( 795 ) 



surfaces in a similar plane of a crystal are, in most cases, concentric 

 and eqniform three-axial ellipsoids whose half axes stand in the 

 relation of V^X^,V'?., and l/A., ; among these the so-called piincipal 

 ellipsoid h, whose axes are V?.^, \/).^ and l/p-, must here be kept 

 more especially in view. 



In the present case we leave unnoticed the rotatory qualities of 

 the crystal, and suppose an infinitely thin plate, cut parellel to a 

 plane of thei'mic symmetry, whose principal directions correspond to 

 the coordinate axes. Let lig. 1 represent the elliptic intersection 

 of the plate with the ellipsoitl h ; the line traced by the melted wax 

 then has the direction of the tangent of tlio elli[)se in the point 

 P{x'y'), given by the radius vector q, whicii may enclose the angle 

 fp with the axis X The flow of heat may thus proceed along q, 

 being the boundary line. In this case the equation for the isothermal 

 line pq is : 



,vx' ?/?/' 



c=\r\. 



Thus for the two sections Op and 0<j cut off on the two axes 

 the result is : 



;/ Q sin <p 



therefore : 



x' Q cos (f 



2i = b. 



0, X, 



cot (f. 



