ON MODES OF CHANGING THE TORMS OF BODIES. 235 



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 em- 

 ployed, 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 appli- 

 cation of fire, as in the mine of Ramraelsberg, in the Hartz, where the stra- 

 tum containing the ore is of such a nature, partly, perhaps, on account of the 

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

 a large quantity of fuel, which is burntin the vast excavation,of which it forms 

 the side, it is rendered so friable as to be worked with ease. More commonly, 

 however, the force of gunpowder is employed, and rocks are generally blasted 

 with great convenience by an explosion of this powerful agent. A hole being 

 bored to the depth of three or four feet, the powder is placed at the bottom, 

 and a wire being introduced, small stones and sand are rammed round it, 

 and the wire is withdrawn, leaving a communication for firing the powder, by 

 means of a train of sufficient length to insure the safety of the workman. 

 It is said that the explosion is more efficacious when the powder does not fill 

 the whole of the cavity; this, however, appears to require confirmation. The 

 chemical powers, which are the ultimate causes of the operation of gunpow- 

 der, belong to a department of philosophy which it is not our business to in- 

 vestigate: but the elasticity of the gases and vapours which are extricated, as 

 modified by the heat which accompanies their production, will be considered" 

 and explained in the subsequent divisions of this Course of Lectures. 



