INSTRUMENTS FOE EXPERIMENTS ON SOUND. 139 



is studded with small white points to enable it to be seen. 

 Now if I put on the second magnet at the other end, 

 turning it into the same circuit, and combine the vibrations 

 of those two, the figure becomes much more complicated. 

 There are three or four ventral segments, two different 

 variations being thrown into the string at the same time. 

 Then compound them still more by turning the second 

 magnet round upon its axis and setting them in two rect- 

 angular directions. A beautiful elliptical figure is formed 

 thus by the vibrations in a horizontal direction at one end and 

 in a vertical direction at the other. This apparatus shows 

 the vibration of a string ; I will next demonstrate another 

 means of combining the vibrations of reeds. This is a 

 very pretty contrivance, contributed by Mr. Pichler, who 

 kindly comes to assist me in exhibiting it. The instrument 

 consists of a wind-chest with means of blowing ; above it there 

 are two reeds, one fixed in the vertical and the other in a 

 horizontal position ; by shifting the bearing of one reed it can 

 be made to double its length, so that the vibrations shall be to 

 each other in any ratio from that of unison down to an octave 

 below. We can pass through all the intermediate figures. On 

 each reed is placed a small mirror, and here is a limelight, the 

 beam from which falls first on the mirror of the upper reed, 

 it returns and is reflected on the second, whence it is thrown 

 on the screen. Whilst the mirrors are still, the spot of light 

 on the screen is motionless ; when we set them vibrating, the 

 one in a vertical direction gives a vertical line of light, when 

 the other in a horizontal direction is added they combine their 

 two harmonic motions and give beautiful curves, which I have 

 already named. The circle denotes unison ; the varied figures 

 are produced by varying phases and velocities at the two 



Strings were the earliest sources of sound and were the first 

 used for acoustical experiments ; indeed, in ancient Greek 

 times determinations were made of the law's of sound from 

 strings, which are generally attributed to Pythagoras, though 

 Mr. Chappell in his erudite work concludes that Pythagoras's 

 information came originally from Egypt or even can be traced 

 back to Babylon. At any rate, by Euclid's time a very 

 perfect knowledge of the laws of strings had been attained. 

 Euclid wrote a work called Sectio Ganonis, the division of the 

 string or monochord by which all these ratios are obtained. 



