INTRODUCTION. 



which supplies a force sufficiently great to force a heavy weight 

 supported on wheels along the tube. This process, which is the 

 opposite of the application of compressed air, is also adapted to 

 the service of telegraphic and postal despatches. 



One physical principle, which is associated with weight, the 

 discovery of which is of great antiquity it takes the name of the 

 great man who discovered it, Archimedes was at the end of the 

 last century applied to produce the ascent of balloons in the air. 

 The art of the aeronaut, greatly improved since Montgolfier's time, 

 has become popular; and each year balloons traverse the aerial 

 regions, in which curious phenomena have been discovered by their 

 means; and in the hands of serious observers, they will end by 

 unveiling many of the mysteries of the atmosphere. 



Meteorology, as yet so backward, cannot fail to utilize its aid. 

 Moreover, during the war between France and Germany, balloons 

 were sent as messengers from the heart of Paris to every part of 

 France, carrying / in their frail cars to the provinces news of the 

 besieged but confident population. Perhaps the day will come 

 when the problem of the direction of aerial machines in their 

 present form, or more probably in a new one, will be partially solved ; 

 when they will be able to tack about or cut through the air, as 

 sailing or steam ships cut through the waves of the sea : then, 

 instead of curious or exclusively scientific experiments such as are 

 now made, real aerial journeys may be taken, and regular expe- 

 ditions susceptible of useful applications. 



III. 



From the applications of the phenomena and laws of weight, we 

 shall pass on to those which result from the phenomena and laws 

 of sonorous vibrations. 



Here we shall find ourselves almost exclusively in the domain 

 of art, with that which moves and charms us with its vivacity, 

 and at the same time with its profundity. Music, indeed, is not 

 only an art, it is a science. Nevertheless it is in relation to neither 

 of these that it borrows aid from applied physics. That wonderful 

 natural instrument, the human voice, being left out of the question, 



