727. _What are the numbers of vibrations in a second that produce the various musical sounds?_ C or Do, 480 vibrations in a second; B or Si, 450 vibrations; A or La, 400 vibrations; G or Sol, 360 vibrations; F or Fa, 320 vibrations; E or Mi, 300 vibrations; D or Re, 270 vibrations; C or Do, 240 vibrations. It is thus seen that the _more rapid_ the vibrations, the _higher_ the note, and _vice versa_. 728. _Why does the length of a wire or string determine the sound that it produces?_ Because the _shorter the string_ the _more rapid_ are its vibrations when struck. 729. _Why does the tension of a wire or string affect its vibrations?_ Because when the string or wire is tight, a touch communicates vibrations to _all its particles_; but when it is loose the vibrations are _imperfectly communicated_. 730. _Why are some notes low and solemn, and others high and quick?_ Because the vibrations of musical strings vary from 32 vibrations in a second, which produces a soft and deep bass, to 15,000 vibrations in a second, which produces the sharpest treble note. 731. _Why can our voices be heard at a greater distance when we speak through tubes?_ Because the vibrations are _confined to the air within the tube_, and are not interfered with by _other vibrations_ or movements in the air; the tube itself is also a _good conductor of sound_. [Verse: "And I will cause the noise of thy songs to cease; and the sound of thy harps shall no more be heard."--EZEKIEL XXVI.] 732. _Is air a good conductor of sound?_ Air is a _good conductor_, but water is a _better conductor than air_; wood, metals, the earth, &c., are also _good conductors_. 733. _Why can we hear sounds at a greater distance on water than on land?_ For various reasons: because the smooth surface of water is a good conductor; because there are fewer noises, or counter vibrations, to interfere with the transmission of sound; and because there are no elevated objects to impede the progress of the vibrations. 734. _Why do sea-shells give a murmuring noise when held to the ear?_ Because what may be called _expended vibrations_ always exist in air where various sounds are occurring. These _tremblings_ of the air are received upon the thin covering of the shell, and thus being _collected into a focus_, are _transmitted to the ear_. 735. _Why can people in the arctic regions converse when more than a mile apart?_ Because there the air, being _cold and dense_, is a very good conductor; and the _smooth surface of the ice_ also favours the _transmission of sound_. 736. _Why do savages lay their heads upon the earth to hear the sounds of wild beasts, &c.?_ Because the earth is a good conductor of sound. For this reason, also, persons _working under ground in mines_ can hear each other digging at considerable distances. 737. _Why can church clocks be heard striking much more clearly at some times than at others?_