278 Mr. W. H. Barlow on the Pneumatic Action [Apr. 16, 



put into the glass tube in which it is held. To provide for the escape 

 of the air passing through the instrument, a small orifice is made in the 

 side of the tube of the speaking-trumpet, so that the pressure exerted 

 upon the membrane and its spring is that due to the difference arising 

 from the quantity of air forced into the trumpet and that which can be 

 delivered through the orifice in a given time. 



There being an initial pressure upon the membrane to prevent vibra- 

 tory action as before described, the strength of the spring and the size 

 of the orifice had to be adjusted, so that while the lightest pressures 

 arising under articulation could be recorded, the greatest pressures 

 should not produce a movement exceeding the limit of the width of the 

 paper. 



It will be seen that in this construction of the instrument the sudden 

 application of pressure is as suddenly recorded, subject only to the modi- 

 fications occasioned by the inertia, momentum, and friction of the parts 

 moved. But the record of the sudden cessation of pressure is further 

 affected by the time required to discharge the air through the escape 

 orifice. 



Inasmuch, however, as these several effects are similar under similar 

 circumstances, the same diagram should always be obtained from the 

 same pneumatic action when the instrument is in proper adjustment ; and 

 this result is fairly borne out by the experiments. 



We are thus enabled to trace to what extent the pneumatic action 

 varies with different articulations ; and it will be seen that although there 

 are instances in which considerable differences in sound do not make 

 much variation in the diagram, yet, as a rule, every change of sound or 

 articulation produces a change in the diagram, and that there are pneu- 

 matic actions revealed by this instrument which are imperceptible to 

 ordinary observation. 



Before referring to the peculiarities of the diagrams, it may be desir- 

 able to say a few words on the quantities of air used in articulation. 



On reference to medical authorities, it appears that the average 

 quantity of air expelled in one respiration is estimated at 40 cubic 

 inches, and that the total air-space of the lungs is estimated to average 

 110 cubic inches. 



I have ascertained by experiment that a balloon made of goldbeater's 

 skin, whose cubic content when full was 523 cubic inches, was filled 

 with twelve ordinary respirations, or at the rate of about 44 cubic inches 

 for each respiration. 



Also that by filling and emptying the lungs as completely as practi- 

 cable, the 523 cubic inches could be filled with six respirations, or about 

 88 cubic inches for each respiration. 



I also made the following experiment to ascertain the average quantity 

 of air used in pronouncing syllables. 



Using the same balloon and speaking into an elastic tube communi- 



