line integral of F = hP,Qi along the boundary is Rǫ 0 P ( +t,y ) dt Rǫ 0 R Q ǫ,y t − ǫ 0 P (x +t,y ǫ ) dt− Rǫ 0 Q x,y t . This line integral measures the ”circulation” at the place (x,y). Because Q(x + ǫ,y) − Q(x,y) ∼ Qx(x,y)ǫ and P(x,y + ǫ) − P(x,y) ∼ Py(x,y)ǫ, the line integral is (Qx − Py)ǫ2 is about the same as ...