  We investigate the long time stability of the Sun-Jupiter-Saturn-Uranus
system by considering the planar, secular model. Our method may be considered
as an extension of Lagrange's theory for the secular motions. Indeed,
concerning the planetary orbital revolutions, we improve the classical circular
approximation by replacing it with a torus which is invariant up to order two
in the masses; therefore, we investigate the stability of the elliptic
equilibrium point of the secular system for small values of the eccentricities.
For the initial data corresponding to a real set of astronomical observations,
we find an estimated stability time of $10^7$ years, which is not extremely
smaller than the lifetime of the Solar System ($\sim 5$ Gyr).
