172 LECTURE XIX. 



metal into the recesses of the die ; it is performed by a strong pressure, f 

 united with a considerable impulse, communicated by a screw like that of 

 a printing press ; and sometimes the impression is formed by the repeated 

 blows of a hammer only. 



Thin plates of silvered copper are moulded into any figure that may be 

 required, by being placed between two corresponding stamps, of which the 

 one is fixed, and the other attached to the bottom of a heavy hammer. The 

 hammer is raised and suffered to fall in a right line, by means of pincers, 

 which open when they have acquired a certain height. Sometimes the 

 contact, produced by the forcible impulse of a die, is sufficiently intimate to 

 cause a thin plate of silver to cohere permanently with a surface of iron ; 

 and this mode of uniting metals is actually employed in some manu- 

 factures. 



The operations of perforating, cutting, turning, boring, digging, sawing, 

 grinding, and polishing, resemble each other, in great measure, with respect 

 to the minute actions of the particles of bodies which they have to overcome. 

 Penetration is generally performed in the first instance by the effect which 

 we have called detrusion, where the magnitude of the penetrating substance 

 is considerable : but when a fine point or edge is employed, it probably first 

 tears the surface where it is most depressed, and then acts like a wedge on 

 the portions of the substance left on each side, with a force so much the 

 greater as the edge is thinner. The resistance opposed by a solid, or even 

 by a soft substance, to the motion of a body tending to penetrate it, appears 

 to resemble in some measure the force of friction, which is nearly uniform, 

 whether the motion be slow or rapid, destroying a certain quantity of 

 momentum in a certain time, whatever the whole velocity may be, or what- 

 ever may be the space described. Hence arises the advantage of giving a 

 great velocity to a body which is to penetrate another, the distance to which 

 a body penetrates being as the square of its velocity, or as its energy ; and a 

 certain degree of energy being required in order to make it even penetrate at 

 all. It is true that when we exchange a slow motion for a more rapid one, 

 by the immediate action of any mechanical power, we can only obtain the 

 same energy from the same power, for we must diminish the mass in the 

 same proportion as the square of the velocity is increased ; but a very small 

 part of the force which is consumed in the operation of a machine of any 

 kind, is employed in generating momentum ; by much the greatest part is 

 spent in overcoming resistances which vary but little with the velocity ; a 

 small portion only of the resistance increasing in proportion to the square 

 of the velocity ; so that by applying a triple force, we may obtain more 

 than a double velocity, and more than a quadruple effect : and besides it 

 has already been observed that when the velocity begins to exceed a certain 

 limit, the effect is increased in a much greater proportion than that of its 

 square. The same work is also performed with less pressure, and less strain 

 on the machinery, where a great velocity is employed. It is on account of 

 the efficacy of velocity in facilitating penetration, that soft substances, mov- 

 ing very swiftly, will readily perforate much harder ones ; and for the 

 saine reason a gunshot wound, and even the loss of a limb, takes place with 

 so little disturbance of the neighbouring parts, that it is sometimes scarcely 

 felt. The advantage of an impulse, however inconsiderable, above a pres- 



