  Description of the TAU-4 installation intended for long-term monitoring of
the half-life value $T_{1/2}$ of the $^{212}$Po is presented. Natural thorium
used as a source of the mother's chain. The methods of measurement and data
processing are described. The comparative results of short test measurements
carried out in the ground (680 h) and underground (564 h) laboratories are
given. Averaged value $T_{1/2}$ =$294.09\pm 0.07$ ns of the $^{212}$Po
half-life has been found for the ground level data set similar one for the
underground data set. The solar-daily variations with amplitudes
$A_{So}=(11.7\pm 5.2)\times10^{-4}$ for the ground data and $A_{So}=(7.5\pm
4.1)\times10^{-4}$ for the underground one were found in a series of $\tau$
values.
