ONE OF THE INTERESTING characteristics of the complete ballistic cycle, executed in the interval between the initial motion of the projectile from the seated position in the bore and the final, abrupt dissipation of its energy in the target, and the characteristic which limits experimental development most severely, is the brief duration of some of its elements, in several of which the factors to be measured are of very great intensity. It has been pointed out that a considerable part of the more effective analytic procedure of ballistic engineering practice is, in nature, purely empirical summary.1 In order that such generalizations may be satisfactorily free from spurious characteristics, it is necessary to establish carefully the quantitative basis for experiment, particularly in the study of the gun cycle itself and of penetration ballistics.
1L. Thompson, “Ballistic Engineering Problems: Empirical Summaries,” U.S. Naval Institute PROCEEDINGS, vol. 56, p. 411, May, 1930.
The more fundamental data of interior ballistics are the pressure distributions, both as time and as space functions, and the time table of projectile displacements. From these, all of the elements of gun and powder design can be developed, if the data accumulated are sufficiently precise and sufficiently general. The time rate of change of projectile displacement at the muzzle, while the actual ejection velocity, is not precisely the initial datum which defines the trajectory at a specific angle of projection, although the difference is too small to be of importance. The value of the displacement rate which lies between the region of positive acceleration by the powder gas and negative acceleration by the air occurs at a point 5-10 calibers or more from the muzzle, at an instant after ejection but before emergence from the expanding volume of powder gases. Associated with this phase of the cycle of the projectile motion are a number of phenomena of considerable importance, such as muzzle flash (the secondary explosion of the expanding gases), the propagation of the blast wave and blast gases, and the erosive wear of the gun at the muzzle by the high-velocity jet. The blast effects are potentially considerable not only in producing deformation of structural material adjacent to the battery, but also, for certain conditions in salvo firing, with respect to the initiation of secondary oscillations of the projectile. The latter disturbances may result in appreciably reduced range and in an enhancement of dispersion. Precision in the estimation and control of the exterior trajectory, both in the low angle fire of ordinary gunnery and for effective anti-aircraft gunnery, requires rather exact knowledge of the variation of air retardation not only with change in velocity but as a function of the shape, and of the orientation in flight of the projectile. Determination of this function and of the factors which prescribe the limits of flight characteristics for a given system are usually considered the major experimental problems of exterior ballistics. It is also required to have accurate knowledge of burning or running times of the fuzes which produce burst of high explosive projectiles, for all types of conditioning with respect to the atmosphere, rate of spin of projectile, and properties of the trajectory. Finally, in order to know the probable effectiveness of gunfire and of attack by bombs it is essential to acquire well-established experimental measures of precision for the important types of conditioning, a problem of some statistical complication, and to have extensive data regarding the penetration of armor and the effect of detonating projectiles. The latter effects are to be obtained not only for solid confinement but when occurring in air and in water.
It is the purpose of this outline to describe certain experiments which have been designed and used for studies pertaining to these problems. It is practicable in the use of some of the optical apparatus to restrict the application to that of calibration standards, employing more convenient, if less precise, recording systems for routine collection of data. For example, an optical chronograph for measuring projectile velocities,2 in the operation of which the projectile merely eclipses a beam of light, has been found to be a very competent standard for the calibration of routine chronographs. The optical chronograph has been set up in a form to give precise data in a small laboratory space and also as a field unit extending over 100-150 feet of the trajectory.
Measurement of velocity aboard ship and at high angles of projection.—An important present problem with respect to velocity determination is that of obtaining fairly exact values for firing conducted from ship batteries and at the proving ground for very high angles of projection above the capacity of the proving ground permanent screen installations; this is especially desirable in the case of firing for range calibration.
2L. Thompson and N. Riffolt, “A Set of Calibration Standards . . .”, Journal of the Optical Society of America, vol. 10, no. 6, p. 705.
An experiment has been conducted recently at the Naval Proving Ground with the object of utilizing coils wound directly on the barrel of the gun as a means for determining bore displacement intervals and ejection velocities. It has been tested for the latter application by use during a calibration program of the U.S.S. Oklahoma, measuring simultaneously the velocities of all guns of the principal battery.
The records are produced by the displacement of a sensitive oscillograph element (recording on a moving film surface), which occurs at the transit of the plane of the gun coil by the projectile. The coils are solenoids of a few turns of heavy wire and are connected in series with a battery and the primary of a transformer having characteristics determined by the external circuits of the gun coil and recording systems, respectively. Introduction of the projectile in the coil, through which the normally steady current i0 is passing, produces a momentary increment in the total flux, with a corresponding set of transient electromotive forces in the primary circuit. The small impulses in the secondary (recording) circuit are of the form shown by the oscillogram on page 377, which was obtained for a three-gun salvo, and which indicates not only the relative velocities but also the corresponding ejection times for the three projectiles. The precision in the measurement of the time interval for the motion between the two coils is not greatly inferior to that of routine velocity measurement by external chronographs, the probable error in velocity being, for a major caliber gun coil set, one-half per cent or less. Actual ejection velocities are obtained empirically from a calibration curve, of the form in Fig. 2; corresponding intervals and the equivalent velocities by the external solenoid chronograph were plotted to identify the calibration, the coil system being an exact duplicate of the one mounted on each gun. The coils may be separated 10-20 feet on a major caliber gun. The actual distance used is ordinarily about thirteen feet. The circuit and elements of the experiment are presented in Fig. 1.
The maximum value of the secondary current is the point of reference in measuring the records. Absolute position of the projectile base at the instant of current maximum is determined for new conditioning by means of external screens, times of contact being superposed on the record. These additional data are required in case of application of the coil set for measurements to obtain the bore displacement function, and are unnecessary for the ordinary application in obtaining ejection velocity.
The primary current value i1(t) as shown in Fig. 3 is a true representation within the precision of the measurement of the experimental records, since its computation involves only the observed secondary current i2(t) and characteristics of the transformer and oscillograph circuits. It will be noted that the position of the projectile cannot vary from round to round with reject to current maximum, at any specific velocity, for the same initial conditioning of the circuit. Thus, since the calibration of Fig. 2 is obtained for an exact reproduction of the service circuit by simultaneous measurement of ejection velocities, using a standard chronograph, the significant errors of the gun coil system are only those of measurement of the time record, of coil separation and of the standard chronograph used for comparison. Numerical integration of equations representing the electromotive forces of the circuit will give the flux increment established when at maximum value. Assuming that the rate of change of flux will be unchanged for rounds of equivalent velocity, with equal magnetomotive forces (ampere turns), the effect of certain changes in circuit design can be examined by means of these equations. It is essential that the resistances R1, R2 of the primary and secondary circuits be small, values not greater than 0.5 ohm being desirable. The effect of a change in velocity (i.e., by powder charge variation) is to produce small change in amplitude of the recorded current but it does not interfere with the measurement of the time intervals. The width of the current trace at the base of the curve, from beginning to maximum value, expressed in units of displacement, is a measure of the relative position of projectile with respect to coil for maximum current.
As the shock from the explosion travels along the bore, the changes in magnetic induction are sufficient to produce a very slight secondary current, as shown by the high-frequency disturbance in the oscillograph record, beginning appreciably before the projectile reaches the first coil. If the coil is not wound tightly on the bore, the relative motion of coil and bore during the vibration of the bore will produce an undesirable distortion of the record. If the muzzle coil is placed near the end of the bore, there is a distinct variation in current which follows the “whip” and “jump” displacement, on which is superposed a high frequency disturbance of small amplitude, shown on page 377. The latter appears to originate in discontinuous changes in the magnetic induction which occur during the flexure of the steel, the number of which varies from 1,000 to 3,000 per second. The effect in question continues for a period of 0.03 to 0.04 second after ejection. The magnitude of the effect varies with the gun, probably with the characteristics determining the amplitude of bore displacement at ejection. It is of further interest that the records for first rounds fired in a program are invariably of greater amplitude.3
A further experiment of this nature is designed to utilize a multiple coil set for obtaining bore displacement of the projectile as a time function. In conjunction with a pressure-time gauge the necessary data can be obtained to define uniquely the so-called bore dissipation and powder burning functions of interior ballistics.
3Note Observations of Sixtus and Tonk, Physical Review, vol. 37, p. 930.
A single exposure camera has been developed at the proving ground for the measurement of the velocity of a projectile, together with a typical record. The film is stationary during exposure. The shutter is of the focal plane type, with a staggered slot. It is opened by a solenoid energized at initial recoil displacement of the gun (by an inertia switch). The camera is set under the trajectory and leveled. The angle of projection is known. As the projectile passes through the field of the camera, a line image is produced on the film. The duration of exposure is measured by the times between oscillograph contacts in an auxiliary circuit, the contactor of which is attached to the shutter. There is greater experimental difficulty in the use of this method for any routine conditioning than occurs with the gun coil method. In this or a modified form, it would be of possible advantage, however, for studies of projectile motion on an experimental range, firing horizontally.
Pressure measurement in gun.—On page 8 are exhibited the essential parts of an apparatus mounted on a 6-inch gun for the determination of pressure variation at a specific point in the chamber. A typical record is also shown. The optical system outlined in Fig. 4 is so arranged that the motion of the gun during recoil (prior to ejection) does not distort the photographic trace obtained on the moving plate (p). Also, there is attached to the surface (R) of the gauge block an optical recoil meter consisting of a steel mirror which is covered with black paper having narrow, parallel slits. As the gun recoils, a second beam of light, focused on this plane, is reflected intermittently and produces the auxiliary record of recoil shown at (D) on page 378. It is accordingly possible to identify the gun displacement at succeeding phases of the pressure-time curve. For the most exact establishment of time distribution of pressure, it is advantageous to substitute for the pendulum P a high speed film mounted on a rotating drum. This widens the record along the time axis and produces a uniform time scale.
The gauge is used primarily as a standard for calibration purposes since it requires piercing of the chamber of the gun. It has the dynamical properties required of an accurate indicating system for a cycle having high frequency components in the curve of pressure variation. Qualitatively stated, these are compact moving part of small inertia, large stiffness, and appreciable damping. The natural frequency of the gauge, as ordinarily used, is of the order 5,000-10,000, depending on the spring unit set in the slot S.
The difference between the two records shown on page 378, where were super-osed in printing, is typical of that produced, in the average, by using two orientations of the charge along the axis of the chamber of the 6-inch gun. This differential, or the corresponding change in velocity, is a factor of some importance in the case of certain guns.
In addition to the study of this source of dispersion, comparison firings have been conducted with the copper “crusher” gauge and with several types of instruments which record by means of the oscillograph. The latter instruments are the only kind very well suited to routine practice.
Among gauges of this class, the piezo-electric crystal with vacuum tube amplification is favorably regarded,4 although there is a legitimate basis for questioning the precision of the results obtained for cycles of the order of duration of those of medium and large calibers. It does not have the ruggedness and freedom from interference desirable in a technique to be made the basis for purely routine collection of data. Perhaps the most acceptable compromise for this application is a type which is, in effect, a microscopic, continuous-contact rheostat attached to a spring- piston unit of fairly high natural frequency. A gauge used in conjunction with the gun-coil system permits obtaining a complete ballistic record for each round, on one oscillograph film. Its dynamical characteristics are such as to insure satisfactory precision in following the pressure variations in any gun of service interest. A calibration is, of course, necessary.
4Arthur W. Ford, “Instruments for Making Ballistic Determination,” Army Ordnance Journal, vol. 7, no. 39
Time of flight of bombs, of anti-aircraft (fuzed) projectiles, etc. (Measurement of long time intervals).—Certain special problems in ballistic design are dependent, for satisfactory reduction, on a determination of rates of displacement and of acceleration of moving parts of gun systems in recoil or counter recoil, (or of auxiliary mechanism such as that for projectile seating, holding of bombs, etc.). Frequently the total interval of observation is too long to obtain satisfactory results with an oscillograph of ordinary characteristics. If the rate of motion of the oscillograph recording surface is decreased sufficiently to cover the entire interval, the precision of measurement may not be adequate. For these measurements and for determination of the time of fall of bombs, time of flight of projectiles to burst, etc., it has been found that an oscillographic apparatus which utilizes an ultra-speed moving picture camera as a recording system is quite satisfactory. Its application to the study of rapidly moving parts of machines, and to events separated by long intervals of time is particularly convenient.
The instrument was first assembled at this proving ground for the determination of times to burst of high explosive projectiles. Time calibration was accomplished by means of a pendulum and an auxiliary lens system superposing the image of the chronometer at one side of the film. A distinct advantage in this experiment is the fact that the recording camera may be kept at rest except for a second or two near the instant at which the events are to occur. In this case the two events are discharge of gun and burst of projectile. To show the burst most clearly in daytime it is desirable to have black smoke, and in the case of fuze tests a high order of detonation insures obtaining good records for times of flight as great as 25-30 seconds, provided that the lens of the camera is of good quality.
In the form of apparatus now employed, a microsplit stop watch is inserted in the field in place of the pendulum; this is a more compact arrangement and facilitates measurement of the records. With a camera taking 200 pictures per second, the probable error in time interval should not be more than 0.01 second. An excellent camera of this type, developed and made abroad, has been offered commercially within the last three years; it is equipped with an independent optical system for the chronometer. The proving ground apparatus had, for practical reasons, used the single optical system. The above-mentioned commercial form does not, however, employ the set of high frequency, solenoid-controlled indicators which constitute the “oscillographic” feature of the apparatus herein described. As many circuits as desired are observable in this manner by incorporating the required number of indicators, and thus as many remote events recorded with respect to time distribution. For highest precision, a calibrated, constant-angular-velocity chronometer may be introduced, and the probable error can be made much smaller.
For the measurement of time-of-fall of bombs, a high frequency relay and the small indicating arm, described in the preceding paragraphs, were placed in the circuit, the indicator appearing in the auxiliary field along with the chronometer. At release of bomb an automatic radio signal operated this relay, through the ground receiving station; the time of impact of the bomb was established by an interpolation of the pictures of the actual motion of the bomb near the ground. The velocity of fall of the bomb can also be obtained for a part of the trajectory near the end. The time-of-fall data are adequate to establish the characteristics of the bomb trajectory if taken for a series of altitudes covering the full range of service application, including low altitudes. By reference to a solution for the equations of motion of the bomb, usually stated with a square law of velocity as the resistance function, the mean time of fall at a specific altitude identifies the corresponding mean ballistic coefficient (for that altitude). By plotting the mean ballistic coefficients representative of the series of altitudes, the ballistic coefficient (or “terminal velocity”) representative of the motion after a specific time of fall can be determined from the faired curve, and, correspondingly, the air resistance value for the same section of the trajectory. Accordingly, bombs can be compared and the essential trajectory characteristics determined, with respect to a basis for sight calibration, by the measurement of mean times of fall for adequate samples at several representative altitudes.
On page 379 are presented a number of records obtained in experimental studies with this apparatus. Strip (a) identifies the time from discharge of a gun to the burst of a high-explosive projectile in its trajectory. A photograph showing blast phenomena occurring in the discharge of gun salvo is reproduced (b); the difference in ejection times for the three projectiles can be measured by means of the relative positions of the images in one of the exposures, as well as an approximation of the order of velocity. Recoil characteristics of the system can be obtained, with time distribution of displacement, from the successive positions of the muzzle. For the latter purpose, the camera is best placed at one side, and nearer the guns.
Muzzle phenomena.—The initial appearance of powder gas at the muzzle is usually as a non-luminous or dull red jet, the latter color originating in the heated particles of unburned powder. Within a few thousandths to hundredths of a second the expanding volume of gas is nearly always ignited, the flame of this secondary combustion being propagated throughout the spherical mass in an interval of 0.01 second or less. It is this event which is known as the “flash” of a gun and which produces the objectionable, blinding light. Development of powder and gun design to reduce flash is thus fundamentally a matter of preventing, or blanketing rapidly, the secondary explosion of the incompletely oxidized gases ejected from the bore.
An inspection of the photographs on pages 380-81 is sufficient to show that the first part of the expanding gas jet (consisting of carbon monoxide, hydrogen, carbon dioxide, and water vapor) is cooled well below the ignition point of the mixture with air; it will be noted that in some cases the boundary is non-luminous for several hundredths of a second after its propagation through air is initiated. During this time flame should appear in the surface areas if the temperature were, in that region, above that for ignition (about 750°C.). In accordance with a suggestion by Cranz, the heat accumulation, necessary for recurrence of the higher temperature, might be developed initially in the vicinity of the characteristic “stationary” pressure maxima in the jet, at which points compressions are high and jet velocities attain local minimum values. These abrupt differences in velocity and pressure are the result of the dissipative reaction between the surface of the high velocity stream and the adjacent medium.
The fragmentary appearance of brilliant luminescence in some of the pictures of first stages of ejection suggests that in certain cases, at least, ignition is actually developed directly by the hot gas from the gun. Although the boundary of the gas volume is below ignition temperature, the subsequently ejected gases are cooled less than the air mixture and may be still above the ignition temperature upon making contact with the explosive mixture near the surface at the forward boundary.
Inert (volatilizing) components introduced in powder lower the temperature of combustion, and a greater proportion of carbon dioxide tends to blanket the gases which produce the secondary explosion, although it does not appear that these are necessarily the principal factors which are involved in a deliberate suppression of flash. A lower temperature of combustion should result in lower muzzle temperatures and if carried far enough should therefore prevent the external combustion (through failure to attain ignition temperature). A long barrel, for a similar reason, may occasionally, or regularly if long enough, produce flashless results.
In the case of accidental variation from round to round with respect to occurrence of the secondary explosion the difference appears to be a matter of varying extent of mixture with air before the entire volume, including the central jet, is cooled below the ignition point.
Where salts are incorporated to produce flashlessness, a favorable result may occur from change in physical properties of the system or, in some cases, from the increased quantity of carbon dioxide. The latter source is noted as an important agency in the discussion by Cranz who points out, further, that mechanical directors for the ejected gases may be used effectively in reducing the volume of flame. It is reasonable to expect that a section of bore added at the muzzle, having deflecting outlet holes, might so dissipate the gas stream transversely that the quantity of gas actually burned outside the muzzle would be considerably decreased, if ignited at all. Photographs taken at the Naval Proving Ground of machine gun ejection with one such attachment show no flash.
Muzzle erosion.—It has been concluded that a principal agency producing muzzle wear is the escape of powder gas through the narrow annular orifice between the rotating band of the projectile and the surface of the bore. In a new gun, this orifice develops, ordinarily, only as the band breaks contact with the end of the bore. However, even in a new gun there will be some earlier escape of gas through a similar orifice (in the vicinity of the phase of maximum pressure). The early escape and expansion of this relatively small quantity of gas is at most a factor affecting wear at points in the bore other than the muzzle. As the gun is fired through a portion of its life, the orifice at the muzzle may be expected to progress inward, so that eventually the powder gases may pass the projectile in considerable quantity before the projectile reaches the muzzle. The velocity of expansion is several times that of the projectile. It has been a subject of controversial nature whether, in fact, this expansion actually does occur through such a moving orifice. It will be noted in certain cases that the gas jet from the muzzle is appreciable before even the nose of the projectile is visible. This represents the result for a gun in a partly worn state.
In these there is clearly to be seen the preceding wisp of non-luminous gas which was found in the case of the worn-gun records. It is concluded, therefore, that this element of gas escapes through the transient orifice around the projectile at and near the phase of maximum pressure, as a result of the deformation of the bore by radial pressure. There is no evidence of further escape of gas from the new-gun muzzle until the band breaks contact with the bore at the muzzle, while in the case of the worn-gun records the projectile nose is entirely obscured by the powder gas before the band reaches the muzzle. It is of interest to note that section of the gas which is reflected from the band of the projectile, and also the luminous element of gas carried in the wake, for 15 to 20 feet beyond the gas volume in which the element formed.
It is a well-known result that the bore erodes more rapidly over the outer surface of the protruding land than at the bottom of the groove of the rifling. There are several factors which appear to determine this result, with no obvious inconsistency in respect to the simple mechanism of gas flow as the principal actual agency of erosion at the muzzle. First, it is clear that the penetration of the isothermals is considerably greater at the corners of the lands, at any specific phase of the cycle, than at any other section of the bore surface. Diagonals from the corners of the lands inward are lines of maximum temperature at any time t and depth d, and there will be a tendency for the lands to approach the isothermals contour as the leveling process continues. The rate of wear is known to be a rapidly increasing function of the temperature of the boundary. Second, the gas flows at lower velocity in the grooves, after the band has broken away from the muzzle. Since an important component of muzzle wear probably occurs as a result of the expansive flow of gas over the surface subsequent to the ejection of the projectile, this is likely to be a factor in lowering the relative rate in the grooves, especially in view of the fact that the retarded flow in the grooves also means less circulation and lower temperature of gas in contact with the surface. Third, there is substantial evidence that the work of the dissipative forces at the surface of contact of the projectile and bore is sufficient to produce steel-softening temperatures in the surface of the projectile at appreciable depths. While the lands will not be heated as much, because of the continued change of surface, the land surface of the area in contact with the generatrix will be severely heated at the boundary, particularly in the vicinity of the muzzle where the forces are greatest and the temperature of the projectile surface is a maximum. Accordingly, it follow that the land surface will be at relatively higher temperatures than the groove, the latter not being in contact with the bourrelet and being heated appreciably only on one side (of the land) by the rotating band reaction. For this reason the further, immediate exposure to the high velocity jet, at the muzzle temperature of the gas, may be expected to erode the grooves less than the upper surface of the lands during ejection (of the gases). The evidence of reference with respect to heating from the surface reactions is that of recovered projectiles which show deep engraving of the steel body of the projectile on one side, a result which at present can be accounted for reasonably only on the assumption of surface softening. Finally, the grooves are almost invariably more thoroughly covered by copper than the surface of the lands, and usually with copper more firmly attached. Under these conditions, the wear of steel is likely to be distinctly less in the area of better protection.
Contrary to some of the recent conclusions summarized in the study of De Bruin,5 it is considered that available evidence supports a relatively simple theory of the mechanism of erosion of the bore at the muzzle, in some respects not unlike that of the older views of Vieille and Lanfroy. The principal wear is attributed to the action of the expansive flow of hot gases over the surface of the bore, first through the annular orifice around the projectile at or near its ejection and subsequently, at lower velocity, after projectile ejection is completed. At the same time it is true that a gas-leak theory is not required to account for some erosion in the chamber, in which circulation occurs with a condition of maximum gas temperature.
5G. de Bruin: Smokless Powder and the Erosion of Fire Arms, Communique de la sts. Anme “Fabriques Nederlandaises D’Explosifs” (Translation by Lieutenant Colonel Aiken Simons, E. I. du Pont de Nemours & Co. Inc.).
Blast effects.—An aspect of the exterior problem associated with gas ejection is that of acceleration of the projectile and of damage to adjacent structures by blast. The projectile is slightly accelerated while passing through the gas volume, emergence taking place at a point of considerable distance from the muzzle. The photographs on pages 380-81 show several typical determinations of the point. A rule which may be accepted for practically all modern guns is that the projectile emerges from the gas volume at a distance of 20-35 calibers from the muzzle.
It is possible to estimate roughly the velocity increment received by the projectile from the blast impulse. It is found to be of the order of a few tenth-foot seconds. Although the result is only a rough approximation, the effect of blast on the projectile in the few thousandths second of its motion through the gas volume is quite negligible, as far as translational velocity is concerned. Since the duration of the impulse from blast, at the same velocity, varies approximately linearly with d while the ratio jet reaction/mass varies as 1 /d, it follows that the order of velocity increment is the same for all calibers, (when firing similar projectiles at similar velocities).
Objects in the vicinity of the muzzle are exposed to high impulsive pressures associated with the wave of compression from the blast. Damage to structural material is best estimated by empirical data obtained in, or calibrated by, actual exposure to full-scale test, since extent of displacement depends on duration of the applied pressure, which is always very short, and static load deflection formulas give no reliable indication. Inasmuch as the damage is likely to be roughly proportional to the total impulse, the data secured by ordinary blast meters are a fairly good index of relative effects to be expected. No blast meter in routine use is competent to measure actual pressures, directly. It is possible, however, to obtain an indication of the pressure by an application of the theory of sound waves of large amplitude. While the waves developed are not, in the vicinity of the muzzle, approximately plane, the assumption of plane wave propagation is justified at only moderate distances from the muzzle. The corresponding values of velocity of wave, pressure increment, and impulse pressure for an equivalent, nondirective source (as in a detonation of a suspended quantity of explosive) are given in the following table taken from C. Cranz, Interior Ballistics; the gas expansion velocities are, of course, not applicable to the jet from the muzzle. So far as this application is concerned, the important consideration is merely that a unique relation exists for a specific source (gun and charge of powder) between the velocity of the wave and the intensity of the blast effect. The experiment is to identify throughout the field the loci of points of equal velocity, that is, equi-damage lines. The extent of actual damage depends on the kind of structure and requires to be determined empirically in any case.
An instrument has been constructed for the convenient measurement of the wave velocity at various points in the field, employing the oscillograph for recording. From a plot of the time-displacement data for the wave front, using several of these instruments, velocities are obtained by graphical differentiation. The instruments may be placed about two feet apart and a series of four is sufficient to give the required data in any specific vicinity, provided that the positions are carefully identified. Pressure, or impulse values, may be taken from the table, entering with measured velocity, and loci of equal pressure values drawn as blast curves.
Study of the motion of high explosive projectiles during and after penetration of armor.—The motion of a projectile from the first stages of impact with armor plate until fuze action is completed and burst occurs is one of the most interesting parts of the ballistic cycle. Photographic observation of the characteristics of motion in the plate have furnished a limited basis for estimating the extent of axial deflection in passing between separated plates, and for the study of the development of fracture which sometimes takes place in a time too short for penetration to become complete.
A typical study of projectile motion in Plate at high obliquity is shown on page 378.
The photographs of projectile burst on page 382 were obtained with a small hand film camera equipped with a solenoid shutter control; correct timing to insure opening just before burst of the projectile behind the armor plate is accomplished by a solenoid plunger which moves initially when the projectile passes through the wires of a chronograph screen before impact with the plate. This operates the shutter, set for about 0.1 second exposure. Since the opening occurs before burst of the projectile, the entire field is exposed prior to appearance of smoke. The only additional image to develop on the photographic plate is accordingly that of the flash of the burst. The smoke adds no appreciable exposure in its area and therefore does not show. The convenience of this single-exposure method, compared with moving picture recording, is that of a large image, with the certainty of an exposure at the exact instant of the burst, and consequent elimination of the obscuring action of the smoke.
IF A STATE IS ENVIRONED must by powerful rival states, and if the danger of war is always present, it must become militaristic and develop a vigorous foreign policy. If, however, it is weak, it must seek neutralization or protection through alliances and understanding with its least hostile neighbors. Under militaristic systems, as Herbert Spencer so clearly showed, domestic problems and institution must be subordinated to the primal necessity of self-preservation, so that internal development and general welfare must be correspondingly slighted. If, on the other hand, a state has neighbors not dangerous to its national existence, it may devote itself fundamentally to domestic problems and seek to build up the economic, social, and cultural interests of the people. The one policy tends to centralization and the subordination of the individual; the other to decentralization and to the national coöperation with individuals in the furtherance of common policies beneficial to national and individual alike. The United States, for example, situated far from Europe and having adjacent no dangerous rivals, has seldom had to engage in foreign wars and hence has tended to neglect foreign policies, but it has developed well defined domestic policies, such as the promotion of democracy and education, the development of natural resources and the strengthening of agriculture, commerce, and manufactures. Its foreign policies have aimed chiefly at the preservation of peace and friendly relationship with other nations.—Dealey.