PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 645 



able with high pressure steam, and as explosions occur, and 

 boiler and tubes burst, with pressures not much exceeding one 

 hundred pounds, a pressure of five hundred to one thousand 

 pounds per square inch of compressed air in a cylinder or reser- 

 voir, can scarcely be entertained as safe. The tests and experi- 

 ments made with steam as to the strength of materials, and the 

 calculations of the most eminent men made upon them, are totally 

 inadequate to give a satisfactory theory of the sustaining strength 

 of vessels with pressures free from the heat and vapor of steam." 



" Cylindrical boilers would sustain the same pressure with 

 steam, were it not that heat separates every particle, while ex- 

 pansion weakens the metal ; besides, explosive gases (the force 

 of which cannot be ascertained) creates a pressure that bursts 

 them, which simple pressure cannot do. Hence the strength of 

 cylinders for holding compressed air at high densities, is abun- 

 dant and safe." 



If such important results as those related both by Yon Rathen 

 and Parsey have been worked out, why, it may be asked, have 

 not the plans of either of them been adopted and reduced to 

 practice ? A complete answer to this pertinent question is, that 

 the practical utility of the motive power they demonstrated, was 

 still limited by the serious want of providing along the route on 

 the track for a continuous supply of the power demanded by ex- 

 haustion, without requiring compressing power at every point 

 wherever replenishment may be needed, and without the neces- 

 sity of coming to a full stop with their motors, in order to receive 

 it. No such plan of operation had entered into the calculations 

 of either of these two inventors. To run an engine, for the pur- 

 pose of moving a train, with such a stock of compressed air as 

 could be packed up in a receiver borne along by and with the 

 engine, as far as that stock would propel it, and then stop aa 

 post-horses do, at a relay tavern, where to replenish the means 

 both of compression and delivery are to be provided, and where 

 reservoirs are to be kept supplied, could not be deemed such a 

 practical advantage in comparison with steam, as would be worth 

 exchanging steam for ; and therefore, their experiments and ex- 

 ploits, so far as practical usefulness is concerned, remain in abey- 

 ance, a dead letter, to this day. 



It was reserved to a later period in the age of discovery, to 

 devise the method which should place the compressed air on the 

 track, in a reservoir extended along the line of the route, acces- 



