138 LECTURES TO SCIENCE TEACHERS. 



2. Distributors. Air . Speaking-tubes, Stethoscopes, &c. 



Wood . Sounding-rods. 

 Metal . Wires. 



3. Pugging of floors, &c. 



4. Reservoirs. Eesonators, Organ-pipes, Sounding-boards. 



5. Dampers of Pianofortes. 



6. Regulators. Organ Swell. 



7. Detectors. The Ear ; Sensitive-flames, Membranes, Phon- 



autographs, &c. 



8. Tuning-forks, Pitch-pipes, and musical scales. 



Now of these we shall consider to-day principally mono- 

 chords, tuning-forks and sirens, under the former head, that 

 of eliciting; in the second place, that of reinforcing and 

 distributing,' resonators of various kinds, and telephones. 

 The latter perhaps might have been more distinctly specified 

 in the title of the lecture, as what Professor Clerk-Maxwell 

 terms distributors. Eefore, however, adverting to the means 

 of eliciting sound, we can hardly avoid mentioning something 

 as to vibration in general. We find it proceeding from 

 ordinary pendular vibration up to the most delicate vibration 

 of ether, on which rests the fundamental hypothesis of light, 

 and we can observe this vibration in various ways. 



A very ingenious instrument is here contributed from 

 abroad which enables you to combine one or more harmonic 

 motions. The string is strained between two elastic terminals, 

 both of which by means of electro-magnets can be set into oscil- 

 latory motion. By putting the first alone into motion we 

 get single vibration ; by joining and coupling up with it that 

 at the other end, which can be rotated round its axis, we can 

 combine another harmonic motion, either in the same direc- 

 tion making complex vibrations, or at right angles, or indeed 

 at any given angle; thus compounding it into various regular 

 figures, ellipses, circles, and other curves such as were produced 

 by Lissajous. 



Taking this end of the vibrating string first, I bring the 

 battery in connection ; then straining the string to the right 

 tension you will see very distinctly that it is vibrating in two 

 segments forming a node at the middle point. The string 



