The following is the approximate expression for the relation between a
change [Delta]s in the length of the half chain and the corresponding
change [Delta]y in the dip:--
s+[Delta]s = x+(2/3x) {y²+2y[Delta]y+([Delta]y)²} =
x+2y²/3x+4y[Delta]y/3x+2[Delta]y²/3x,
or, neglecting the last term,