134 OF gun-metal.

than 15 0 may be owing to the bombardiers that load themortar; or to the recoil not being in the direction of therange: the too great length of the chafe is the cause of boththese circumstances; for before the shell can quit the mortar,it has recoiled a space equal to half the difference between thediameters of the tnortar and of the shell; the direction of thelatter is then altered by slrikingagainst the mortar, whichis frequently much damaged.

77. she chase should be so short that the bombardiermay easily adjust the powder and the shell; and that the shellin quitting the mortar may not be able to strike against it,and alter its recoil. The chafe of the largest mortars shouldbe a diameter and half of the bore in length, which wouldgreatly facilitate the loading: upon this principle the chafe ofsmall mortars might be longer: but as the striking of theshell against the mortar depends on the difference betweentheir diameters, the length of the chafe must in all mortarsbe proportionate to the diameter of the bore, the properdegree of windage having been previously determined.

78. From the most accurate experiments that have beenhitherto made, it appears that mortars will be of the bestproportion, when the length of the chafe does not exceed adiameter and half of the bore, and when the windage is23: 22. If the windage be lessened, the chafe must also beshoitened, as in mortars used in proving powder; wherevery little windage being allowed, the length of the chase isonly a diameter and a quarter of the bore.

79. The best length of the chafe in mortars, for facili-tating the operations of the bombardiers and preventing theshock of the shell, and the obliquity of the recoil beingdetermined ; it is necessary to have recourse to expedientsto give the greatest initial velocity to the shells, which isoften required on service without lengthening the chase ; thisis effected:

1. By constructing the chamber of such a figure thatthe powder being collected close to the vent, more ofthe fluid inay be generated in equal times: the sphericalfigure is the best for answering this purpose, as has beenexplained in the Treatife on Powder.

2- By opposing a great resistance to the explosion ofthe powder, without increasing the weight of the shell,by which means more of the elastic fluid will be gene-rated before it is in motion; this resistance may be aug-mented by placing a tompion of wood over the earth