222 On the Construction of Buildings 
Herschell, that the glass and the contained liquid, in order to give a 
musical tone, must vibrate regularly in unison as a system, and if any 
considerable part of a system is unsusceptible of regular vibration, the 
whole must be so.” 
transmitted to a fluid medium. The converse of this must also 
be true, i. e. when a sound passes from a fluid to a solid in con- 
tact with it, if this latter medium be not uniform and homoge- 
neous in its structure. Thus every musical performance is mod- 
ified essentially in its quality by the nature of the structure 1n 
which it is given; and hence the importance of attention, in this 
particular, to the choice of materials, and manner of constructing 
the walls of an apartment built for musical effect. | 
some of the principles just stated can, also, be explained 
many facts and phenomena in the natural wor Seis 
‘he deep and awful silence which reigns in the elevated regions 
of the globe is owing, not only to the lack of the ordinary sounds 
of animated nature, but to the diminished density of the air act- 
ing, as we have seen, both to enfeeble and modify the powers of 
speech, and deaden the force of such sounds as actually exist. 
e period of night seems peculiarly adapted to the formation 
and transmission of sound, especially musical sounds. If we may 
credit the reports of travellers, the tones of those birds in the 
equatorial regions which sing at night are singularly plaimtive 
and melodious, as we know to be the case with the mocking bird, 
the whippoor-will and the nightingale. To certain sensitive 
minds, almost all sounds, at this season, partake of a musical chat- 
acter; to such there is melody in the running waters of a bros; 
the hum of insects is a song ;—the voice of falling water mingles 
with the rising wind and the distant surging of the ocean to oe 
amighty chorus. The hush of nature, even, in the silent elo- 
quence of night, is woven into harmony, and 
“The mute still air 
Is music slumbering on her instrument.” 
In the case just mentioned, the sound is excited in a solid and 
u 
_ But the attention of the most unimaginative cannot fail, at such 
times, to be arrested by the prevalence of sounds of which they 
ere that 
of the great cataract of the Orinoco, when heard at night, int 
plains which surround the mission of Apures, was three UM” 
louder than during the day. The explanation given by this emir 
nent traveller, and repeated by Mr. Herschell, is as follows: 
“In a hot day, when a warm current of air ascends from the heated 
ground and mingles with the cold air above of a different pier gp a 
transparency of the atmosphere is so much affected that every 
