$2\mathrm{.}7$ A simple pendulum is constructed from an aluminium sphere attached to a light rod. A second pendulum is constructed of the same length but with a brass sphere. The diameters of the two spheres are the same. The two pendulums are set in motion at the same time with the same amplitude of oscillation. After $10$ min the amplitude of oscillation of the aluminium pendulum has decreased to one$-$half its initial value. By what factor has the amplitude of oscillation of the brass pendulum decreased at this time? $($Assume that the damping force acting on a pendulum is directly proportional to the velocity of the sphere. The densities of aluminium and brass are $2\mathrm{.}7{10}^{3}kg{m}^{-3}$ and $8\mathrm{.}5{10}^{3}kg{m}^{-3},$ respectively.$)$