  In this note we describe explicitly, in terms of Lie theory and cameral data,
the covariant (Gauss--Manin) derivative of the Seiberg--Witten differential
defined on the weight-one variation of Hodge structures that exists on a
Zariski open subset of the base of the Hitchin fibration. Dedicated to Tony
Pantev on the occasion of his 60th birthday.
