The latter quantity is determined by experiment. A similar rule may be written out from eq. (3) for mines differing but little from common mines.
5. The quantity of gunpowder required to throw out a cubic yard of material has been calculated from a great number of mines fired in different kinds of soil. The following table gives the quantities required according to Lebrun and Macaulay, respectively the French and English authorities on the subject:[8]
| Number. | Description of Earth, Rock, or Masonry. | Weight per cubic foot. | Charge, Gumpertz and Lebrun.v | Charge, Macaulay. | Proportional value of charge. |
|---|---|---|---|---|---|
| lbs. | lb. oz. | lb. oz. | |||
| 1 | Light sandy earth (common earth, Lebrun) | 85 | 1.8 | 1.13 | 1.12 |
| 2 | Hard sand | 111 | 1.10¾ | 2.0 | 1.25 |
| 3 | Fat earth mixed with sand and gravel (common earth, Macaulay) | 116 | 1.5⅓ | 1.10 | 1.00 |
| 4 | Wet sand | 118 | 1.12 | 2.2 | 1.30 |
| 5 | Earth mixed with stones | 118 | 1.14 | 2.4 | 1.40 |
| 6 | Clay mixed with tufa | 124 | 2.1 | 2.8 | 1.55 |
| 7 | Fat earth mixed with pebbles | 143 | 2.4 | 2.12 | 1.69 |
| 8 | Rock | 143 | 3.0 | 3.10 | 2.25 |
| 9 | New or old moist brickwork or masonry | 2.2 | 1.30 | ||
| 10 | Inferior brickwork or masonry | 2.11 | 1.66 | ||
| 11 | Good, new ditto | 3.10 | 2.25 | ||
| 12 | Good, old ditto | 4.1 | 2.50 | ||
| 13 | Roman ditto, or other equally good in warm climates | 4.11 | 2.90 |
6. For common mines in ordinary earth a convenient rule, very generally used, and which gives results nearly the same as those deduced from the table, is:
The charge of gunpowder in pounds is equal to one tenth the cube of the line ne of least resistance in feet, or
| C lbs. = (1/10)l3 ft. (5) |
OVERCHARGED AND UNDERCHARGED MINES.
7. For overcharged and undercharged mines in which the L. R. R. and crater radius differ materially in length the results deduced from the preceding equations are not applicable. For such mines the following equations, due to Gumpertz and Lebrun, are in common use, viz.:
For an overcharged mine,
| C = C´(11/6)[l + (7/8)(r - l)]3. (6) |