The difficulty of enabling a large number of men to exert their force at the same instant of time has been almost obviated by the use of sound.  The whistle of the boatswain performs this service on board ships; and in removing, by manual force, the vast mass of granite, weighing above 1,400 tons, on which the equestrian figure of Peter the Great is placed at St Petersburgh, a drummer was always stationed on its summit to give the signal for the united efforts of the workmen.

An ancient Egyptian drawing was discovered a few years since, by Champollion, in which a multitude of men appeared harnessed to a huge block of stone, on the top of which stood a single individual with his hands raised above his head, apparently in the act of clapping them, for the purpose of insuring the exertion of their combined force at the same moment of time.

57.  In mines, it is sometimes necessary to raise or lower great weights by capstans requiring the force of more than one hundred men.  These work upon the surface; but the directions must be communicated from below, perhaps from the depth of two hundred fathoms.  This communication, however, is accomplished with ease and certainty by signals:  the usual apparatus is a kind of clapper placed on the surface close to the capstan, so that every man may hear, and put in motion from below by a rope passing up the shaft.

At Wheal Friendship mine in Cornwall, a different contrivance is employed:  there is in that mine an inclined plane, passing underground about two-thirds of a mile in length.  Signals are communicated by a continuous rod of metal, which being struck below, the blow is distinctly heard on the surface.

58.  In all our larger manufactories numerous instances occur of the application of the power of steam to overcome resistances which it would require far greater expense to surmount by means of animal labour.  The twisting of the largest cables, the rolling, hammering, and cutting large masses of iron, the draining of our mines, all require enormous exertions of physical force continued for considerable periods of time.  Other means are had recourse to when the force required is great, and the space through which it is to act is small.  The hydraulic press of Bramah can, by the exertion of one man, produce a pressure of 1,500 atmospheres; and with such an instrument a hollow cylinder of wrought iron three inches thick has been burst.  In rivetting together the iron plates, out of which steam-engine boilers are made, it is necessary to produce as close a joint as possible.  This is accomplished by using the rivets red-hot:  while they are in that state the two plates of iron are rivetted together, and the contraction which the rivet undergoes in cooling draws them together with a force which is only limited by the tenacity of the metal of which the rivet itself is made.