1831.] on Vibrating Elastic Surfaces. 337 



them undisturbed, and with equal intensity for any length of 

 time. 



68. By making a broad mark, or Fig. 13. 

 raising a little ledge of bees' wax, or 



a mixture of bees' wax and turpentine, 



it was easy to confine the pool of water 



to the middle part of the plate, fig. 13, where, of course, the 



crispations were most powerfully produced. Such a barrier is 



often useful to separate the wet and dry parts of the glass, 



especially when a violin bow is used as the exciter. 



69. In other experiments, deal laths, two, three, or four feet 

 long, one inch and a half wide, and three-eighths or more of 

 an inch in thickness, were used instead of the glass plates. 

 These could be made to vibrate by the fingers and wet rod (67), 

 and by either shifting the bridges or changing the lath, an 

 almost unlimited change of isochronous vibrations, from that 

 producing a high note to those in which not more than five or 

 six occurred in a second, could be obtained. The crispations 

 were formed upon a glass plate attached to the middle of the 

 lath, by two or three little pellets of soft cement*. 



70. Obtained in this way the appearances were very beauti- 

 ful, and the facilities very great. A glass plate, from four to 

 eight inches square, could be covered uniformly with crispa- 

 tions of the utmost regularity ; for, by attaching the plate with 

 a little method, and at points equidistant from the centre of the 

 bar, it was easy to make every part travel with the same velo- 

 city, and in that respect differ from and surpass the bar which 

 sustained it. The conoidal heaps constituting the crispation 

 could be so enlarged by slowness of vibration, that three or 

 four occupied a linear inch. The glass plate could be removed, 

 and another of different form or substance, and with other 

 fluids, as mercury, &c., substituted in an instant. 



71. In using laths, it is necessary to confine the parts bear- 

 ing upon the bridges, either by slight pressure of the fingers, 

 or by loops of string, or by weights. The exciting glass rod 

 need not necessarily rest upon the middle of the bar or plate, 

 but may be applied with equal effect at some distance from it. 

 Long laths may be made to subdivide in their mode of vibra- 

 tion, according as the rod is applied to different places, and the 



* Equal parts of yellow wax and turpentine. 



z 



