ON MODES OF CHANGING THE FORMS OF BODIES. 179 



which consists of a 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 off 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 col- 

 lected 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 be- 

 sieging a town, are now generally superseded by the introduction of 

 artillery, although they may perhaps still afford, in some cases, a more 

 economical and equally powerful mode of operation. The same mo- 

 mentum, and the same energy, may be given to a battering ram at a less 

 expense than 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 sus- 

 tain without giving way, independently 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 firmly, and sometimes 

 a screw, turned by means of wheelwork, has been used for gaining a force 

 sufficient to overcome their adhesion. In all such cases, however, the effect 

 of percussion has a considerable advantage ; and even if other means are 

 employed, it is of use to begin with lessening the firmness of the adhesion 

 by the blows of a hammer ; and in this manner a screw may be extracted, 

 which is so firmly attached by its rust as to be immoveable by other 

 means. 



The expansive force of heat is frequently of great service in dividing 

 rocks, or in destroying old buildings. This is sometimes done simply by 

 the application of fire, as in the mine of Rammelsberg in the Hartz, where 

 the stratum containing the ore is of such a nature, partly, perhaps, on ac- 

 count of the combustible matter which enters into its composition, that, by 

 the effect of a large quantity of fuel which is burnt in the vast excavation, 



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