A piece is said to fire at random-shot, when the breech rests upon the bed of the carriage, so that the ball is carried to the greatest possible distance. But as, in this method of firing, the ball cannot be directed to any determinate object, it is rarely used in the sea-service, and only when the shot cannot fail of doing great execution in the place whereon it falls.
Besides the two ranges above described, there is the ricochet[[45]], invented by the Marshal de Vauban.
To fire a piece by way of the ricochet, the cannon is only charged with a quantity of powder sufficient to carry the shot along the face of the works attacked. The shot, thus discharged, goes rolling and bounding, killing, maiming, or destroying all it meets in its course, and creates much more disorder by going thus slowly, than if thrown from the piece with greater violence.
When ricochet firing is used, the pieces are elevated from 3 to 6 degrees, and no more; because if the elevation is greater, the shot will only drop into the work, without bounding from one place to another. They are to be loaded with a small charge, and directed in such a manner as just to go over the parapet[[46]].
It was the opinion of engineers formerly, that by charging the pieces high, the ball was thrown to a greater distance. Hence the pieces were charged with two thirds, or even the whole weight of the shot, in order to impel it with greater velocity; but it has been discovered since, that the half, or one third of the weight of the ball, is the fittest charge for the piece[[47]].
If the whole quantity of powder, employed to charge the cannon, could take fire at the same instant, it is apparent that the velocity, communicated to the shot, would increase in proportion to the additional quantity of powder. But though the time of its inflammation is very short, it may yet be conceived as divided into many instants. In the first instant, the powder begins to dilate and impel the shot forward; and if it has force enough to expel it from the piece before the whole charge is inflamed, that part which is left to take fire afterwards will produce no effect at all on the shot. A charge of extraordinary force does not therefore accelerate the velocity of the bullet: and hence it follows that the piece ought to be charged with no more powder, than will take fire whilst the ball is passing through the chace of the cannon.
It may not be amiss to observe here, that the range of cannon is greater in the morning and at night, than at noon; and in cold, than in hot weather. The reason is, that at these times the air being less heated, gives less way to the dilatation of the powder, which being by this means confined, as it were, to a smaller sphere of action, must have a stronger effect in proportion[[48]].
“When the lengths of cannon are proportional to the height of the charge, the shot will be discharged with the same velocity, whatever the calibre may be; and since the ratios of the velocities of shots, issuing from pieces of different lengths, loaded with different charges of powder, will be of great use in the construction of cannon, we have collected them in the following table, where the numbers at the top express the length of the pieces by the diameter of their shots. That is, the first is 12 diameters; the second 15, and so on. The first perpendicular column expresses the charges, in respect to the weight of the shots: thus, ¼, ⅓, ½, ⅔ imply that the weight of the charge is ¼, ⅓, ½, ⅔ of the weight of the shot. The other numbers, in the same horizontal lines, express the distance in feet moved over by the velocities of the shot, uniformly continued in a second of time.
| A Table of Velocities. | ||||||||
|---|---|---|---|---|---|---|---|---|
| 12 | 15 | 18 | 21 | 24 | 27 | 30 | 36 | |
| ¼ | 1043 | 1052 | 1058 | 1063 | 1066 | 1068 | 1071 | 1074 |
| ⅓ | 1186 | 1200 | 1210 | 1217 | 1222 | 1224 | 1229 | 1234 |
| ½ | 1406 | 1434 | 1452 | 1465 | 1475 | 1482 | 1488 | 1497 |
| ⅔ | 1568 | 1613 | 1641 | 1662 | 1677 | 1688 | 1698 | 1711 |
“We made use of the diameter of a 9 pound shot, which being 4 inches, is more convenient in the calculation; and this diameter expresses the height of the charge when it is a quarter of the weight of the shot, and the rest in proportion.