Mechanical Science. 371 



immersed body is not withdrawn rapidly, but slowly, the waves 

 are not produced, but at the instant when the body is detached from 

 the water. 



The duration of the experiments on the primitive waves varied 

 from one to six seconds ; and the results were found by M. Bidone to 

 accord with the theory of M. Poisson, when such experiments could be 

 made, as satisfied the conditions serving as a foundation for the theory. 

 M. Bidone terminates his memoir by remarks on the figure of waves 

 obtained by striking the surface of water with prismatic segments, hav- 

 ing triangular, square, and elliptical bases ; they present phenomena 

 similar to those exhibited by apertures, with their edges of the same 

 forms. On comparing, for example, the square base of the prismatic 

 segment to the wave formed by this segment, it was seen that the wave 

 had the form of a quadrangle with rounded angles, and that the 

 summits of the angles of the square base corresponded to the middle 

 of the sides of the quadrangle. 



The first part of the memoir of M. Bidone contains a verification 

 of a formula, given by INI . Eytelwein of Berlin, for calculating the 

 velocity of water in a rectilinear canal ; the section of the current 

 and its perimeter, and the inclination of the canal taken at the upper 

 surface of the water, or at the bottom of the canal parallel to that 

 surface, being known. The accordance between the velocities ob- 

 served and calculated according to the formula is remarkable. 

 The difference is at most, only a forty-eighth of the first quantity. 



4. On a Phenomena of Shadows, by M. Mongez. — When the sun 

 is free from clouds, the shadow of bodies is surrounded by a penum- 

 bra, very sensible, though much more obscure than the shadow ; when 

 two bodies, each producing a shadow, are made to approach. each 

 other, at the moment preceding the contact the shadows advance 

 towards each other, and change their form at the point of contact; 

 the shadow of a right line thus becomes a curve, and that of a globe 

 like the summit of a paraboloid. M. Arago attributes the effect to 

 the superposition of the penumbras accompanying the bodies ; thus 

 if the intensity of the penumbras was only half that of the shadow, 

 it would be doubled at the instant when the two were superposed, 

 and thus produce an obscure part of equal depth with the shadow, 

 which being added to it, would alter its form in that place. — Bill. 

 Univ. xxiii. 323. 



5. On the Vibration of Air. — M. F. Savart has published a variety 

 of experimental researches into the nature u\ the vibrations performed 

 by air, both in tubes and also in spaces of irregular form, but bounded 

 by solid bodies ; the latter are entirely new, and, with the former, 

 possess great interest to those who delight in this branch of science. 

 We cannot give a better idea of the nature of these results than by 



2 B 2 



