236 NOTES. 



If we write 



p *x=Pxx- % ina2 &amp;gt; & c -&amp;gt; &c., 

 Py*=P y *-2mpy, &c., &c., 

 this becomes, in virtue of the equation of continuity, 



p T^T 



r 



; j 7 .................. j 



dx dy dz 

 which agrees in form with (2) of Art. 175. 



The quantities p xx , p yz , &c. determine the apparent stress of the 

 medium at the point (x, y, z), as distinguished from the actual statical 

 stress specified by p^^ p yie , &c. The apparent stress is what is really 

 observed in all experiments in fluids. It is in fact the stress which 

 must hold at any point of the continuous model above spoken of, in 

 order that the model may work similarly to the original. 



In any case where we could assume that the oblique component of the 

 stress across any plane is small enough to be neglected, we should find, 

 calculating the rate of change of the momentum contained in a tetra- 

 hedral space such as that in the figure of Art. 3, 



p **=p yy =p **&amp;gt; =-p&amp;gt; sa y- 



The equations of motion then assume the form (2) of Art. 6. 



