ELASTICITY OF REGULARLY CRYSTALLIZED BODIES. 149 
systems consists of two lines crossed rectangularly, one of which, a 2, 
places itself always on the axis of greatest elasticity, and this line serves 
as the second axis to the hyperbolic curves which compose the noda{ 
system. Doubtlessly these curves are not entirely similar in the different 
plates; but Ihave not beenable to perceive any very remarkable difference 
between them, unless that it appears that their summits gradually ap- 
proach by a very small quantity, in proportion as the plates more nearly 
approach containing the intermediate axis in their plane. 
Fourtn Series.—Plates cut round the diagonal AD, and perpendi- 
cular to the plane BC Y Z; figs. 13 and 14. 
These plates present much more complicated phenomena than those 
we have hitherto observed. Except for the first and the last, neither 
of the two nodal systems consists of lines crossed rectangularly, which 
shows that this kind of acoustic figure can only occur on plates which 
contain at least one of the axes of elasticity in their plane, since Nos. 
2, 3, 4, 5, which are inclined to the three axes, present only hyperbolic 
lines, whilst No. 1, which contains two of the axes of elasticity, and 
No. 6, which contains only one, are susceptible of assuming this kind 
of division. 
- In this series, neither of the modes of division remains constantly the 
same for the different degrees of inclination of the plates: setting out 
from the plate No. 1, one of the systems gradually passes from two 
crossed lines to two hyperbolic branches, which are nearly transformed 
into parallel straight lines in No. 6; on the contrary, the other system 
appears in No. 1 under the form of two hyperbolic curves, the summits 
of which approach nearer and nearer until they coalesce in No. 6, where 
they assume the form of two straight lines which cut each other at 
right angles and this contrary course in the modifications of the two 
systems is such, that there is a certain inclination (No.3) for which the 
two modes of division are the same, although the sounds which cor- 
respond to them are very different. 
_ As in the preceding series, and for the same reasons, the sound of 
each nodal system goes on always ascending in proportion as the plate 
more nearly approaches containing the axis of greatest elasticity in its 
plane. 
Firtu Serres.—Plates cut rownd the diagonal A E, and perpendicular 
to the planer s t; figs. 5. 
‘ bodice all the plates which may be cut round the diagonal A E of 
_ the cube fig. 5, there are three each of which contains one of the axes 
S _ of elasticity, and which consequently we have already had occasion to 
_ observe; thus the plate No. 3, fig. 8, which passes through the diagonal 
A B, and through the edge A Y, contains the diagonal A E in its 
