INTRODUCTION. 



If the weight of bodies is frequently, from the point of view 

 of work, an obstacle to be overcome, it is also a useful auxiliary 

 which machines of all kinds continually and necessarily use : here 

 we are in the domain of Applied Mechanics rather than that of 

 Physics. Of these applications, we shall only refer to a few of the 

 most striking. In some it is the energy of the bodies which fall 

 under the action of gravity or weight, rather than their dead weight 

 itself, which produces the desired effect. In other cases, it is the 

 play of relatively minute actions which, thanks to the properties of 

 fluids, gives rise to effects which may be called prodigious : the 

 hydraulic press, for instance, Pascal's idea, which was only realized a 

 century after his time, shows us the muscular power of a single 

 arm increased a hundredfold by the powerful machine flattening 

 and crushing the most resisting materials, and lifting enormous 

 weights to considerable heights. Moved by steam, the hydraulic 

 press has raised gigantic iron-plated tubes, weighing not less than 

 two millions of kilogrammes, to a height of thirty or forty metres. 

 These now form the tubes of the famous bridge over the strait 

 separating the Isle of Anglesea from the county of Carnarvon. 



Another new invention has permitted the undertaking and bring- 

 ing to a successful termination that grand work the Mont Cenis 

 Tunnel, under the masses of the Col de Frejus, a work which is 

 being repeated under the Saint-Gothard. We refer to the use of 

 air compressed by a fall of water into reservoirs, by which it is 

 forced into the tunnel. Thus transformed, the force of gravity puts 

 into motion the boring tools which pierce the rock ; then, when 

 gunpowder has completed the work, the air, escaping from compres- 

 sion, replaces the impure and smoky atmosphere of the gallery. 

 Thus where steam has failed, the mechanical compression of air 

 obtained by a waterfall, that is, by weight, triumphs. 



Compressed air also renders possible the rapid construction and 

 foundation of piers of bridges thrown across arms of the sea or 

 wide rivers ; and, on some subterranean railways, sends the train 

 from one end to the other, like a pellet out of a pop-gun. In 

 Paris, London, and New York it transmits despatches between out- 

 lying telegraphic stations and the central one. A vacuum made by 

 a powerful pneumatic machine on one end of a piston moving in 

 a tube, brings into play the pressure of the air at the other end, 



