , the firing assembly 20 is also remote and independent of operation of the sinker bar 18 in the event the sinker bar snags or does not otherwise reach the bottom of the tubing 14 . the backup firing mechanism is an alternate structure which is conveniently threaded to the lower end of the tcp gun assembly 15 which will be described in greater detail on reference to fig2 of the drawings . in fig2 of the drawings , the numeral 21 identifies a subassembly which has a threaded connection with the tcp assembly 15 and vented sub opening into the casing 10 below the packer . thus , the entire backup initiator assembly 20 suspends below the bottommost gun and is spaced sufficiently therefrom that it is able to operate without apprehension that misfiring of any of the equipment above will prevent safe and certain operation . the sub 21 is hollow , having an internal passage 22 . the passage 22 extends upwardly to the assembly 15 above , supporting a primer cord 23 which extends along the passage . the sub 21 is threaded to a suitable housing 24 by threads 25 , and a suitable seal is perfected by seals at 26 . the housing also is provided with a passage through the center , enabling the primer cord to extend through it in the same fashion . the housing 24 has an enlarged central opening at 27 to receive a primer assembly 28 . the primer assembly is seated adjacent to the end of the primer cord 23 . they are placed adjacent to one another to assure that the explosive in the primer assembly 28 intiates detonation of the primer cord . the primer 28 is a cylindrical assembly which is held in the housing 24 by means of a threaded bolt 29 . the bolt 29 threads to the lower end of the housing 24 to lock the primer in location . the entire mechanism 20 disassembles near the primer 28 to enable it to be replaced after using . to this end , there is a firing pin housing 30 which is below the housing 24 . the housings thread together at a threaded connection 31 . joinder of the two housing forms a closed , sealed chamber , therebeing seals 32 between the two housings to assure isolation of the chamber for the primer 28 . the bolt 29 is axially hollow at 33 to receive a pointed firing pin 34 impacting against the primer 28 . the firing pin travels axially through a passage in the firing pin housing 30 . the firing pin 34 has a tip which is shaped to strike and intiate detonation from the force of a blow on the primer 28 . it has an elongate shaft sized to fit within the passage 33 along the firing pin housing 30 . the firing pin housing 30 is sealed relative to the firing pin 34 by means of a seal ring at 35 . thus , the area near the primer 28 is isolated from pressure on the exterior of the tool . this prevents building up an air cushion in advance of the firing pin which might slow down the firing pin as it travels toward the primer . proceeding down the tool body , the firing pin housing is perforated at 38 to deliver external pressure into an annular chamber 39 . the chamber 39 ( around the firing pin ) therefore sustains the same pressure which is on the exterior of the initiator 20 . the firing pin housing 30 has an internal shoulder at 40 which supports a collet spring retainer 41 . the retainer 41 fits around the firing pin . the firing pin 34 has a enlarged portion at 42 . the enlargement 42 is notched with an external shoulder at 43 . the shoulder 43 catches against enlargements 44 on a set of collets 45 . the collets are received against the collet retainer 41 , abutting against the shoulder . the enlargement 44 locks the firing pin 34 against upward movement . this locking position prevents movement of the firing pin until the collets are released . collett release permits radial outward expansion of the collets 45 . in particular , this is achieved by moving the enlargement 44 radially outwardly to thereby disengage the shoulder 46 securing the firing pin against movement . this operation is controlled by a collet latch sleeve 48 . in the illustrated position ; the collet latch sleeve 48 is held in a latching position by means of a collet spring 49 . the spring 49 bears against the collet spring retainer 41 . when the spring is sufficiently compressed , the enlargement 44 rides over the inside shoulder of the collet latch retainer sleeve 48 . thus , radial outward movement of the collet assembly is forbidden until the spring 49 is compressed . this sequence of operation does not occur until upward movement of a shear pin piston 50 occurs . the shear pin piston 50 is received in a passage or chamber 51 on the interior of the housing 30 . for freedom to move upwardly , thereby compressing the spring 49 , the collet assembly must be released to enable movement of the firing pin . upward movement may require venting of the chamber 51 , and to this end , a port to the exterior is formed by a hole 52 to relieve pressure buildup around the piston 50 . the piston 50 is axially hollow so that the chamber 51 is not pressure isolated either above or below the piston 50 . the piston 50 supports the retainer ring 48 . the outer components are attached by means of a cylindrical skirt with suitable holes to communicate with the port 52 and hence the exterior of the tool . the piston 50 is not free to move because a screw 53 supports a shear pin 53a to prevent movement . this shear pin locks the piston in place until the shear pin is broken . after it has been broken , it can be conveniently replaced on removal of the screw 53 . as described to this junction , the piston 50 only moves after shearing the pin 53a . moreover , such movement also releases the collet latch mechanism that holds the firing pin in place . this is accomplished by compressing the spring 49 and enabling the enlargement 44 on the collets to deflect radially outwardly , unlatching the firing pin . hydrosatic pressure drives the firing pin to strike the primer previously described . when the primer 28 is stuck , it detonates the primer cord 23 and thereby initiates detonation of the shaped charges carried in the assembly 15 above . in the chamber 51 , an actuator rod 54 is positioned near the piston 50 . the rod 54 controllably strikes the piston 50 and shears the pin 53a . this movement is controlled by the apparatus below the actuator rod 54 . the actuator rod as originally installed is not free to travel . a ratchet sleeve 55 fits around the rod 54 . the sleeve 55 is slotted for a portion of its length to define a set of flexible collet fingers 56 . the collet fingers extend along the actuator rod 54 and thread to it on the exterior surface . this threaded connection is illustrated at 57 . the threads are cut so that the collet fingers will thread to the actuator rod 54 when they are in their unflexed position . thus , the two members are threaded together and assume the position shown in fig2 of the drawings by threaded engagement . the sleeve however , moves relatively upwardly with a ratcheting movement . this requires that the thread beween the firing pin 54 and the collet fingers at 56 incorporate a buttress thread enabling the collet fingers to ride over , flexing radialy outwardly and relaxing as they ride over the crown of the threads on the actuator rod . that is to say , ratcheting movement permits the actuator rod to move relatively downwardly and yet locks the two members together so that relative upward movement of actuator rod is forbidden . so to speak , this extends the length of the actuator rod . the arming sequence of the device utilizes this feature , namely , arming by pulling the actuator rod 54 downwardly , ratcheting the sleeve as downward movement occurs and locking the two members together so that they more upwardly as a unit . disassembly of the tool is permitted so that the sleeve 55 can be threaded by hand for resetting . to this end , the housing 30 terminates at a thread 58 which in turn enables connection with a short threaded sub 59 . the sub 59 is perforated at 60 to introduce well pressure to the region near the actuator rod 54 . the collet fingers 56 support an enlargement 61 at the lower end of the fingers . this enlargement faces an abutting threading member 62 that will be described . this locks the enlargement 61 against downward motion , initiating the ratcheting movement which permits the actuator rod 54 to move downwardly . the actuator rod 54 is driven by an actuator piston 63 . the piston 63 is pressure driven in a manner to be described . it is received within a surrounding cylinder 64 , and has a cooperative seal 65 . the cylinder 64 is a liner placed inside of a surrounding housing 66 . the housing 66 threads to the sub 59 at a threaded connection 67 . moreover , the cylinder 64 is also threaded on the interior of the housing 66 by means of a threaded connection at 68 . the piston 63 is equipped with the actuator rod 54 above the piston and also includes a locking rod 70 extending below the piston 65 . the cylinder 64 has an external shoulder 71 which receives a coil spring 72 . the spring 72 bears against a sleeve 73 . the sleeve is forced downwardly . in the downward position , the sleeve holds a set of collets 74 in a locking position . the collets 74 include an enlargement 75 which locks in a conformed groove 76 on the piston lock rod 70 . the collets are captured against the bottom end against the cylinder 64 . thus , they cannot move upwardly or downwardly . in the illustrated position , the lock rod 70 is fixed , not free to move . when the sleeve 73 moves upwardly , the collet enlargement 75 is released thereby permitting movement of the rod 70 . more will be noted concerning this later . the housing member 66 is provided with multiple passages at 77 , and a push rod 78 is positioned in each passage . typically , two or three push rods will suffice for operation . the range of travel of the push rod is relatively short , and it is prevented from falling out of the passage 77 by means of a snap ring 79 . the several rods 78 force the sleeve 73 upwardly , releasing the collet grip on the rod 70 to enable the rod 70 to move . in the illustrated position , the rod 70 is temporarily locked in position . the housing 66 is threaded to a housing member 80 . it defines an internal chamber 81 which is charged with dry nitrogen gas . the members 66 and 80 are jointed at a threaded connection 82 . seals are also included at 83 to prevent leakage of the gas charge . at one end , the chamber 81 has the exposed push rods 78 . at the opposite end , the chamber encloses a gas piston 84 . the piston 84 travels along the chamber to contact the push rods to drive them upwardly . when this occurs , the push rods upset the sleeve 73 , unlatching the collets and enabling movement of the rod 70 previously mentioned . the gas piston 84 is fixed in location temporarily by means of a shear pin 85 . the shear pin is received in a radial hole in the member 80 , the member 80 extending therebelow . the member 80 has a step formed on the exterior to receive a threaded skirt 86 , the skirt threading to the member 80 at a set of threads 87 . suitable seal rings prevent leakage along the shear pin 85 . moreover , the member 80 and 86 are threaded and sealed so that the chamber 81 is protected against leakage . after the piston 84 moves upwardly , it defines a chamber below the piston . the chamber is filled with oil introduced into a passage 88 . an orifice restrictor 89 is located in the passage 88 to restrict flow in both directions through the passage 88 . the skirt 86 previously mentioned in an appendage of an upper end of another housing sleeve 90 defining an oil chamber 91 . this chamber is located below the piston 84 . oil is introduced from the chamber 91 through the restrictor 89 below the piston 84 . the chamber 91 is filled through a removable plug port 92 . oil in the chamber 91 is above an oil piston 93 . the piston 93 is captured within the housing member 90 . it is drilled axially to define the chamber 91 on the interior and enables the piston 93 to travel the length of the chamber . the piston 93 is forced upwardly by pressure under the piston 93 received in the chamber 94 within the housing 90 . the housing 90 closes at the lower end . a bolt 95 is threaded axially into the housing member 90 to hold the piston 93 above the lower end thereby enabling the chamber 94 to be exposed to external pressure . pressure on the exterior is introduced through a port or passage 96 opening into the chamber 94 from the exterior of the tool . movement of the piston 93 is thus dependent on pressure applied below the piston , forcing the piston upwardly . when the piston 93 moves upwardly , oil is forced under pressure through the restrictor 89 , and moves the gas piston 84 upwardly . more will be noted concerning this in review of operation of the device . operation of the firing initiator 20 shown in fig2 can best be understood by tracing one sequence of its operation . the sequence begins at the bottom of the tool subject to initial conditions . the operation results in detonating the primer cord extending lengthwise of the firing initiator means 20 for detonation of the shaped charges . the initial conditions where operation of this apparatus come into play will be first described . the initial conditions assume that the tcp gun assembly has been landed in the packer 13 and has been sealed at the packer . moreover , the firing mechanism 17 has been used unsuccessfully , typically by dropping the sinker bar on it . at this juncture , no firing has occurred and the shaped charges are still positioned adjacent to the formation 11 . between the plug 12 and the packer 13 , a particular pressure is maintained and will be described as hydrostatic pressure . this is the pressure in this region between the plug 12 and the packer 13 . the tubing string 14 is able to conduct fluid ( gas or liquid ) under pressure . fluid flows into the region around the tcp gun assembly 15 to increase the pressure below the packer 13 . assume that the initial pressure below the packer 13 is substantial , perhaps 5000 psi . this defines the hydrostatic pressure in the vicinity of the means 20 . pumps at the surface introduce a fluid under high pressure through the tubing 14 past the packer 13 and into the regions above the plug 12 . the pressure in this area is raised to a specified level which can be controlled from the surface . assume that hydrostatic pressure is 5000 psi in this region . the pumps are operated until higher pressure is accomplished , and the device of the present invention is armed by this higher pressure . there are certain shear pins which can be selectively changed to require different pressures to operate . more will be noted concerning this hereinafter . for purposes of description , assume that the operating pressure requires 8000 psi while the hydrostatic pressure without aid of pumps is 5000 psi below the packer 13 . this indicates a boost of 3000 psi for operation of the device . this large change is a safety factor , namely that the firing initiator 20 is not armed until this change in pressure is first achieved and a subsequent pressure drop is also achieved . accidental firing is thus prevented by properly sizing shear pins utilized in the device . assume for the moment then that 8000 psi is required to initiate operation of the device . as pressure rises , the pressure near the tool 20 increases and this pressure is introduced into the chamber 94 at the bottom end of the tool . this compresses hydraulic oil in the oil chamber 91 . the oil flows at a limited or restricted rate through the restrictor 89 . it flows upwardly through the passage 88 . in the preferred embodiment , the flow rate of the restrictor 89 is adjusted and hence , one or more passages can be used and one or more restrictors can also be used . in any event , at a controllable rate , the pressure below the piston 84 is increased . ultimately , a sufficient pressure is achieved to shear the pin 85 . as will be understood , the device is constructed so that one or more shear pins positioned around the piston 84 are broken collectively . this is accomplished when the pressure forcing the piston 84 upwardly exceeds a certain level and hence , an adequate shearing force is accomplished . when this happens , the piston 84 is free to move upwardly . while it moves upwardly , the piston compresses the gas in the chamber 81 . assume that the pressure driving the piston is adequate to drive the piston 84 the full length of the chamber 81 . when it does travel this length , it strikes the push rod 78 . again , in the preferred embodiment , redundant construction utilizes up to about four of the push rods 78 . this assures that failure of one push rod does not cause the device to fail in operation . the piston 84 moves upwardly , striking the pin 78 , forcing all of the pins 78 upwardly . they force the sleeve 73 upwardly against the spring 72 . this moves the sleeve 73 to a location enabling the collets 74 to flex radially outwardly which is accomplished . when this occurs , this frees the rod 70 for movement . at this point of operation , the rod 70 is free to move and hence , the piston 63 has been freed to move . the piston 63 is exposed to pressure from the exterior of the tool through the port 60 . in this example , the pressure on the exterior of the tool is 8000 psi . that acts on the top side of the piston 63 to push the piston 63 downwardly . the piston travels downwardly , pulling the actuator rod 54 with it . when the actuator rod moves downwardly , the rod pulls the sleeve 55 downwardly but the sleeve can not move and hence ratcheting action occurs between the rod 54 and the collet fingers 56 around it . this ratcheting action changes the relative location of the sleeve 55 . thus , the sleeve 55 does not move but locks around the actuator rod . if the rod 54 moves downwardly by any distance whatsoever , the sleeve 55 moves relatively upwardly by the same measure . after movement , the members 54 and 55 are in an armed condition . that is , the piston 63 has traveled downwardly by some distance ( assume 2 &# 34 ; as an example ) and the sleeve 55 has moved upwardly on the actuator rod 54 by the same distance . at this point , the tool is now armed . before this , the means 20 could not be fired because it was not armed . assume that this pressure is held on the exterior of the tool for several minutes . whatever the interval , this assures that arming does occur because the pressure necessary to arm the tool has been accomplished . after arming , well pressure is then relieved . flow through the tubing string 14 drops pressure below the packer 13 . assume as an example that the pressure drops rapidly to the hydrostatic pressure maintained between the plug 12 and the packer 13 . assume that this is 5000 psi , the pressure used in the example above . pressure drop will ultimately be accomplished whereupon the pressure which forced the oil piston 93 upwardly is no longer available . the piston 93 then is free to move downwardly . since it was in a pressure balanced condition , it is forced to move downwardly , enlarging the oil chamber 91 and removing oil from below the gas piston 84 . this oil flows downwardly through the passage 88 and is metered through the restrictor 89 . this permits the piston 84 to move downwardly and enlarge the chamber 81 reducing nitrogen pressure . however , there is a more rapid pressure drop above the piston 63 . recall that with arming of the tool , the piston 63 as forced downwardly ; since the port 60 evacuates the space above the piston 63 , the piston 63 is then free to move rapidly upwardly . that is , while the pressure may drop below the piston , it drops much faster above the piston 63 . there is a net upward force acting on the piston 63 to drive it upwardly forcefully . the piston 63 drives the actuator rod 54 upwardly . it has been lengthened because the ratchet action moves the sleeve 55 upwardly . with upward movement of the piston 63 , the piston 50 is struck and forced upwardly . to move upwardly , the shear pin 53a must be sheared . when the piston 50 is forced upwardly , it moves the locking sleeve 48 around the enlargement 44 upwardly against the coil spring 49 . this releases the collets 45 around the firing pin . the firing pin is then free to move upwardly . the firing pin 34 is forced upwardly by the hydrostatic pressure until the pin 34 strikes the primer 28 . the primer is then detonated and ignites the primer cord 23 . the primer cord 23 burns , igniting the many shaped charges to assure that the tcp gun assembly 15 properly operates . thereafter , the fired gun assembly can be safely retrieved . the structure shown in fig2 is easily rearmed . thus , the threaded connection is broken at 31 to enable a new primer 28 to be placed in the illustrated position and to also permit new primer cord to be located in the passage 22 . that portion of the equipment is easily reassemblied . the shear pin 53a is likewise replaced upon retracting the firing pin to the original and illustrated position . likewise , the shear pin 85 is replaced . this enables the tool to be quickly disassemblied and reassemblied with new shear pins , primer cord and the parts positioned in the illustrated position to enable subsequent operation . an important factor to note is the selection of shear pin strength . the shear pins are selected for different strengths . this makes certain the firing sequence and prevents accidental firing by hydrostatic pressure at the bottom of the well . as an easy example , the shear pins can be selected to require practically any level of pressure above hydrostatic or bottom hole pressure to trigger operation . in one example , assume that bottom hole pressure is 5000 psi while the pressure required to operate the means 20 is 9000 psi . the well can be pressured up to 10 , 000 psi as an example . the shear pins can be selected to shear at about 9000 psi . overpressuring adds certainty of performance . overpressuring then assures proper operation with safety . moreover , this is accomplished taking into account bottom hole pressure and thus makes the initiator means 20 independent of bottom hole pressure . likewise , it is independent of annular pressure . while the foregoing is directed to the preferred embodiment , the scope thereof is determined by the claims which follow .