  The second-order moment of the nuclear charge density($R^2_c$) is dominated
by the mean square radius(msr) of the point proton distribution($R_p^2$), while
the fourth-order moment($Q^4_c$) depends on the msr of the point neutron
one($R_n^2$) also. Moreover, $R^2_n$ is strongly correlated to $R^2_c$ in
nuclear models. According to these facts, the linear relationship between
various moments in the nuclear mean field models are investigated with use of
the least squares method for $^{40}$Ca, $^{48}$Ca and $^{208}$Pb. From the
intersection points of the obtained straight lines with those of the
experimental values for $R^2_c$ and $Q^4_c$ determined through electron
scattering, the values of $R_p$ and $R_n$ are estimated. Since relativistic and
non-relativistic models provide different lines, the obtained values of $R_n$
and the skin thickness($R_n-R_p$) differ from each other in the two frameworks.
