It has been supposed by some, that a more com plete inflammation of the powder would take place, if it were ignited at the bottom of the hole ; but it has been found by experience, that this is not essen tial. It even appears from experiment, that gull powder is more fully inflamed by applying the fire near the top, for in firing Ordnance, considerable quantities of the uninflamed grains of the powder have been collected, by placing a piece of cloth at a distance from the gun, and allowing the shot to pass throu?h it.
Quick-lime, suddenly slacked, has been suggested, as a mode of rending rocks; this process would in practice be found very deective. In some situa tions where the explosive effects of gunpowder could not be applied with safety, as in the. interior of a building, or for such like purposes, this mode may be resorted to with good effect.
Blasting with gunpowder, under water, is neces sarily performed in various operations of the Engi-1 neer, particularly in the excavation of the founda tions of piers, and in deepening the entrances of harbours. This is performed by inserting the charge of powder into. the perforated rock, by means of a case or cartridge made of tin-plate. The lower part is made to fit the bore in the rock, as nearly as may be ; from this a small pipe of the same metal is car ried to the surface of the water, with the priming powder. Here wlidding is unnecessary, the pres sure of the water upon the tin case superseding the necessity of any ; and the explosive effects are gene rally greater, in proportion to the charge, than those in the open air. It may here be observed, that ex
plosions under water have, in some instances, been proposed as a mode of attack in marine warfare; and it is presumed, that explosions, at considerable depths, might occasion such an impression on the water, and -so disturb the equilibrium of the atmo spheric pressure, as to be capable of sinking large ships, or floating batteries.
It may also be mentioned, that a new instrument, called the Blasting Screw, has been lately applied with considerable success to the rending or splitting of large trees and logs of timber. It consists of a screw which is wrought into an auger-hole, bored in the centre of the timber ; here the charge of powder is inserted, and the orifice of the hole in the log is then shut up or closid with the a match, or piece of cord, prepared with saltpetre, is intro duced into a small hole left in the screw for this purpose, by which the powder is ignited. The appli cation of this screw to the purposes of blasting is not very obviously necessary, because, from what we have seen, it would appear that the auger-hole being charged with powder and sand, would answer every purpose. One great objection to the process of blasting applied to the rending of timber is, the ir regular and uncertain direction of the fracture, by which great waste is sometimes occasioned. It may, however, be necessary to resort to this mode of breaking up large trees, when cut down and left in inaccessible situations, where a great force of men and of implements cannot easily be procured or ap plied.