Show that a tilted free liquid surface undergoes shear stress

[Est120]

i really can't understand the answer of this question, is the question 1.3 in fluid mechanics by Frank ,M White

For the triangular element in Fig P1.3, show that a tilted free liquid surface, in contact with an atmosphere at pressure pa, must undergo shear stress and hence begin to flow.

i repeat i know the answer ,there is a solutionary of this book but i CANT understand why does the pressures at the bottom of the triangle are higher ,and why did the solutionary did n0t put Area in the formula since F=P*A ,
someone please explain this

 

[Chestermiller]

The shear stress on the free surface is obviously zero. But, the shear stress on a plane of constant y in the x direction is not zero. So, it depends on the orientation of a surface whether there is shear stress on it or not.