THE EARTH. 2Z0 



Odours like\vise are diffused only by the means of air ; with- 

 out this fluid to s\nm in, they would for ever remain torpid in 

 their respective substances ; and the rose would affect us with 

 as little sensations of pleasure, as the thorn on which it grew. 



meet with an extended solid body of a regular surface, an ear placed in tli» 

 passage of these reflected waves, will perceive a sound similar to the originiu 

 i>ound, but whicli will seem to proceed from a body situated in a similar posi- 

 tion and distance behind the place of reflection,* as the real sounding body is 

 before it. This reflected sound is commonly called an Echo, which, however, 

 cannot take place at less than fifty-five feet ; because it is necessary that the 

 distance should be such, and the reverberated or reflected sound so long it 

 arriving, that the ear may distinguish clearly between that and the original 

 sound. 



It is in general kno^vn, that caverns, grottoes, mountains, and ruined build. 

 iDgs, return this image of sound. Image we may call it, for in every respect 

 it resembles the image of a visible object reflected from a polished surface. 

 Our figures are often represented in a mirror without seeing them ourselves, 

 while those standing on one side are alone sensible of the reflection. To bo 

 capable of seeing the reflected image of ourselves, we must be directly in a 

 line with the image. Just so it is in an echo ; we must standiu the line in which 

 the sound is reflected, or the repetition will be lost to us, while it may, at the 

 same time, be distinctly heard by others who stand at a small distance to one 

 side of us. 'i'here is a very extraordinary echo at a ruined fortress near Louvain, 

 in Flanders. If a person sing, he only hears his own voice, but then he hears 

 it with surprising variations, sometimes louder, sometimes softer, now more 

 near, then more distant. There is an account in the Memoirs of the French 

 Academy, of a similar echo near Rouen. 'Ihe building which returns it is a 

 semicircular court-yard ; yet all the buildings of the same form do not pro- 

 duce the same effects. We find some music halls excessively adapted for 

 sounds, while others, built upon the same plan, in a different place, are 

 found to resist the tones, instead of enlarging them, in a very disagreeable 

 manner. 



As we know the distance of places by the length of time a sound takes to 

 travel from them, ro we may judge of the distance of an echo by the length 

 of the interval between our voice and its repetition. The most deliberate 

 echoes, as they are called, are ever the most di.^tant; while, on the contrary, 

 tlinse that are very near, return their sound so very quick as to linve the 

 iiitHrval almost imperceptible : when this is the case, and the echo is so very 

 near, the voice is said to be increased and not echoed ; however, in fact, the 

 increase is only made by the swiftly pursuing repetition. Our theatres and 

 concert-rooms are best fitted for music or speaking, when they enlarge the 

 sound to the greatest pitch at the smallest interval : for a repetition which does 

 not begin the word till the speaker has finished it, throws all the sounds into 

 confubinn. Thus the theatre at the Hay-market, in London, enlarges the 

 sound very much ; but then at a long interval after the singer or speaker. 

 The old theatre at Drury-lane, before it was altered, enlarged the sound hot 



* Sound is reflected in the same direction as li^lit from a mirror; that is, 

 the angle of reflection is equal to the angle of incidence. 



