222 , LECTURE XIX. 



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

 sion than in the printing press: hammers, or long wooden beams, placed ver- 

 tically, 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 hammering: 

 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 pre- 

 serve its ductility, it must be frequently annealed by exposure to heat. Anvils 

 and vices are necessary appendages to the hammer; their use depends princi- 

 pally 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 ap- 

 plied a well known law of hydrostatics to the construction of a very useful press, 

 which is simple, powerful, and portable. 



Extension is seldom performed by forces that tend immediately to increase 

 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 em- 

 ployed 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, are rolled into plates, and a thin plate 

 of silver being soldered to a thicker one of copper, the compound plate is sub 

 mitted again to the Action of the press, and made so thin as to be afforded at 

 a qioderate expense. The glazier's vice U a machine of the same nature, for 

 forming window lead: the softness of the lead enables it to assume the re- 



