  The Yakutsk array dataset in the energy interval $(10^{17},10^{19})$ eV is
revisited in order to interpret the zenith angle distribution of an extensive
air shower event rate of ultra-high energy cosmic rays. The close relation of
the distribution to the attenuation of the main measurable parameter of
showers, $S_{600}$, is examined. Measured and expected distributions are used
to analyze the arrival directions of cosmic rays on an equatorial map including
the energy range below $10^{18}$ eV, which was previously avoided due to the
reduced trigger efficiency of the array in the range. While the null hypothesis
cannot be rejected with data from the Yakutsk array, an upper limit on the
fraction of cosmic rays from a separable source in the uniform background is
derived as a function of declination and energy.
