254 Mr HOPKINS ON AERIAL VIBRATIONS 



stretched across it, is suspended by a fine wire or thread from the upper 

 extremity of the tube, in such a manner that it can be heightened or 

 lowered at pleasure. The other parts of the apparatus are merely such 

 as are adapted for facility and -accuracy of arrangement of the tube 

 and plate. 



25. The air in the tube is put in a state of sonorous vibration 

 by means of the plate, which is made to vibrate by drawing the bow 

 of a violin equably across its edge in a direction perpendicular to its 

 plane ; the vibratory motion of the air is communicated to the membrane 

 suspended in the tube, and the degree of motion is indicated by the 

 agitation of a small quantity of light dry sand sprinkled upon it*. 

 Suppose the tube to be open at the upper end B, and let the membrane 

 be drawn up near that extremity. Tf the sand indicate a considerable 

 motion when the plate is vibrating, let the membrane be gradually 

 lowered ; a position will thus be found in which the sand has little 

 or no apparent motion, thus indicating the existence of a node. On 

 lowering the membrane still further, the sand will again become strongly 

 agitated, and will then come to another place of rest, (or at least of 

 minimum vibration), and so on till it reach the lower end of the tube. 

 These alternations of points of rest and motion can of course only take 

 place when the tube is sufficiently long in comparison with the length 

 of an undulation produced by the vibrating plate, to admit of them. 

 These nodal points are thus found to be at equal distances from each 

 other, the distance of the upper one from the top of the tube being less 

 than half that between the nodes. This is independent of the length of 

 the tube. These results are accordant with our theory, (Art. 22, I.), from 

 which it appears that this constant distance between two consecutive 



nodes must be -. 

 2 



If we call the distance of the upper node from B, -— C, C denotes 

 what I have termed the displacement of the nodes. 



altering the tension by means of the small cylinder round which the end b of the membrane 

 passes, or by moving the small bridge cd, and thus altering the length of the vibrating part. 



* This was the method adopted by Savart in such a variety of caseSj in which he wished to 

 ascertain the intensity of sonorous vibrations in air. 



