ACOUSTICAL DISCOVERIES. 69 



what corresponds to beats in music. Here is a very pretty instrument 

 for showing the same thing. In this case we may consider that we 

 have two free reeds, one attached to the end of the other, on the same 

 principle as the pendulums attached one to the end of the other. 

 There are two thin strips of metal soldered together end to end with 

 their planes at right angles to one another. Fastening one of these in 

 a vice, we may set them vibrating ; the motion of the free end will be 

 the result of the combination ot the motions of the two strips taken 

 separately. On lengthening the lower strip, it will make a smaller 

 number of vibrations in a given time, and in this way any combination 

 of two rectangular motions may be obtained. 



I must also call your attention to another instrument invented by Sir 

 Charles Wheatstone for producing these figures by the motion of two 

 cranks, in two planes, at right angles to one another. The ends of the 

 cranks are fixed by a hinge to one end of a rod, the middle of the rod 

 turns in a fixed socket, so that the free end describes the same curves 

 as the end to which the crank arms are attached. There are several 

 points worthy of attention in this instrument, especially the arrange- 

 ment by which the number of revolutions of the two wheels which 

 drive the cranks may be made to bear any given ratio to one another. 

 I must now pass to another subject which is well illustrated by appa- 

 ratus in this exhibition, and which was worked at and developed by 

 Sir Charles Wheatstone, viz. : The production of sound by exciting 

 vibrations in tubes by means of gas flames. 



If a small gas-jet of suitable form be placed just within the lower end 

 of a tube open at both ends, vibrations will be excited in the tube, and 

 those vibrations which correspond to the length of the tube will be 

 reinforced by it, and a musical note will be heard. On raising the 

 flame into the tube the sound ceases, but on making the flame larger, 

 the sound is again produced and is louder than before. The sound 

 gets louder as the flame is raised, and the most intense sound is pro- 

 duced when the flame approaches to the position of the node. 



Working at this subject, Sir Charles Wheatstone produced an instru- 

 ment which is called a chemical harmonicon or gas-jet organ. It 

 consists of a key-board, to the keys of which are attached small gas 

 burners coming from a tube containing hydrogen gas. Each burner 

 is placed just within the lower end of a gas tube, and the tubes are of 



