330 



TRANSACTIONS OF SECTION A. 



3. The Effects of Air Currents on Sound. By Professor F. It. Watson. 



This problem is part of the larger one of curing the echoes and reverberation 

 in the auditorium at the University of Illinois. Theory indicates that if the 

 ventilating system be arranged so that a broad sheet of warm air move over the 

 head of the speaker and be drawn out at the rear of the auditorium, it will act 

 as a partition and, more or less completely, reflect and refract the speaker's utter- 

 ances to the auditors. Lord Rayleigh has shown that reflection at the boundary 

 of gaseous media depends on the difference in the velocities of sound in the 

 two media. Taking V= ^y p/ p» an increase in temperature or moisture de- 

 creases p and hence increases V. Total reflection may occur for very oblique 

 incidence. With constant temperature, changes in j> do not affect V, as p / p re- 

 mains constant. If carbon dioxide is present, y is less and V is decreased. He- 

 fraction also depends on the ratio of velocities, so that the sound which pene- 

 trates the moving stream is bent in the direction of the current with possibility 

 of total reflection at the upper boundary of the stream. Furthermore, air 

 in motion carries sound with it. These factors act to turn the sound to the 

 audience, and, though each effect is small, the total effect may be large. Experi- 

 ments verify theory and show that reflection takes place from hot gases rising 

 from gas-jets so as to set up stationary waves. 



4. The Vernier Arc : A New Form of Micrometer. By J. W. Gordon. 



The model exhibited presents the micrometer on a very large scale, but is 

 nevertheless capable as it stands of yielding measurements correct to one five- 

 hundredth of an inch. On a scale suitable for the eyepiece of a microscope or 

 telescope the accuracy of the reading would be limited only by the magnifying 

 power of the eye lens. 



The instrument consists of (1) a scale and divided arc mounted in the focal 



Vernier Arc and Scale. 



plane; and (2) a diffraction grating mounted behind the eye lens and imme- 

 diately in front of the eye point. 



The effect of so mounting a diffraction grating is to produce in the field 

 of the instrument two images formed by diffracted light of every object lying 

 in the field. These images flank their original on either side; their orientation 

 being determined by that of the diffraction grating. On a bright field these 

 flanking images are too feebly illuminated to be at all conspicuous, but on a dark 

 field they are easily seen. 



Advantage is taken of the phenomena of the flanking image in the follow- 

 ing way : A narrow black field is traced in the centre of the field of the instru- 

 ment and, in apposition with this dark field on one side of it, a scale is ruled 



