The most approved method of constructing the .mould of a gun is in dry sand, and this is the me thod now practised in Britain, Guns cast in loam do not come from the Mould with a surface so cor rectly resembling of the model as those cast is dry sand, and in order to render the surfaee correct, and to remedy defects, it was always necessary to subject them to the process of turning. In ens carefully east in dry sand, the process of turning might be dispensed with, the gun would then be the outer skin of metal which, ha ving cooled mere rapidly than the other parts, is the hardest; this outer skin is also less liable to rust than the surface laid bare by turning. The mould of a guts in dry sand, at the same time that it is more accurate, is also sooner made and chied than a loim mould. Dry sand-moulding is a part of the busi: ness of the loam-moulder.
The sand for dry sand-mouldings is made by mix ing a quantity of sharp refractory sand with water in which day has been diluted. After the mixture is thoroughly made, if a handful is grasped, and on opening the hand the sand retains the form given .it„ then the consistence of the mixture is good. The sand should have the following qualities : lit, It should not be fusible by time heat of melted cast iron; if it were, it would adhere to the metal, and snake the surface of the gun rough. -2cIly,it must be sharp, and composed of angular particles ; If the particles of the sand were round, it would not hold together on taking out the model. Silly, It must not contain too much clay, for in that cue it would crack in drying. ably, It must contain a certaie proportion of clay to retain the form that the model impresses on it.
The length of each piece of the model should be a very little greater than the given length of the corresponding part of the gun; because the length of the mould is the length of the gun whilst hot ; and this is longer than the length of the gun when it comes to the temperature of the atmosphere, at which temperature the dimensions of the guns are given. It has been estimated that some kinds of cast-irou contract six-hundredths of an inch in a foot, in passing from the liquid state to the tempera ture of the atmosphere. This contraction is not caoeidereble enough to be taken into tonsideration in the diameter of the pattern. The ShriPlang of
the sand in drying. thong not censideratite *lids Ince wise to make the piece shorter, and is another mo tive for making the pattern a little longer than the dimensions taken from a gun at.the usual tempera ture. The patterns of the trunions are attached to the pattern of the second reinforce by screws, so as to be unscrewed and separated when the pattern is to be lifted out of the sand.
The gun-box, in which the dry sand mould is to be formed, is of cast iron, and cast in sand. It consists of portions ; each of' these portions has flanges by which it is fixed to the others, and the whole, when connected, together, form the gun-box. In the flanges are holes through which bolts are passed; the bolts are secured by wedge formed keys ; thus the differ ent parts of the box are firm ly held together. The two portions of the gun-box which contain the breech-ring and cascabel are single, not being divided longitudinally. Each of the other five transverse portions is divided longitudi nally into two. A handle is fixed to each portion of the box, for the purpose of mov ing it. The upper transverse portion AA contains the gun head. In each of the two portions BB, which contain the second reinforce, there is a lateral projection for the trunions. The figure repre sents the gun-box with the breech lowermost, in the posi tion in which it is placed when the metal is poured in.
To make the mould, the pattern of the breech is first placed on a board, and the corresponding por tion of the gun-box is put over it, and sand is ram med between the pattern and the box. The fiat ex posed surface of the sand is painted over with black mg, which consists of charcoal and clayed water, that there may be no adhesion with the sand of the next portion of the mould. The pattern of the first reinforce is now fitted into the pattern of the breech, and the corresponding portions of the first reinforce box adjusted on the flange of the breech box. 'Sand is well rammed, in small quantities at a time, be tween the pattern and the box ; and the upper fiat surface of the sand is painted over with blacking. The mould is completed by adding the remaining pieces of the model and of the box, one above ano ther, ramming the sand, and painting the transverse surface of the sand at the top of each division of the box with blacking. The sand must be strongly rammed and equably, that every part of its surface may be able to resist the pressure of the liquid me tal. Three little wedges are interposed between the two adjacent transverse portions of the box, that the sand may project a little, so that after it is dry it may be flush with the box ; if this were sot done, there would be an interval between the adjacent surfaces of the sand, through which the metal would pass and form a fin.