Derivation of potential of two closed curves s and s'

[faheemahmed6000]

I am reading Maxwell's "a treatise on electricity and magnetism" and I came across a formula of "potential of two closed curves s and s' "

$M= \iint\dfrac{cos\varepsilon}{r} dsds'$
where:

$M=$ potential of two closed curves s and s'

$\varepsilon=$ angle between elements ds and ds'

$r=$ distance between elements ds and ds'

s and s' are two closed curves.

The book has a derivation of this formula in "article 423" but its extremely hard to understand because of lack of vector notation.

Can someone give me an easy to understand derivation of this formula using modern vector notation?