618 POTTERY 



cast iron is made fast to the wooden beams -which support all the upper mechanism of 

 the mill-work. The bearing m! is disposed in an analogous manner ; but it is sup- 

 ported against two cast-iron columns, shown at L" L", in fig. 1651. The guide tubes 

 M" are bored smooth for a small distance from each of their extremities, and their in- 

 terjacent calibre is wider, so that the vertical shafts touch only at two places. It is 

 obvious, that whenever the shaft if is set agoing, it necessarily turns the wheels M and 

 M', and their guide tubes M" ; but the vertical shaft may remain either at rest, or re- 

 volve, according to the position of the lever-click or catch K, at the top, which is made 

 to slide upon the shaft, and can let fall a finger into a vertical groove cut in the sur- 

 face of that shaft. The clamp-fork of the click is thus made to catch upon the hori- 

 zontal bevel wheel M', or to release it, according as the lever K is lowered or lifted up. 

 Thus each millstone may be thrown out of or into gear at pleasure. 



These stones make upon an average 11 or 12 turns in a minute, corresponding to 3 

 revolutions of the water-wheel, which moves through a space of 3 feet 4 inches in the 

 second, its outer circumference being 66 feet. The weight of the upper stone, with its 

 iron mountings, is about 6 cwts. when new. The charge of each mill in dry material 

 is 2 cwts. ; and the water may be estimated at from one-half to the whole of this weight ; 

 whence the total load may be reckoned to be at least 3 cwts. ; the stone by displace- 

 ment of the magma, loses fully 400 pounds of its weight, and weighs therefore in 

 reality only 2 cwts. It is charged in successive portions, but it is discharged all at 

 once. When the grinding of the siliceous or felspar matters is nearly complete, a 

 remarkable phenomenon occurs : the substance precipitates to the bottom, and assumes 

 in a few seconds so strong a degree of cohesion, that it is hardly possible to restore it 

 again to the pasty or magma state ; hence, if a millstone turns too slowly, or if it be 

 accidentally stopped for a few minutes, the upper stone gets so firmly cemented to the 

 under one, that it is difficult to separate them. It has been discovered, but without 

 knowing why, that a little vinegar added to the water of the magma almost infallibly 

 prevents that sudden stiffening of the deposit and stoppage of the stones. If the mills 

 come to be set fast in this way, the shafts or gearing would be certainly broken, were 

 not some safety provision to be made in the machinery against such accidents. Mr. 

 Hall's contrivance to obviate the above danger is highly ingenious. The clutch I, I', 

 Jig. 1651, is not a locking-crab, fixed in the common way, upon the shaft i, ; but it is 

 composed, as shown \nfigs. 1654, 1655, 1656, 1657, of a hoop, u, fixed upon the shaft 

 by means of a key, of a collar v, and of a flat ring or washer x, with four projections, 

 which are fitted to the collar v by four bolts, y. Fig. 1655 represents the collar v, seen 

 in front ; that is, by the face which carries the clutch teeth ; and.%. 1656 represents 

 its other face, which receives the flat ring x, fig. 1657. in four notches corresponding 

 to the four projections of the washer-ring. Since the ring u is fixed upon the shaft i, 

 and necessarily turns with it, it has the two other pieces at its disposal, namely, the 



collar v, and the washer X, because they are 

 always connected with it by the four bolts y, so 

 as to turn with the ring M, when the resistance they 

 encounter upon the shaft i/ is not too great, and 

 to remain at rest, letting the ring u turn by itself, 

 when that resistance increases to a certain pitch. 

 To give this degree of friction, we need only 

 interpose the leather washers s, c', fig. 1654 ; and 

 now, as the collar coupling-box, v, slides pretty 



O\^ J /^\ N <-i freely upon the ring u, it is obvious that by 



/j \ \^ J p tightening more or less the screw-bolts y, theso 



washers will become as it were a lateral brake, 

 to tighten more or less the bearing of the ring 

 , to which they are applied : by regulating this 



pressure, everything may bo easily adjusted. When the resistance becomes too great, 

 the leather washers, pressed upon one side by the collar v, of the washer x, and rubbed 

 upon the other side by the prominence of the ring u, get heated to such a degree, that 

 they are apt to become carbonised, and require replacement. 



This safety clutch may be recommended to the notice of mechanicians, as suscep- 

 tible of beneficial application in a variety of circumstances. 



Great Porcelain Mill. The large felspar and kaolin mill, made by Mr. Hall, for 

 Sevres, has a flat bed of hornstone, in one block, laid at the bottom of a great tub, 

 hooped strongly with iron. In most of the English potteries, however, that bed 

 consists of several flat pieces of chert or hornstone, laid level with each other. There 

 is as usual a spigot and faucet at the side, for drawing off the liquid paste. The 

 whole system of the mechanism is very substantial, and is supported by wooden 

 beams. 



The following is the manner of turning the upper blocks. In fig. 1650 the main 



