Strehlke on ilie Acoustic Figures of Plates. 305 



S S' and A (or, instead of A, at A E or E'), and caused to 

 vibrate at B. The long diameter C C' of D C D' C', as may 

 be anticipated from the above, is parallel to the principal axis 

 of the hyperbolas in Fig. 2 ; and this is also the case with the 

 lateral ellipse B B' F F'. But if, instead of supporting the 

 plate, it is fixed between a screw, at a point of F E, which 

 is nearer to the centre than E, the long diameters of the curves 

 are in the direction of D D', and the other parts of the figure 

 are changed accordingly. 



If, in Fig. 6, the plate is supported at S S', and at the 

 middle of H B, Fig. 7 is formed, in which E E' and D D' are 

 always in the direction of the principal axis in the hyberbola 

 of Fig. 2. 



EXPERIMENTAL ILLUSTRATION OF THE EQUALITY BE- 

 TWEEN THE RADIATING AND ABSORBING POWERS OF 

 THE SAME SURFACE. 



BY THE REV. WILLIAM RITCHIE, M.A., F.R.S., 



Prof, of Nat. and Exper. Philos., Royal Institution. 



"DROFESSOR Leslie has shown, by a series of ingenious 

 experiments, that those surfaces which radiate heat most 

 copiously also absorb it in the greatest quantity ; but, as far as 

 I know, it has not been experimentally demonstrated, at least 

 in a manner adapted to illustration before a large audience, 

 that the radiating and absorbing powers are exactly equal to 

 each other. The simplicity of the following mode of illustra- 

 tion, and the clear conviction which it brings to the minds of 

 an audience, have induced me to give it a place in the Journal 

 of the Royal Institution. 



The instrument consists of a large differential thermometer, 

 with cylindrical chambers made of the thinnest tin-plate, similar 

 to what I formerly described in the transactions of the Royal 

 Society *. The horizontal branch, A B, of the glass tube is 



PhiL Trans., 1827, p. 123. 



