Jet des bombes, Fr. This word has been adopted instead of Tir, which formerly expressed the course that a shell took when it was thrown out of a mortar by the power of gunpowder.
We sometimes use the words flight and range, to express the same action and progress.
The jet or flight of a bomb usually forms a curved line; but many engineers assert, that when the mortar is placed horizontally, it describes the three movements that are made by a cannon ball, viz. The violent or strait forward one, the mixed or curved, and the natural one, which is perpendicular.
It is particularly incumbent upon the officers who superintend the mortar duty, to ascertain, by a correct observation of the eye, the exact distance to which he means to throw the bomb. With this view he must give as many degrees of elevation as may be found necessary by the judgment he has formed.
In order to obtain some degree of certainty he first throws a bomb, by way of experiment, and he increases or diminishes his degrees of elevation according to the distance it runs, and from the spot on which it falls.
These are the only rules which are generally followed by those officers who have the direction of mortars. However, according to St. Remi the French bombardiers frequently make use of tables in order to calculate precisely the different lines of extent according to the different elevations of the mortar, particularly with respect to the degrees of the square rule from 1 to 45·
Although this method has been sanctioned by various and innumerable experiments, it has nevertheless been exposed to some censure. Mr. Blondel has written a treatise on the subject. This engineer asserts, that he has discovered a way of firing true, which exceeds all former inventions.
We are of opinion, that the best method must be that which is founded upon practical and daily experience. Those men who are in the continual habit of exercising in mortar duty, and who can form just calculations, especially with regard to the quality and quantity of gunpowder, will always be esteemed in preference to the most profound theorists.
According to the experiments which have been made by bombardiers with respect to the flight of bombs, a mortar is said to propel or urge forward in proportion to the quantity and quality of the gunpowder, by which it is charged.
A mortar, for instance, which has twelve inches calibre, and which is loaded with two pounds of mealed gunpowder gives a difference in its flight of 48 feet from one degree to another; and 2160 feet in its greatest extent under the elevation of 45 degrees.