170 LECTURE XIX. 



mangle for linen acts nearly in a similar manner. In calendering mills, 

 the force of a spring is employed for exerting a pressure on the block with 

 which the materials are glazed. 



The copper plate printing press, and the machine for copying letters, are 

 composed of two rollers parallel to each other, pressing on the substance 

 which is interposed, and which is brought into its situation partly by the 

 friction of the surface of the roller and partly by external force. 



The rollers by which sugar canes are pressed, are in general situated 

 vertically, the middle one of three being turned by horses, by mules, or by 

 water, and the canes being made to return round it so as to pass through 

 both interstices in succession. It appears to be of some advantage in presses 

 of this kind that all the rollers should be turned independently of their 

 action on the materials interposed, since the friction of two rollers may 

 tend to draw the materials into the space between them, with more regu- 

 larity and greater force, than the action of a single roller would do. For 

 this reason, it may be advisable to retain the toothed wheels turning the 

 rollers, even when their axes are not firmly fixed but held together by 

 an elastic hoop. (Plate XVIII. Fig. 231.) 



In oil mills, a still greater momentum is applied to the purpose of com- 

 pression than in the printing press : hammers, or long wooden beams placed 

 vertically, are raised by a water wheel, and suffered to fall on wedges 

 which act very forcibly on the materials contained in bags on each side. 



Compression is also sometimes performed by the operation of hammer- 

 ing : thus, cast brass is generally hammered before it is used, in order to 

 increase its strength ; the hammer renders it so much stiffer, that if it is 

 necessary to preserve its ductility, it must be frequently annealed by 

 exposure to heat. Anvils and vices are necessary appendages to the 

 hammer ; their use depends principally on their firmness, which is chiefly 

 derived from weight in the one case, and from strength in the other ; and 

 pincers may be considered as portable vices. 



For the purpose of producing a continued pressure on such substances 

 as have a tendency to contract their dimensions under the operation of a 

 press, a spring has been interposed between the press and the materials, 

 which is capable of pursuing them with a certain degree of force : the 

 utility of such an arrangement must, however, be extremely limited. 

 Mr. Bramah has applied a well known law of hydrostatics to the construc- 

 tion of a very useful press, which is simple, powerful, and portable. 



Extension is seldom performed by forces that tend immediately to in- 

 crease the dimensions of the substance only : it is generally procured by 

 reducing the magnitude of the substance in another direction, sometimes 

 by means of pressure, but more effectually by percussion. The rollers of 

 the press employed for laminating metals are turned by machinery, and 

 are capable of being moved backwards and forwards in order to repeat the 

 operation on the same substance ; their distance is adjusted by screws 

 which are turned at once by pinions fixed on the same axis, in order that 

 they may be always parallel. In this manner lead, copper, and silver, ate 

 rolled into plates, and a thin plate of silver being soldered to a thicker one 

 of copper, the compound plate is submitted again to the action of the 



