Intelligence and Miscellaneotis Articles, 437 



follow it are detected without difficulty by means o£ the sensitive 

 flame. 



All of these experiments by Lord Rayleigh have been repeated 

 by me. The source of sound used is Galton's adjustable whistle, 

 through which a blast is sent from a cylinder of compressed air 

 of oxygen. The sensitive flame is fed from a similar cylinder of 

 compressed coal-gas, the pressure of the supply being carefully 

 regulated in each case by means of a water manoineter-gauge. 

 The whistle is capable of giving a pitch as high as 18,000 or 20,000, 

 but as this limit is approached the intensity becomes too much 

 diminished, and practically the best pitch it yields is about 13,000 

 vibrations per second. Lord Eayleigh's whistle is slightly different 

 in construction, and probably better than the Galton whistle. 

 But there is no difliculty in obtaining good results with this pitch. 

 The greatest practical difficulty is that of keeping the sensitiveness 

 of the flame exactly right, the slightest variation of pressure 

 making it inconstant, and causing it to give misleading indications 

 \Ahen the attempt is made to apply it to purposes of measurement. 



I have attempted by means of the whistle and flame to verify 

 acoustically the experiment in light first performed by Grrimaldi 

 and analysed by Dr. Thomas Toaug, that of producing diffraction- 

 bands by transmitting waves in the same phase through two 

 small openings, and exploring the air with the sensitive flame for 

 the hyperbolic lines of maximum and minimum motion. The 

 whistle, giving forth waves 10-5 inch in length, was placed 34 inches 

 from the screen of cardboard, whose vs'idth was two feet. Near 

 the middle of this were cut two vertical slits, 3 inches apart, 

 and each ^ inch wide. The position required by theory for the 

 hyperbolic bands was determined, the screen being at right angles 

 to the direction of the whistle from its middle point. The middle 

 line of maximum motion behind the screeii was detected without 

 difficulty. It was discontinuous, as might be expected when the 

 wave-length is so considerable in comparison with the distance 

 between the apertures. The nearest hyperbolas on the two sides 

 of this were found in their right position, and traced back rather 

 more than a foot from the screen, but they were not so well defined 

 as the middle line. The next pair of hyperbolas was also found, 

 but with poor definition. By using slits a half inch in width 

 results were perhaps a little better, though in neither case could 

 any measurements approximate to exactness. 



Fresnel's celebrated experiment of producing interference-bands 

 by reflexion of light from two mirrors inclined at an angle of 

 nearly 180° was tried by Professor A. M. Mayer and myself con- 

 jointly, using sound-waves. A large plate of glass was rested on 

 the table, and another plate inclined to it at an angle of 152°, the 

 whistle being 67 inches from the flame, 4 inches from the inclined 

 mirror, and 13 inches above the table. Six interference-bands 

 were detected by means of the flame, their mean distance apart 

 being 4 inches. By subsequent calculation this result was found 

 correct to within a tenth of an inch. An important source of 



