US3016012A

US3016012A - Inertia operated mine firing device

Info

Publication number: US3016012A
Application number: US71741A
Authority: US
Inventors: Joseph D Turlay
Original assignee: Individual
Current assignee: Individual
Priority date: 1949-01-19
Filing date: 1949-01-19
Publication date: 1962-01-09
Anticipated expiration: 1979-01-09

Description

Jan. 9, 1962 J. D. TURLAY 3,016,012

' INERTIA OPERATED MINE FIRING DEVICE Filed Jan. 19, 1949 v 2 Sheets-Sheet 1 awe/whom Jill Turlay Jan. 9, 1962 J. D. TURLAY 3,016,012

INERTIA OPERATED MINE FIRING DEVICE Filed Jan. 19, 1949 2 Sheets-Sheet 2 gwuemiw 3,016,012 Patented Jan. Si, 1962 3,016,012 INERTIA OPERATED MINE FIRING DEVICE Joseph I). Turlay, 2211 Pierce St., Flint, Mich. Filed Jan. 19, 1949, Ser. No. 71,741 8 Claims. (Cl. 102-17) (Granted under Title 35, US. Code (E52), see. 266) The present invention relates to mine firing devices, and more particularly to an inertia operated firing device which is arranged to be operated to fire the mine in response to a sudden lateral blow as may be caused by a vessel colliding with the mine and in which operation of the device is prevented in response to oscillatory or relatively slow translational movement of the mine as a result of wave or tide action.

An inertia firing device of this character is particularly well suited for use with a subfioating mine such, for ex ample, as that-disclosed and claimed in my copending application for Subflo-ating Mine, Serial No. 519,418.

In accordance with the construction and arrangement of the firing device of the present invention, the aforedesoribed operation thereof is accomplished by the provision of a compound pendulous switch arran ement wherein the contacts of the switch are activated in respons to relative motion between the two pendulously supported members of the compound pendulum when alateral shock of predetermined intensity is received by the mine and wherein both of the members are arranged to move as a unit in response to slow oscillatory and translational movement of the mine whereby actuation of the contacts is prevented in response to the unitary movement of the pendulous members.

An object of the present invention is to provide an inertia responsive firing device for a subfioating mine in which the device is operated in response to a lateral blow such as that received by the mine when struck by a passing vessel and in which operation of the device is prevented in response to oscillatory or slow translational movement of the mine such as that resulting from wave action.

Another object resides in the provision of a new and improved inertia controlled switch which is operated in response tolateral shock of predetermined intensity re ceived thereby and in which operation of the switch is prevented in response to slow oscillatory or translational movement of the switch.

An additional object resides in the provision of a new and improved compound pendulously supported switch device wherein the switch is operated in response to relative motion between the pendulously supported members of the device.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood. by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

FIG. 1 isa sectional view of a sub-floating mine and showing the inertia controlled firing device of the present invention assembled therein;

FIG. 2 is an end view of the mine shown in FIG. 1;

FIG. 3 is a top plan view of the inertia controlled firing device of the present invention and showing a portion of the supporting meanstherefor;

FIG. 4 is a sectiona-l view takenalong the line 44' of FIG. 3; and a FIG. 5 is a plan view'par tly broken away of the firing contacts and Supporting meanstherefor, the casi-ng being removed.

Referring more particularly to the drawings in which- 12 secured thereto and provided with an aperture 13 adapted to allow the introduction of an explosive charge 14 composed of TNT, torpex or the like disposed within an explosive chamber 15 formed by a wall or partition 16 extending laterally across the casing and secured inte riorly thereto in any suitable manner as by welding the parts together. The aperture 13 is adapted to be sealed by a cover or cap 17 secured thereto as by the bolts 18, a suitable gasket 19 being arranged between the cover" and the end' portion 12 to insure a watertight joint therebetween.

There is also provided within the casing of the mine a well 21 extending inwardly into the chamber 15 and having a receptacle 22 secured thereto within which is arranged an explosive booster charge 23, the receptacle being preferably braced to the casing of the mine as by the member 24. The well 21 is provided with a shoulder or flange 25 to which is secured as by the bolts 26, a detonator extender mechanism indicated generally by the numeral 27, a watertight connection between the extender mechanism and the casing of the mine being maintained preferably by a gasket 28 arranged between the extender mechanism and the aforesaid shoulder 25. The partition 16 is provided with a recessed portion 29 having side walls in substantial alinement with the casing 31 adapted to receive and support a battery 32, a suitable wrapping of insulating material 33 being disposed between the battery and the casing in the manner illustrated to maintain the battery securely in a fixed position with respect thereto. The battery 32 is enclosed by a cover 34 secured thereto as by the screws a resilient pad or cushion 36 of material suitable for the purpose such, for example, as rubber being preferably arranged between the cover and the battery. 1

There is also provided within the casing of the mine a circular support 37 to which is secured as by the bolts 38 a plate or base 39 having secured thereto as by the resilient mountings 41, an inertia controlled firing mechanism indicated generally by the numeral 42 arranged in substantial coincidence with the center of free rocking movement of the mine within the water. Supported by the plate 39 in any suitable manner are a plurality of control relays and thermal devices for arming the mine, controlling the operation of the depth control mechanism and for firing the mine in response to the operation of the inertia controlled mechanism 42 as the mine is subjected to lateral movement of predetermined character such, for example, as may be caused by the impact of a vessel striking the mine or relatively sud-den movement of the water caused by the moving vessel, or in the event that the sup ply of power required to maintain the mine at a predetermined depth of submersion has been exhausted. The firing of the mine in response to a countermine shock received through the surrounding water is prevented by the resilient mountings 41 which reduce the movement ofthe firing mechanism 42 sufficiently to prevent the operation of the firing mechanism.

There is also secured to the plate 39 a switch 43 having a movable control element'44 adapted to be actuatedfrom the exterior of the mine casing when a detachable plug 45 provided in the casing of the mine insubstantial alinement with the control element 44 has been removed.

The switch. 43 is operated momentarily prior to the launching of the mine thereby to insure that the various electroresponsive elements comprising the firing and. depth control circuits are in a predetermined initial condition before a cycle of operations thereof is performed.- The inertia controlled firing mechanism and the aforesaid electroresponsive devices are protected during: the assembly, testing and transportation of the firing and depth control mechanism by a plate 46 and a pairof brackets 47, the plate and brackets being disposed respectively below and above the plate 39 and secured thereto by the bolts 48.

There is also provided within the casing 11 a recess or well 49 within which is disposed at hydrostatically controlled depth regulating device indicated generally by the numeral 51 secured in watertight relation with respect to the casing of the mine as by the nuts 52 and gasket 53. The device 51 is adapted to apply a control to the depth regulating circuit by way of the conductors within the cable 54 selectively in accordance with the instant depth of submersion of the mine within the water.

The tail of the mine is enclosed by a casing 55 comprising an upper portion 56 and a lower portion 57 secured thereto in any suitable manner as by welding the parts together and provided with an annular member 58 secured exteriorly thereto. There is provided on the tail portion of the casing 11 an annular member 59 to which the member 58 is secured as by the bolts 61 and maintained in watertight relation with respect thereto as by the gasket 62. The lower portion 57 of the casing is provided with an aperture 63 adapted to be enclosed by a plate 64 secured thereto as by the bolts 65, a watertight connection being insured therebetween by a gasket 66. Secured to the lower portion of the plate 64 is a cup-shaped casing 67 adapted to support a reversible motor 68 secured thereto as by the bolts 69, the electrical power required for the operation of the motor being supplied thereto by way of the conductors within the cable 71. The motor is adapted to drive a screw propeller 72 at a reduced rate of speed by means of the provision of a reduction gearing within the cup-shaped casing 67. The upper portion 56 of the casing is provided with a cup-shaped member 73 secured thereto as by welding the parts together and preferably additionally braced to the portion 56 of the casing as by the annular support 74. The screw propeller 72 is secured to the shaft 75 by a nut 76, the shaft 75 also causing the actuation of a rotary pump within the support 77 thereby to effect a variation in the specific gravity of the mine in accordance with the extent of operation of the pump during the time that the valve indicated generally at 78 has been moved to an open position thereby to establish communication between the pump and the tubes or

ducts

79 and 81.

The casing 55 is partly filled with a ballast fluid suitable for the purpose such, for example, as a fluid composed of one part of lard oil and three parts of kerosene adapted to be forced outwardly through the

tubes

81 and 79 by the aforesaid pump into the surrounding water or adapted to be supplied with a quantity of water drawn inwardly through the

tubes

79 and 81 by the pump, as the case may be, the direction of flow of the fluid being controlled by the direction of operation of the pump during the time the valve 78 is open. The tube 81 is provided at the lower end thereof with a ball check valve disposed within the casing 82 in communication with the ballast fluid, means being provided to permit the flow of fiuid in either direction past the ball valve when the pump is in operation, the direction of movement of the fluid past the valve being controlled by the direction of rotation of the pump. The outer end of the tube 79 is secured to the cap 56 at a nipple 83, a watertight joint between the tube and the nipple being eifected by the coupling 84.. The nipple is preferably threaded to receive an external connection whereby the pump may be primed, ifdesired, prior to the launching of the mine. There is arranged on the exterior portion of the casing 56 a plurality of brackets 85 adapted to support a cupshaped annular member 86 having a strainer 87 arranged across the opening thereof thereby to exclude the entrance of foreign sea growths and the like from the ballast control system.

Secured to the casing 56 in any well known manner as by welding the parts together are a plurality of fins 88 preferably composed of sheet metal having bent over portions adapted partially to enclose the fins at 89 and 91. Supported by the fins in coaxial relation with respect to the shaft 75 is preferably provided a cylindrical member 92 adapted to reduce the turbulence of the water as the screw propeller operates. The ballast fluid is introduced within the casing through a filler hole 93 adapted to be sealed by the plug 94, the filler hole preferably being arranged adjacent the valve 78 whereby the operation of the valve 78 may be made manifest during the testing of the device.

The casing 11 is preferably reinforced by the

circular members

95 and 96, the member 96 being adapted to receive and support a plurality of weights 97 detachably secured thereto as by the bolts 98. When it is desired to plant the mine in a body of salt Water, the weights are secured to the member 96 and the mine is thus roughly adjusted to a specific gravity substantially equal to the specific gravity of the salt water within which the mine is to be planted, the weights being of sufiicient mass to render the specific gravity of the mine substantially the same as the specific gravity of fresh water when the weights are removed, whereby the mine is also adapted to operate with equal facility in either fresh water or salt water. By providing a plurality of detachable weights 97 in the manner disclosed, the mine is adapted to float at a predetermined depth of submersion in either salt water or fresh water, as the case may be, controlled by the setting of a single depth control dial it being merely necessary to remove the weights when the mine is to be planted in fresh water.

The explosive charge 14 is preferably poured into the explosive chamber 15 in a liquefied condition through the aperture 13 when the mine is arranged with the aperture uppermost. The aperture is then sealed by the cap 17 and the mine is turned over with the cap lowermost. The explosive charge solidifies in the position shown in FIG. 1 in close engagement with the domeshaped end portion 12 of the mine casing and in contact with the lower portion of the recessed portion 29 of the partition 16. An arrangement is thus provided whereby, by employing a dome-shaped cap for the leading end of the mine casing, the mine is adapted to withstand the violent shock of impact of the mine against the surface of a body of water when the mine is launched from an aircraft in flight, without damage or injury thereto, the explosive charge being employed additionally to support the partition 16 and absorb the shock of impact of the battery 32 against the partition as the mine strikes the water.

For further details of construction and operation of the foregoing components of mine 10 other than the inertia device hereinafter more fully to be described, reference may be had to the aforementioned copending application.

The inertia firing control mechanism employed with the mine of FIG. 1 comprises a supporting base 315 secured to the plate 39 yieldably as by the resilient mountings 41, the mountings 41 being attached to the base and plate in any suitable manner as by the rivets 316. The base 315 has a hemispherical portion 317 and a plurality of upturned edges 318 thereby to strengthen the base and increase the resistivity thereof to deformation as the result of the shock received during the planting of the mine. The base 315 is provided with a support 319 secured thereto in any suitable manner as by Welding the parts together and having slidably arranged therein a plunger 321, the plunger having a shoulder 322 thereon adapted to be continuously maintained in engagement with a complementary shoulder 323 arranged within the support 319 by a spring 324 at all times except when a heavy shock is received during the planting of the mine. The opposite end of the spring 324 is arranged within a recessed portion 325 of a cap or plug 326 threaded within the support 319. The support 319 extends downwardly within an aperture 327 centrally arranged within the plate 39 whereby the lower portion of the base 315 is normally supported by the resilient mountings 41 at a distance from the curved portion 328 of the plate 39 sufficient for the portion 328 of the plate to be employed as a stop for the base 315 when the base 315 moves into engagement with the portion 328 of the plate 39 as the result of the shock received during the planting of the mine from the aircraft in flight.

The plunger 321 is tapered as at 329 thereby to form a conic section upon which is pivotally mounted a mass 331 suitably recessed at 332 to coact with the apex of the aforesaid conic section of the plunger 321 and pivotally support the mass thereon. The mass is of such a shape that the center of gravity thereof is slightly lower than the point of support and sudden translational movement of the plunger 321, therefore, causes the mass 331 to tilt.

The upper surface of the mass 331 comprises a conic recessed portion 333, a circular flat surface 334 coaxially arranged with respect to the conic recessed portion and a curved annular surface or seat 335 intermediate the flat surface and the conic recessed portion. A circular member 336 is tapered and rounded as at 337 thereby to coact with the curved annular surface 335 of the mass 331 and maintain the member 536 in predetermined spaced relation with respect thereto at all times when the device is not subjected to sudden translational movement of a predetermined character. The member 336 is secured to a circular plate 338 having a cylindrical member 339 secured thereto and extending both upwardly and downwardly therefrom substantially as shown. The lower portion of the cylindrical member 339 comprises a laterally extending flanged portion 341 to which is secured as by the screws 342 a circular mass 343 extending downwardly therefrom and having a truncated spherical surface 340 with a radius of curvature sufiiciently less than the radius of curvature of the inside surface of the supporting base 315 whereby the shock of planting the mine is adapted to cause the circular mass to engage the inside surface of the base 315 and the mass 331 to be arrested by the mass 343 and the spring 324 to be compressed by the shock received by the mine as the mine strikes the surface ofthe Water, thereby preventing damage or injury to the pivot support 321 at the tapered portion 329 thereof. The mass 343 is also provided with a tapered recessed portion 344 of sutlicient size to permit considerable pivotal movement of the mass 343 as the device receives a'translational impulse of predetermined character. In a similar manner the mass 331 is recessed as at 345 to permit tilting of the mass 331 about the pivot support therefor. When the'firing mechanism is tilted sufiiciently for the recessed

portions

344 and 345 of the masses, 343 and 331 respectively to engage the slideable supportfor the pivot member 321, the

members

343, and 331 are tilted with respect to each other sufficiently to close a pair of firing contacts, as will more clearly appear as the description proceeds.

A structure is thus provided in which the

elements

331 and 343 comprise a compound pendulum adapted to oscillate as a unit about the pivot support 321 in response to relatively slow translational impulses received thereby and in which relative motion between the

masses

331 and 343 is effected as the result of the lateral component of a blow or sudden impulse of predetermined character received by. the device by reason of the center of gravity of the mass 343 and supporting means therefor being disposed ata greater distance below the. pivot 332 than the center of gravity of the mass 331 and. by reason of the moment applied to' the mass 331 by the horizontal component of the force applied thereto by way of the pivot member 321. Rotational movement of the mass 343. and supporting means therefor including the circular member 336, with respect to the mass 331, causes a plunger 346 slideably arranged within the member 336 to be moved downwardly by the combined force of gravity and the tension of the contact spring 347 thereagainst. When this occurs, the contact spring 347 is brought into engagement with the contact element 348 thereby closing the firing control circuit comprising the conductors 349 secured thereto. The

contact members

347 and 348 are electrically insulated from each other and from the plate 338 as by the insulating

devices

351 and 352, the insulating device 352 preferably being provided with an aperture through which the plunger 346 is adapted to move. Disposed about the

contact members

347 and 348 is a guard 353 thereby to prevent excessive flexing of the

contact members

347 and 348 sufiicient to change the adjustment thereof regardless of the violence of the shock to which the device may be subjected. The aforesaid guard and contact members are secured to the plate 338 as by the screws 354. Secured to the base 315 as by the screws 355 is a hemispherical cover 356, a gasket 357 being arranged therebetween thereby to seal the spherical chamber Within which the inertia switch contacts are disposed.

A cylindrical projecting member 358 having the upper end thereof closed at 3 59 is clamped to the upper portion of the cover 356 as by the nut 361, a watertight connection therebetween being assured by a gasket 362 disposed between a shoulder 363 on the member and a comple mentary recessed portion 364 within the cover. The projecting member 353 is composed of insulating material suitable for the purpose such, for example, as Bakelite and is provided preferably with a pair of inserts 365 adapted to receive the screws 366 and thereby establish an external circuit connection to the conductors 349 respectively secured thereto. There is arranged beneath the heads of the screws 366 a plate 367 having a downwardly projecting eye 368 therein adapted to engage and support one end of the spring 369. The other end of the spring 369 is secured to an eye 371 formed in the bracket 372 secured to the plate 338 as by the screws 373. An arrangement is thus provided in which the spring 369 is employed to lessen the pressure on the pivot 321, prevent excessive rotation of the pendulum about the pivot support, and apply a restoring force to the elements of the compound pendulum in a direction to assist the force of gravity in restoring the pendulum to an initial vertical position.

It is, of course, understood that

contact members

347 and 348 are connected in the firing circuit of mine 10 whereby the explosive charge of the mine is fired as the contact members are closed. It will further be understood, however, that the contact members may otherwise be employed to control other mine or analogous circuits upon discriminating response of the inertia switch to the same or similar shocks received by or movements imparted to the apparatus associated therewith.

It will be appreciated from the foregoing, that an inertia. controlled firing mechanism has been provided which is well adapted to fulfill the aforestated objects of the invention for use in floating mines or other marine ordnance devices and which is particularly adapted for use in the firing circuit of the sub-floating mine disclosed in my copending application for Subfioating Mine, Se-

rial No. 519,418.

Obviously may modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of theappended claims the invention may be practiced otherwise than as specifically described.

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the paymentof any royalties thereon or therefor.

What is claimed as new and desired to be secured by Letters Patent of the United States is:

1. In a floating mine, in combination, a pivot support, a first inertia member pivotally supported by said support, a second inertia member having-a greater mass and a lower center of gravity than the'first named inertia member, a tubular member axially dependingfrom the second inertia member and tiltably supporting the second inertia member on the first inertia member, a pair of normally open firing contacts on said second inertia member, means for closing the firing contacts selectively in accordance with the degree of tilt of the first named inertia member with respect to the second inertia member, and resilient mounting means for said pivot support.

2. In a subfioating mine, in combination, an inertia firing mechanism resiliently supported by said mine, said firing mechanism comprising a substantially spherical casing having a pivot support slideably arranged therein, a stop for said pivot support, means for yieldably urging the pivot support against the stop, a mass pivotally arranged on said pivot support and having an annular recessed portion and a substantially flat surface within said recessed portion and coaxial therewith, a second inertia member, means for tiltably supporting said second inertia member with the center of gravity thereof in vertical alinement and below the center of gravity of the first named inertia element, said supporting means including an annular complementary support adapted to engage and coact with the annular recessed portion of the first named inertia member and maintain the inertia members in sub stantial vertical alinement when the mine is in a state of rest, a pair of normally open firing contacts on said support, and means in engagement with said relatively fiat surface of the first named inertia member for closing said firing contacts in response to a relative tilt between the inertia members as a lateral impulse of predetermined character is received by the device.

3. A mine firing device of the character disclosed comprising a casing, means for resiliently supporting said casing, a pivot support slideably arranged within the easing and having a stop thereon, means for urging said stop against the casing at all times except when a sudden shock in a vertical direction is received, a circular mass pivotally mounted on said pivot support, a second circular mass adapted to be arranged about said first named mass and at a relatively short distance therefrom, means for tiltably supporting the second mass on the first named mass and in coaxial relation therewith at all times except when a lateral impulse of predetermined character is received, a resilient member secured to said supporting means and adapted to reduce the rotative movement of the second mass relative to said casing, a pair of firing contacts on said support adapted to be closed when the degree of tilt between the first named mass and said second mass has reached a predetermined value, and means for arresting the downward movement of the first named mass and the second mass in a direction to cause said pivot support to slide within the casing before sufficient pressure is applied to the pivot support by the first named mass to cause damage or injury thereto as said sudden shock is received.

4. In a firing device for a floating mine of the character disclosed, a pivot support yieldably arranged within the mine with the axis thereof vertical, a circular mass pivotally supported on said pivot support and having the center of gravity thereof below the point of support of the mass and relatively close thereto, a circular recessed seat on said mass in alinement with the axis of said support, an annular inertia member of greater diameter than the diameter of said mass, means including a tubular member adapted to engage the seat within the mass and tiltably support the inertia member with the center of gravity thereof below the center of gravity of the mass and in vertical alinement therewith when the device is in a state of rest, a pair of normally open firing contacts arranged on said tubular member, and a slideable member within said tubular member in engagement with said mass and adapted to cause the firing contacts to be closed when the mass is tiled by a predetermined amount with respect to said inertia member in response to a laterial impulse of predetermined character received by the device.

5. In a freely subfloating mine, an inertia controlled device within the mine for firing the mine in response to a lateral impulse of predetermied character received thereby comprising a pivotally supported mass, an inertia member tiltably supported on said mass and having the center of gravity thereof below the center of gravity of the mass, a pair of normally open firing contact elements secured to said inertia member, means including a pin interposed between the mass and one of the pair of contact elements for initially maintaining said cont-act elements open and for causing said firing contact elements to be closed as the mass tilts with respect to the inertia member in response to a sudden lateral movement of the mine within the water, means including a pair of flexible conductors for establishing an external electrical connection of said contact elements, and mean for yieldably resisting rotative movement of the inertia member with respect to said mine suificiently to prevent damage or injury to the conductors as the result of rotative movement of the mine within the water.

6. In a subiloating mine, in combination, a pointed support mounted in said mine, a first inertia member tiltably mounted on the point of said support, a second inertia member tiltably mounted on top of said first inertia member, said first inertia member having the center of gravity thereof slightly below the point of support therefor and said second inertia member having the center of gravity thereof below the center of gravity of the first inertia member thereby to provide a different moment of inertia for each of said inertia members, said inertia members tilting in unison in response to wave action and tilting at different angles with respect to each other in response to a lateral blow, a switch member mounted on the second inertia member for closing an electrical circuit and means for maintaining said switch member in an open circuit position when the inertia members move in unison and for releasing said switch member to a closed circuit position when the inertia members move with respect to each other.

7. A firing device for a mine of the character disclosed comprising, in combination, a casing resiliently supported in said mine, a pointed support yieldably mounted for vertical movement in said casing, a first inertia member balanced on said pointed support and having the center of gravity slightly below the point of support, said first inertia member having an annular seat on the top thereof, a second inertia member loosely encasing the first inertia member and having the center of gravity thereof below the center of gravity of the first inertia member, said second inertia member having a tubular portion fixed thereto and extending axially therebeneath to rest on said annula seat, a pin abutting the first inertia member and extending through the tubular portion of the second inertia member, and switch means mounted on said second inertia member and engaging said pin whereby said switch is initially held in open circuit position as the inertia members move in unison in response to wave action, said switch moving to closed circuit position as the inertia members separate at the juncture of the tubular member and the annular seat in response to a lateral blow received by the mine.

8. In a floating mine, in combination, a pointed support, a first inertia member mounted for tilting motion upon the point of said support, said first inertia member being slightly below the point of support thereof, a second inertia member having an annular supporting surface tiltably engaging the top of said first inertia member and having the center of gravity thereof below the center of gravity of said first inertia member, said annular supporting surface providing a firm support for said second inertia member on said first inertia member whereby the members are moved in unison by wave action, saidmembers tilting to diflferent angular positions with respect to each other when the mine receives a lateral shock, an axial bore through said second inertia member, a pair of electrical contact elements mounted one of said contact elements and the tape the first on the top of the second inertia member and insulated member is increased as the members move to difierent therefrom, and a pin extending through said axial bore angular positions in response to a lateral shock.

and engaging the first'inertia member at one end thereof a and engaging one of said contact elements at the other 6 References cued m the file of this Patent end thereof to maintain the contact members disengaged UNITED STATES PATENTS whereby the Contact elements are released by said pin 1,471,628 Palmer Oct 23, 1923 for mutual engagement when the distance between said 2,293,949 Potter Aug 25, 1942