This section is from "The Encyclopaedia Britannica". Also available from Amazon: Great Books of the Western World (60 Volumes).

The following exercises will show the application of the ballistic table. A slide rule should be used for the arithmetical operations, as it works to the accuracy obtainable in practice.

Determine the time t sec. and distance s ft. in which the velocity falls from 2150 to 1600 f/s.

(a) of a 6-in. shot weighing 100lb, taking n = 0.96,

(b) of a rifle bullet, 0.303-in. calibre, weighing half an ounce, taking n = 0.8.

V. | v. | T(V). | T(v). | t/C. | S(V) | S(v) | s/C. |

2150 | 1600 | 28.6891 | 27.5457 | 1.1434 | 20700.53 | 18587.00 | 2113.53 |

d. | w. | C. | t/C. | t. | s/C. | s. | |

(a) | 6 | 100 | 2.894 | 1.1434 | 3.307 | 2113.53 | 6114 (2038 yds.) |

(b) | 0.303 | 1/32 | 0.426 | 1.1434 | 0.486 | 2113.53 | 900 (300 yds.) |

Determine the remaining velocity v and time of flight t over a range of 1000 yds. of the same two shot, fired with the same muzzle velocity V = 2150 f/s.

S. | s/C. | S(V). | S(v). | v. | T(V). | T(v). | t/C. | t. | |

(a) | 3000 | 1037 | 20700.53 | 19663.53 | 1861 | 28.6891 | 28.1690 | 0.5201 | 1.505 |

(b) | 3000 | 7050 | 20700.53 | 13650.53 | 920* | 28.6891 | 23.0803 | 5.6088 | 2.387 |

* These numbers are taken from a part omitted here of the abridged ballistic table.

In the calculation of range tables for direct fire, defined officially as "fire from guns with full charge at elevation not exceeding 15°," the vertical component of the resistance of the air may be ignored as insensible, and the actual velocity and its horizontal component, or component parallel to the line of sight, are undistinguishable.

Fig. 1.The equations of motion are now, the co-ordinates x and y being measured in feet,

(26) | d2x | = -r = | -gp | , |

dt2 | C |

(27) | d2y | = -g. |

dt2 |

The first equation leads, as before, to

(28) t = C{T(V) - T(v)},

(29) x = C{S(V) - S(v)}.

The integration of (24) gives

(30) | dy | = constant - gt = g(&FRAC12;T - t), |

dt |

if T denotes the whole time of flight from O to the point B (fig. 1), where the trajectory cuts the line of sight; so that &FRAC12;T is the time to the vertex A, where the shot is flying parallel to OB.

Integrating (27) again,

(31) y = g(&FRAC12;Tt - &FRAC12;t2) = &FRAC12;gt(T - t);

and denoting T - t by t′, and taking g = 32f/s2,

(32) y = 16tt′,

which is Colonel Sladen's formula, employed in plotting ordinates of a trajectory.

At the vertex A, where y = H, we have t = t′ = &FRAC12;T, so that

(33) H = ⅛gT2,

which for practical purposes, taking g = 32, is replaced by

(34) H = 4T2, or (2T)2.

Thus, if the time of flight of a shell is 5 sec., the height of the vertex of the trajectory is about 100 ft.; and if the fuse is set to burst the shell one-tenth of a second short of its impact at B, the height of the burst is 7.84, say 8 ft.

The line of sight Ox, considered horizontal in range table results, may be inclined slightly to the horizon, as in shooting up or down a moderate slope, without appreciable modification of (28) and (29), and y or PM is still drawn vertically to meet OB in M.

Given the ballistic coefficient C, the initial velocity V, and a range of R yds. or X = 3R ft., the final velocity v is first calculated from (29) by

(35) S(v) = S(V) - X/C,

and then the time of flight T by

(36) T = C{T(V) - T(v)}.

Denoting the angle of departure and descent, measured in degrees and from the line of sight OB by φ and β, the total deviation in the range OB is (fig. 1)

(37) δ = φ + β = C{D(V) - D(v)}.

To share the δ between φ and β, the vertex A is taken as the point of half-time (and therefore beyond half-range, because of the continual diminution of the velocity), and the velocity v at A is calculated from the formula

(38) T(v) = T(V) - | &FRAC12;T | = &FRAC12;{T(V) + T(v)}; |

C |

and now the degree table for D(v) gives

(39) φ = C{D(V) - D(v)},

(40) β = C{D(v) - D(v)}.

This value of φ is the tangent elevation (T.E.); the quadrant elevation (Q.E.) is φ - S, where S is the angular depression of the line of sight OB; and if O is h ft. vertical above B, the angle S at a range of R yds. is given by

(41) sin S = h/3R,

or, for a small angle, expressed in minutes, taking the radian as 3438′,

(42) S = 1146h/R.

So also the angle β must be increased by S to obtain the angle at which the shot strikes a horizontal plane - the water, for instance.

A systematic exercise is given here of the compilation of a range table by calculation with the ballistic table; and it is to be compared with the published official range table which follows.

Continue to: