184 Mr. C. J. Woodward on an Apparatus to 



satisfactory result ; though a mistake^ pointed out in the July 

 Number of this Journal, renders the reasoning less convincing. 

 When the correction is made it will be found that the ratio of 

 the specific heats, when ^^ = 5000, is not 1'4, but much smaller. 

 For some value of 6^ between 100° and 5000°, probably about 



c' 

 500°, the value of - will become 1*4 ; but then ? will be some- 

 thing near '4, and about three fifths of the gas will be in a 

 state of dissociation. This we cannot allow ; for though we 

 might explain the near equality of the pressures and specific 

 heats of different gases by supposing that 6^ is the same, or 

 nearly the same, for each, yet the constancy of the specific 

 heats with respect to temperature will not now hold even ap- 

 proximately. The difficulty as to the ratio of the specific heats 

 therefore still remains. .*; 



XXIII. On an Apparatus to ilkistrate the Interference of two 

 Plane Waves, By C. J. Woodward, B.Sc* 



THE efiect produced by the simultaneous propagation of 

 two or more plane waves is in a few simple cases easily 

 realized ; in others, howevei*, the mind is somewhat harassed 

 in ascertaining what will be the resultant wave made up of 

 certain given elements. The apparatus I am about to describe 

 is intended to assist the student by enabling him to draw for 

 himself the required resultant wave ; or the apparatus may be 

 used in the class-room to illustrate the general subject of in- 

 terference. 



So far as I know, the only apparatus of the kind in use by 

 lecturers is one based on the method described by Dr. Young 

 (Natural Philosophy, Lecture xxxiii.) ; but here, to obtain 

 any variety of effects, a series of dissected waves are required, 

 and these are troublesome to make or expensive to purchase ; 

 whereas with the arrangement I now exhibit to the Society, 

 when once the framework of the apparatus is made, combina- 

 tions of any desired waves may be drawn, with merely the 

 trouble incidental to cutting out the required waves in card- 

 board or thin tin. 



The apparatus consists of a board, A A, about 3 feet long, 

 2 feet wide, and 1 inch thick, set upright on a base-board B. 

 A strip of cardboard, 0, is fastened by drawing-pins to the 

 board ; and on this is drawn the compound wave resulting 

 from the two component waves W, W^ These component 

 waves are cut out of strong cardboard or tin, and can be easily 



* Communicated by the Physical Society. 



