438 



Intelligence and Miscellaneous Articles. 



uncertainty, however, in this experiment arises from the waves 

 proceeding directly from whistle to flame. Even if a screen is 

 interposed, enough space has to be left below it to allow for the 

 passage of sound-rays reflected from the two mirrors. From the 

 lower edge of the screen, therefore, waves are diffracted and may 

 interfere with either or both sets of waves reflected fi'om the 

 mirrors. The trouble from this source caused the abandonment 

 of this plan of experiment. 



A modification of the Fresnel experiment is that of using but a 

 single mirror, \^ hich may be rested horizontally on the table, and 

 allowing the waves reflected from it to interfere with those radiated 

 directly from the whistle. The effect is obviously the same as if 

 they proceeded from two sources, but interference-bands can be 

 produced on only one side of the median line. In the accompany- 

 ing diagram AM is the plane of the mirror, S the source of sound, 

 and S' the virtual source from which the reflected waves may be 

 regarded as coming. Let the nozzle from which the flame issues be 

 placed first at A and then lifted vertically. The flame will flare 

 at the points B, C, D, &c., whose distances respectively from S 

 and S' differ by an even number of half wave-lengths. Midway 



between A and B, B and C, &c., are points of complete interference 

 where the flame should burn quietly. The distance AB is approxi- 



The accompanying table gives a compa- 



mately equal to =1^7 X. 



risen between the results of theory and experiment, in which the 

 height of the whistle above the table, MS, is 10 inches ; the distance 

 AM is 36 inches, and the wave-length, X, is 1-05 inch. The suc- 

 cessive measiirements are of distances above the table at which the 

 flame became quiescent. The first column is calculated from the 

 foruuila ; the others are the records from five sets of experiments. 



Theory. 



I. 



II. 



III. 



IV. 



V. 



■945 



•9 



10 



•9 



10 



10 



2-835 



2-8 



2-7 



2-7 



2-8 



2-9 



4-725 



4.7 



4-6 



4-7 



4-9 



4-7 



6-615 



6-7 



6-7 



6-7 



6-9 



6-7 



8-505 



8-9 



9-0 



8-8 



9-0 



91 



10-395 



11-0 



11-3 



110 



11-2 



11-3 



12-285 





13-6 





13-6 



13-6 



