OF GUN-METALt i$i

the metal is running into the moulds, at the beginning,the middle, and end of the process, the several pieces, whencold, will be homogeneous.

HZ. If an excessive heat, whether produced by fire ora combination of solvents, be attended with bad effects; adeficiency in that respect is not less pernicious': for inde-pendent of the imperfect mixture, the metal does not setproperly in the moulds; whence proceed the cavities andflaws frequently observable on the outlides of guns afterproof: these can neither be attributed to the intrinsic qualityof the bronze, nor the want of proportionable thickness,but to the defect of heat and mixture; fince the externalflaws do not extend and communicate with the bore : fromthe same cause proceed the asperities visible on the ornamentsand bas-reliefs.

The deficiency, in point of heat, does not always arisefrom too small a quantity of wood being used to heat thefurnace, since with the same quantity it takes sometimes36 or even 48 hours, to effect what at other times 16 or x8are sufficient to do. This can only be accounted for froma want of elasticity in the atmosphere, which prevents thewood from burning with proper activity; or from the neg-lect of the workmen in removing the cinders, which partlystops the reverberation of the flame ; thus the fire not beingso intense, it is longer before the metal acquires the properdegree of fusion.

1x6. There are some accidents which baffle the utmost:caution, as they proceed from variations in the state of theatmosphere. If the melted matter while running into themould be exposed to a cold wind, it may be suddenly con-gealed, which will prevent a due tenacity in the particles whensolid. After the mould has been heated, if the atmospherebecome suddenly loaded with vapour, the mould will im-bibe humidity ; and the melted metal will extract the mois-ture, which rushing out and bubbling vehemently, will dis-turb its setting. On the contrary, when the mould ts per-*fectly dry, repeated experience proves that the metal onrunning into a hot recipient, sets better, and becomes moretenacious and dense, but less hard than when the recipientis cold.

1 iy. Since cavities proceed from the springiness of thecopper, or from the coldness and moisture of the atmospherqer mould; they are found in all parts of the gun indiscri-K 4 minately,