416 Mr. C. A. Bell on the Determination 



rods or tubes under examination ; and the lengths of these 

 were gradually reduced by cutting and grinding, until each 

 gave a note differing from that of the steel by about 4 or 5 

 vibrations per second. In cutting an ordinary glass knife 

 was used, but the trimming of the ends, and small alterations 

 of length, were effected by lightly pressing the ends against 

 a sheet of moderately coarse emery-cloth clamped on the face 

 of a circular wooden disk which was rapidly rotated in a 

 lathe. During the comparison of tones, the steel rod was 

 simply balanced on the corner of a table, and the glass rod 

 held at its centre in the hand, both rods being struck endwise 

 by a light wooden mallet. 



After this preliminary adjustment, the bundle of rods was 

 laid aside until its temperature might be assumed to be that 

 of the room. Each rod was then passed through a hole in a 

 short cylindrical cork placed at such a point — not necessarily 

 the centre — that the vibration was of maximum persistence, 

 and mounted on a board side by side with the similarly sup- 

 ported steel rod. 



A cone or funnel was fixed on the board so that its wider 

 end embraced one pair of the rod ends, while its narrower 

 end was connected by rubber tubing with a short length of 

 glass tube pushed well into the ear-cavity. Beats due to the 

 rod vibrations were thus heard with the greatest distinctness. 

 To throw the rods into vibration, two small "pneumatic 

 hammers 9J were arranged to deliver light blows on their 

 distant ends. Each hammer was formed of a cylinder of 

 wood about 5 cms. in length, cut from an ordinary pen-handle, 

 lying within a slightly wider tube of glass in which it could 

 just slide freely. A perforated cork or a section of rubber 

 tubing, forced into the glass tube, kept the cylinder lying 

 normally just within one of its ends ; the other drawn-out 

 end was connected bv thin rubber tubing with a bag such 

 as is used for pneumatic bells, which could be placed in any 

 convenient position. By tapping on this bag with the finger 

 the little wooden cylinder was sharply projected from, and 

 as sharply drawn back into, the glass, after impact on the 

 rod end ; whilst by pressing gently on the bag the hammer 

 was caused to rest against the rod, at once checking its 

 motion. The rods could thus be thrown into vibration either 

 alternately or simultaneously, the force of the blows delivered 

 being under complete control. This little device, which I 

 have found superior to any other mechanical or electrical 

 arrangement, has proved of eminent service in a number of 

 acoustical experiments. Thus in working with tuning-forks 

 it makes it unnecessary for the operator to approach them — 



