defined as $$\xi=\frac{(E/1\,\textrm{GeV})^{\alpha} (1+r/r_M)}{(r/r_M)^{\beta}}\cdot \theta  \ \textnormal{,}$$ where $\alpha$ and $\beta>0$. We choose $E$ in GeV and $\theta$ in degrees. Using this variable we get for all regions of the electron energies and lateral distances almost identical shapes of the normalized distributions of $\xi$ for a given azimuth angle $\varphi$ and shower age $s$. Thus, the function $f_{\theta} (\theta ; \varphi , r / r_M , E , s )$ of five variables reduces to a function $F_{\xi} (\xi ; \varphi , s )$ of only three variables: $\xi , \varphi$ and $s$, where $F_{\xi} (\xi ; \varphi , s )d\,\xi$ is the fraction of the number of electrons with parameter $(\xi, \xi +d\,\xi)$, the angle $\varphi$ at level $s$, with respect to the total number of them with any $\xi$.\
Using a procedure of minimizing differences between histograms binned for various energies and distances in different $\varphi$ ranges we have found the best values of $\alpha$ and $\beta$ (see Appendix Eq. A6).\
Fig. 7 shows the distributions $F_{\xi} (\xi ; \varphi , s )$ for two values of $\varphi$ , at $s = 0.7$ and $1.3$ (at $s=1$ the picture is similar with less fluctuations). For each $\varphi$ there 