io4 SCIENTIFIC APPARATUS. 



in Longman and Broderip's Tuning Machine, which is exhibited, 

 and which does not materially differ from the contrivances of 

 Pythagoras. 



Perronet Thompson followed the same system, in adjusting the 

 so-called quarter-tones of his Enharmonic Organ. He used a 

 monochord strung with steel wire, the adjustable' weight rising 

 regularly to 250 Ibs. 



Of late the Siren, in one or other of its forms, has been princi- 

 pally employed. A rotating disc, with oblique holes, is brought 

 into close contact with a similar fixed disc, the obliquity of whose 

 holes lies in the opposite direction ; rotation is thus produced by 

 the current of air, and a musical note by the alternate passage and 

 obstruction of the stream; while a screw-counter registers the 

 rapidity of rotation for a given pitch. 



It is worthy of comment that the two mechanical principles 

 thus laid down, have both been found ancillary to the measure- 

 ment of the far more rapid vibrations of light ; the toothed wheel 

 in M. Fizeau's, and the siren in M. Foucault's experiments, recently 

 repeated by M. Cornu with increased accuracy and elaboration. 



Other modes of measurement are the Optical and the Graphical, 

 severally connected with the apparatus of M. Lissajous, and the 

 Vibration-Microscope of Helmholtz ; with the curvetracers of 

 Mr. Donkin and Mr. Tisley, and the Phonautograph. In the two 

 former cases the figures obtained serve also to illustrate the phe- 

 nomena of circular and elliptical polarization. 



Lastly, the method of Beats and Interferences affords the best 

 practical method of measurement for the purposes of tuning and 

 comparison. These have of late been fully discussed in the great 

 work of Helmholtz. 



HISTORY AND ETHNOLOGY. 



It is of the highest moment here, as in other branches of science, 

 to consider developments, now common and familiar, in the light 

 of their history and distribution. It has been found practicable, 



