C. GENERAL PHYSICS. 197 



ceive the harmonic beating* on the ear alone. This fact can 

 readily be confirmed by the aid of one of Helmholtz's reso- 

 nators. A further slight lowering of the velocity of the disk 

 brings out the beats of the next lower harmonic, and so on, 

 until the velocity has been so diminished that even the beats 

 of the lowest or fundamental harmonic are perceived; and 

 then all of the component sounds are beating in unison ; but 

 yet the effects they produce on the ear are very different, 

 for the higher harmonics, notwithstanding their feebler in- 

 tensities, must be heard more distinctly, because their inter- 

 mittences are furthest removed from the numbers that cause 

 their sensations to blend. 



The fact that the durations of the residual sensations di- 

 minish as the numbers of vibrations producing the sounds 

 increase, leads to the knowledge of a new and curious phe- 

 nomenon in the physiology of audition, viz., that the timbre, 

 or quality, of a composite sound begins to change at the in- 

 stant the vibrations outside the ear have ceased ; for from 

 that instant the residual sensation becomes more and more 

 simple in its character, until at last only the simple sound of 

 the fundamental harmonic remains in the ear, and soon after 

 this sensation also vanishes. Thus, after the vibrations of a 

 C reed-pipe, of 128 vibrations per second, containing twenty 

 harmonics, have ceased, the residual sensation of the twen- 

 tieth harmonic, or that highest in pitch, disappears in the 

 T7^ T of a second ; but the sensation of the fundamental or low- 

 est harmonic remains in the ear $ of a second after the sen- 

 sation of the highest has vanished ; and the fundamental re- 

 mains -^ of a second after the cessation of the sensation of 

 the harmonic next above it. 



OX THE CONNECTION" EXISTING BETWEEN THE DIRECTION OF 

 CLEAVAGE AND THE CONDUCTIVITY OF HEAT, AND EXPAN- 

 SION BY HEAT, IN CRYSTALS AND LAMELLATED ROCKS. 



M. Jannettaz has recently determined the following impor- 

 tant laws of the propagation of heat in crystallized bodies : 



1. If a crystal have a plane of cleavage, heat is propagated 

 with greater facility in directions parallel to that plane than 

 in directions perpendicular to it. 



2. If a crystal cleave in several directions in planes per- 

 pendicular to each other, the greatest axis of thermal con- 



