THE PHONOGRAPH, OR VOICE-RECORDER. 287 



" to construct an instrument which should record the pneu- 

 matic actions " accompanying the utterance of articulated 

 sounds " by diagrams, in a manner analogous to that in 

 which the indicator-diagram of a steam-engine records the 

 action of the engine." He perceived that the actual aerial 

 pressures involved being very small and very variable, and 

 the succession of impulses and changes of pressure being 

 very rapid, it was necessary that the moving parts should 

 be very light, and that the movement and marking should 

 be accomplished with as little friction as possible. The 

 instrument he constructed consisted of a small speaking- 

 trumpet about four inches long, having an ordinary mouth- 

 piece connected to a tube half an inch in diameter, the 

 thin end of which widened out so as to form an aperture 

 of 2j inches diameter. This aperture was covered with 

 a membrane of goldbeater's skin, or thin gutta-percha. A 

 spring carrying a marker was made to press against the 

 membrane with a slight initial pressure, to prevent as far as 

 possible the effects of jarring and consequent vibratory 

 action. A light arm of aluminium was connected with the 

 spring, and held the marker; and a continuous strip of 

 paper was made to pass under the marker in the manner 

 employed in telegraphy. The marker consisted of a small, 

 fine sable brush, placed in a light tube of glass one-tenth of 

 an inch in diameter, the tube being rounded at the lower 

 end, and pierced with a hole about one-twentieth of an inch 

 in diameter. Through this hole the tip of the brush pro- 

 jected, and was fed by colour put into the glass tube by 

 which it was held. It should be added that, to provide 

 for the escape of air passing through the speaking-trumpet, 

 a small opening was made in the side, so that the pressure 

 exerted upon the membrane was that due to the excess 

 of air forced into the trumpet over that expelled through 

 the orifice. The strength of the spring which carried the 

 marker was so adjusted to the size of the orifice that, while 

 the lightest pressures arising under articulation could be 

 recorded, the greatest pressures should not produce a move 

 paent exceeding the width of the paper. 



