Because the density of dry air improves the _sound-conducting power_ of the atmosphere. The transmission of sounds is also assisted by the direction of the winds. 738. _Why may the scratching of a pin at one extremity of a long pole be heard by applying the ear to the opposite extremity?_ Because wood is a good conductor of sound, and its atoms are _susceptible of considerable vibration_. It is, therefore, chosen in numerous instances for the construction of _musical instruments_. [Verse: "The morning is come unto thee, O thou that dwellest in the land: the time is come, the day of trouble is near, and not the sounding again of the mountains."--EZEKIEL VII.] Deaf persons have been known to derive pleasure from music by placing their hands upon the wood-work of musical instruments while being played upon. 739. _Why is the hearing of deaf persons assisted by ear-trumpets?_ Because ear-trumpets _collect the vibrations of the air_ into a focus, and make the sounds produced thereby more intense. 740. _Why are sounding-hoards used to improve the hearing of congregations?_ Because, being suspended over, and a little behind, the speaker, they _collect the vibrations_ of the air, and _reflect_ them towards the congregation. 741. _What are echoes?_ Echoes are sounds _reflected_ by the objects on which they strike. 742. _Why do some echoes occur immediately after a sound?_ Because the reflecting surface is _very near_; therefore the sound returns immediately. 743. _Why do some echoes occur a considerable time after a sound?_ Because they are at a considerable distance, and the sound takes time to travel to it, and an equal time to return. 744. _Why do some echoes change the tone and quality of sound?_ Because the reflecting surface, having vibratory qualities of its own, _mingles its own vibrations with that of the sound_. 745. _Why are there sometimes several echoes to one sound?_ Because there are various _reflecting surfaces_, at different distances, each of which returns an echo. [Verse: "And God said, Let the waters under the heaven be gathered together onto one place, and let the dry land appear: and it was so."--GEN. I.] 746. _Are sounds reflected only by distant objects?_ Sounds are doubtless reflected by _walls and ceilings_ around us. But we do not perceive the echoes, because they are so near that they occur at the same moment with the sound. In lofty buildings, however, there is frequently a _double sound_, making the utterance of a speaker indistinct. This arises from the echo following very closely upon the sound. 747. _Why, when we are walking under an arch-way or a tunnel, do our voices appear louder?_ Because the sounds of our voices are _immediately reflected_. And as a _gas reflector increases the intensity of light_, so _a sound reflector_ will _increase the apparent strength of our voices_.