360 Mr. Tomlinsoris Experiments and [May 



undulse reflect the light well, and the space within this 

 rounded square does so imperfectly. I should also observe 

 that the surface of the mercury resembled for the most part 

 the case of an engine-turned watch, and the apparent revo- 

 lution of the star was in the direction of the linger, from 

 right to left, or from left to right, and the number of the 

 radii seemed to depend on the rapidity with which the 

 finger was passed round the edge of the glass, as well as on 

 the size of the vessel used, and the extent of the mercurial 

 surface. 



9. Now, it is known that a body, a foot glass, for instance, 

 in sounding, contains a certain definite number of nodal 

 points or divisions, the vibrating portions between the 

 divisions performing their vibrations independently of each 

 other. These nodal points are points of rest, or at least, of 

 minimum vibration, and the parts of the mercurial surface, 

 which are comparatively quiescent during the vibration, are 

 lines which pass from the circumference to the centre of 

 the mercury, while the undulating rings, being in motion, 

 give an apparent motion to the star formed by the nodal 

 lines. 



10. Dr. Thomas Young, in his Lectures on Natural 

 Philosophy, (vol. i. p. 385.) states, " that a vibrating glass 

 or bell divides, in general, into four portions vibrating 

 separately, and sometimes into six or eight; they may 

 readily be distinguished by means of the agitations excited 

 by them in a fluid contained in the glass." Now, I have 

 observed several and I think better modes of discovering 

 the nodes in a glass or bell. For instance ; procure # two 

 foot glasses, as perfectly in unison as possible, and about 

 three inches in diameter ; place on the glass, the nodes of 

 which are to be ascertained, a piece of very slender copper 

 wire, terminated at each end with a sort of half loop or 

 hook, so as to be free to move in the plane of the circle 

 described by the rim of the glass, and yet not to fall off 

 during vibration. Place the glasses on wood, or other good 

 conductor of sound, and vibrate with the moistened finger 



* If two unisonant foot glasses cannot be procured, or if it be inconvenient to 

 accord them by water or mercury, the wire can be placed on one glass, and the 

 vibration produced by passing the moist finger round the exterior half, and the 

 result will be the same. 



