( 563 ) 
1 @ 
dent of r, The mean value of »° for a full period is ir and by 
TT 
differentiating this with regard to 2 or to r, we should get?’ in the 
denominator. 
The terms in (6) which correspond to the part 
2 (Dix Dox 4: Diy Doy + diz doz) 
in Dd? may likewise be neglected. Indeed, if these terms are to 
Wc t 
contain no factors such as cos 2 DE or sin 2 awk 7 there must be 
between 6, and ds, either no phase-difference at all, or a difference 
which is independent of r. This condition can only be fulfilled, if 
a system of waves, proceeding in the direction of QP, is combined 
with the vibrations excited by Q, in so far as this ion is put in 
motion by that system itself. Then, the two vectors d, and 6, will 
have a common direction perpendicular to QP, say that of the axis 
of y, and they will be of the form 
Hij 
Diy = q cos Nn (: —F + a) 
c Hi 
Day == Ean COS Nl (’ = V +e). 
The mean value of d1y day is 
1 ge 
ee Be cos n(& — €) » 
2r 
and its differential coefficient with regard to 2 has r® in the denom- 
inator. It ought therefore to be retained, were it not for the 
extremely small intensity of the systems of waves which give rise 
to such a result. In fact, by the restriction imposed on them as to 
their direction, these waves form no more than a very minute part 
of the whole motion. 
S 4. So, it is only the terms in (2), with the coefficient 5, 
with which we are concerned. The corresponding forces are 
Obs.) dds 
Lhe V2 Md (be TE aje EEN (7) 
dw " oy 
and 
ms Hb Oy Ou Batak artnr be thea vee AS 
