Firing rockets from cannon can only be practised under certain circumstances. The observations already made on the granulation of gunpowder will have prepared the reader for this announcement. When fired from a cannon under the old régime, the rocket was projected at high velocity, and the case of the rocket was destroyed by the very force which set it in motion. A rocket suitable for artillery should be cast of gun metal, with a frame of considerable strength. In form it should nearly approximate to an expansive bullet; but, instead of the limited length of one and three quarters diameter; it should approach to four diameters; two of which, at least, should be appropriated to the cylinder behind the head. The head is charged with composition more densely driven than is customary in the ordinary rocket; the tubes in the cylinder are also charged with a composition equally dense. The outer frame of the rocket is cast with suitable projections to fit the grooves of the gun: the spiral of these grooves is considerable, being one turn in every three feet, in order to impart to the rocket an effectual spinning motion when in a low state of velocity. The rocket properly constructed is then placed in the rocket-gun, and fired in the usual way; but it is essential that the gunpowder used should be of a suitable quality: its combustion must be as slow as possible, a starting velocity of from 500 to 800 feet per second being sufficient to ensure the flight of the self-sustaining projectile to the end of its range. This principle may be extended from a light firebrand, as already stated, to that of a rocket charged in the head with the most deadly and destructive fulminate. It may appear absurd to speak of fulminates being projected; since all experiments show that fulminates, even when adulterated, will not stand the concussion of a discharge, but invariably ignite in the gun, however carefully placed or packed in the shell which contains them: for this reason fulminates have never been successfully used. But if the fulminate is placed in the head of a rocket, this objection may be obviated. The gradual manner in which velocity is given to a rocket does not subject it to violent displacement during its flight; neither need the concussion in the gun be severe, owing to the nature of the gunpowder used, which in its gradual expansion is analogous to steam: thus the field for the application of fulminates is opened to an unlimited extent.