334 LECTURE XIX. 



cylindrical sieve, placed in an inclined position, and turned by machinery. 

 (Plate XVIII. Fig 238.) 



When the flour is made into bread, the dough requires to be kneaded : for 

 this purpose a machine is sometimes used, in which four or more bars, parallel 

 to the axis of motion, are turned round, by means of a walking wheel. The 

 dough is placed in a circular trough, in which the bars revolve not quite in the 

 middle, so as to approach in each revolution to one of its sides, and thus the 

 dough is perpetually compelled to change its form. 



A machine of nearly the same construction is employed for levigating flints, 

 after they have first been made red hot, and plunged into cold water, in order 

 to render them friable. They are mixed, when it is necessary, with other 

 large stones, and the water, in which the process is performed, carries oft" the 

 powder, and deposits its coarser parts in a short time, while the finer remain 

 much longer suspended, and are thus separated from the rest. 



When a mechanical structure is to be demolished, or a natural substance to 

 be broken into smaller parts, we have often occasion to employ the collected 

 force of men, the powers of machinery, or the expansive force of chemical 

 agents. Battering rams, or wooden beams, suspended by ropes, and armed 

 with iron, which were used by the warriors of antiquity in besieging a town, 

 are now generally superseded by the introduction of artillery, although 

 they may perhaps still aftbrd, in some cases, a more economical and 

 equally powerful mode of operation. The same momentum, and the same 

 energy, may be given to a battering ram at a less expense tlian to a cannon 

 ball; but it is probable that the efficacy of a cannon ball is chiefly owing 

 to the augmentation of its velocity beyond that limit, which is the utmost 

 that the substance to be destroyed can sustain without giving way, inde- 

 pendently of the mass of the body which strikes it. 



For demolishing smaller aggregates, pincers, hammers, and crows, are 

 generally sufficient ; to these sometimes more complicated instruments are 

 added. Thus, for example, several machines have been invented for draw- 

 ing out ship's bolts. A hook which grapples like the common instrument 

 for drawing teeth, has been applied for holding them fi«mly, and sometimes 



