364 Prof. R. W. Wood on the Condensation 



of a rectangular plate supported at the boundary move in 

 the direction of the applied transverse forces. 



If z denote V 2 w, then V 2 ^, = V 4 w, is positive over the 

 plate if the applied forces are everywhere positive. At a 

 straight portion of the boundary of a supported plate z = 0, 

 and this is regarded as applicable to the whole boundary 

 of the rectangular plate, though perhaps the corners may 

 require further consideration. But if \/ 2 z is everywhere 

 positive within a contour and z vanish on the contour itself, 

 z must be negative over the interior, as is physically obvious 

 in the theory of the conduction of heat. Again, since \/ 2 w 

 is negative throughout the interior, and w vanishes at the 

 boundary, it follows in like manner that w is positive 

 throughout the interior. 



It does not appear that an argument on these lines can be 

 applied to a rectangular plate whose boundary is clamped, 

 or to a supported plate whose boundary is in part curved. 



P.S. In connexion with a recent paper on the " Flow of 

 Compressible Fluid past an Obstacle " (Phil. Mag. July 1916) , 

 I have become aware that the subject had been treated with 

 considerable generality by Prof. Cisotti of Milan, under 

 the title " Sul Paradosso di D'Alembert" (Atti E. Istituto 

 Veneto, t. Ixv. 1906). There was, however, no reference to 

 the limitation necessary when the velocity exceeds that of 

 sound in the medium. I understand that this matter is now 

 engaging Prof. Cisotti' s attention. 



XL. The Condensation and Reflexion of Gas Molecules. By 

 R. W. Wood, Professor of Experimental Physics, Johns 

 Hopkins University *. 



[Plate VII.] 



IN a previous communication f I have shown that a jet of 

 mercury vapour, in which the molecular motion is 

 restricted to one dimension, is reflected from a flat plate 

 of glass approximately according to the cosine law. The 

 mercury vapour after reflexion was condensed as a metallic 

 film on the wall of the bulb, which contained the reflector at 

 its centre and was immersed in liquid air. 



Removal of the bulb from the low-temperature bath caused 

 the film of solid mercury to melt and collect into small drops, 

 which made measurements of its thickness in different gases 

 impossible. 



* Communicated by the Author. 



t Phil. Mag. August 1915, " Experimental. Determination of the Law 

 of Reflexion of Gas Molecules." 



