338 Dr. Weber on Savart's Experiments on the 



agitations, and to become perfectly acquainted with its velo- 

 city. How could these objects be better attained than by the 

 sounding vibration of an organ-pipe, a bell, or disk, before 

 which the membrane was expanded, so that every vibration 

 transmitted by the air of those sounding bodies necessarily 

 agitated the membrane ? 



In this manner he made the following double series of ex- 

 periments. Once he held the extended membrane before 

 an organ-pipe, which he was enabled by means of a stopper to 

 lengthen or shorten at will, by which means the same mem- 

 brane was successively agitated by very various rapid concus- 

 sions. In the second place, he stretched a membrane of a very 

 hygrometrical substance, viz. of paper, and made it gradually 

 imbibe an increasing quantity of aqueous vapour. In this 

 manner he was enabled to agitate a membrane of very different 

 degrees of elasticity by constantly equally-quick concussions, 

 ex. gr. by means of a sounding plate or bell held before it. 



The membranes thus mediately agitated show, according to 

 Savart's observations, the following similarity to sounding 

 plates : 



1. In mediately agitated membranes vibrating divisions are 

 formed, which are separated by quiescent or slightly moved 

 lines, as in the sounding plates. 



2. These quiescent fines may undergo such distortions as 

 Chladni has observed in the sounding plates. On the other 

 hand, there are the following differences between the motions of 

 mediately agitated membranes and sounding plates. First, In 

 mediately agitated membranes the divisions near the edge are 

 as great as those within, whilst in sounding plates the divisions 

 near the border are not half so great. Secondly, In mediately 

 agitated membranes the different distortions of the quiescent lines 

 are produced by the different width of the agitating undulations^ 

 (i.e. by different high tones of the organ-pipe placed before the 

 membrane,) whilst the observation in the sounding plates, as is 

 generally known, is, — that when the quiescent lines are some- 

 what distorted, the breadth of the waves of the undulation pro- 

 ceeding from the plate is either not altered at all, or very im- 

 perceptibly, which may be easily known from the height of 

 the tone. 



If we hold a square membrane, the elasticity of which is 

 not changed, before the opening of an organ-pipe provided 

 with a stopper and made to vibrate, we may effect by means 

 of the stopper the circumstance that the sand thrown off" from 

 the divisions represented in Plate VI. fig. 1. No. 1. remain 

 only on the boundary line. If the tone of the organ-pipe be- 

 comes a trifle higher, the form of this boundary line upon 



which 



