204 -WELLS'S NATURAL PHILOSOPHY. 



„ „ By the action of muscles vre can vary the tension of these 



How can the ^ , , , . , , , 



tones of the membranes, and make the openmg between them large or 



voice he ren- gjuaH, and thus render the tones of the voice crave or acute.* 

 aerea grave or " 



»cute? 445. The loudness of the voice depends 



Upon what mainlv upon the force with which the air is 



does the loud- •' i- 



ness of the voice exDclled from the luno;s. 



depend? J- _ o 



The force which a healthy chest can exert in blowing ia 



about one pound per inch of its surface ; that is to say, the chest can con- 

 dense its contained air with that force, and can blow through a tube the 

 mouth of which is ten feet under the surface of water. 



What is the vo- 446. In coughing, the top of the windpipe, 

 coughing? °^ or ^^^^ glottis, is closed for an instant, during 

 which the chest is compressing and condensing 

 its contained air ; and on the glottis heing opened, a 

 slight explosion, as it w^crc, of the compressed air takes 

 place, and hlows out any irritating matter that may be 

 in the air-passages. 



447. Sound, to some ertent, appears to always accompany 

 generally ac- the liberation of compressed air. An example of this is seen 

 lib™^'l"(T '^f ^^^ *^^*^ report which a pop-gun makes when a paper-bullet 

 compressed air ? is discharged from it. The air confined between the paper 

 bullet and the discharging-rod is suddenly liberated, and 

 strikes against the surrounding air, thus causing a report in the same man- 

 ner as when two solids come into collision. In like manner an inflatea blad- 

 der, when burst open with force, produces a sound like the report of a pistol. 



„ , . ., 448. The sound of falling water appears in a great mcasuro 



To what is the . , ^ .^ , , . ^ , 



Bound of falling to be owmg to the lormation and burstmg of bubbles. U hen 



water due? ^j^g distance which water falls is so Umitcd that the end of 



• The power which the will possesses of determining with the most perfect precision 

 the exact degree of tension which these membranes of the glottis, or vocal chords shall 

 receive, is extremely remarkable. Their average length in man is estimated at 7o-10Cths 

 of an inch in a state of repose, while in the state of greatest tension it is about 93-lOOtha 

 of an inch. The average length of the membranes in the female is somewhat less. Each 

 interval, or variation of tone which the human voice is capable of producing is occasioned 

 by a different degree of tension of these membranes ; and as the least estimated number 

 of variations belonging to the voice is 240, there must be 240 different states of tension 

 of the vocal chords, or membranes, every one of which can be at once determined by the 

 will. Their whole variation in length in man being not more than one fifth of an inch, 

 the variation required to pass from one interval of tone to another will not be more than 

 l-I '200th of an inch. 



It is on account of the greater length of the vocal chords, or membranes of the glottis, 

 that the pitch of the voice is much lower in man than in woman : but the difference does 

 not arise until the end of the period of childhood, the size of the larynx in both sexes being 

 about the same up to the age of 14 or 15 years, but then changing rapidly in the malo 

 Bex, and remaining nearly stationary in the female. Hence it is that boys, as well as 

 girls and women, sing treble ; while men sing tenor, which is about an octave lower than 

 treble, or bass which is lower still. — Dr. Carpenter, 



