LECT. XIX. PROPAGATION OF SOUND. 351 



sage, also vibrates in consequence of the sonorous undula- 

 tions excited in the air. Experiment proves, that a sound 

 produced in a solid body is much more distinct to our ear 

 if transmitted by means of a solid immediately : n contact 

 with the ear, or through the intervention of another similarly 

 interposed body, all other conditions being equal. If, on 

 the contrary, the sound be produced by sonorous undula- 

 tions primarily excited in the air, as in all wind instruments, 

 it becomes the more distinct in proportion as the quantity of 

 air in communication with the auditory passage, is within . 

 certain limits, more considerable. The ear-trumpet acts on 

 this principle. When these two modes of propagation are 

 combined in the same organ, it cannot be doubted but that 

 they powerfully concur in perfecting the function. These 

 principles are applicable to the explanation of the use of 

 the pinna, and of the external auditory passage. We per- 

 ceive in these parts an instrument analogous to an ear- 

 trumpet, which, by its peculiar form, collects and reflects 

 towards the axis of the canal, a greater number of sonorous 

 waves; and which, at the same time, acts like a reservoir 

 of air, aud increases the sound. All stringed and wind in- 

 struments yield a more intense sound by the effect of the 

 presence of this reservoir. Savart's bell, which, after having 

 been put in vibration, is brought near an air receiver, and 

 the monster tuning fork of Marloye, placed on a large air- 

 chest, place beyond doubt the effects produced by the re- 

 sonance of masses of air which surround sonorous bodies. 

 An analogous experiment can be made by bringing a tuning 

 fork near to the ear, or introducing it within the mouth : in 

 the latter case, the sound acquires a great intensity. In all 

 these instances, the sound is augmented by sonorous waves 

 reflected' by the walls of the receivers; and, at the same 

 time, by vibrations communicated to the mass of air, and 

 to the walls of the reservoir. In all cases, however, it is 



