Masses, $M_1$, $M_2$ and $M_3$ are connected by strings of negligible mass which pass over massless and frictionless pulleys ${\mathrm{P}}_1$ and ${\mathrm{P}}_2$ as shown in the figure. The masses move such that the portion of the string between ${\mathrm{P}}_1$ and ${\mathrm{P}}_2$ is parallel to the inclined plane and the portion of the string between ${\mathrm{P}}_2$ and $M_3$ is horizontal. The masses $M_2$ and $M_3$ are $4.0 \mathrm{kg}$ each and the coefficient of kinetic friction between the masses and the surfaces is $0.25$. The inclined plane makes an angle of $37°$ with the horizontal and the mass $M_1$ moves downwards with a uniform velocity. Find the tension in the horizontal portion of the string.
(Take $g=9.8 \mathrm{m} {\mathrm{s}}^{-2}$, $\sin 37°$$\cong$ $\raisebox{1ex}{$3$}\!\left/ \!\raisebox{-1ex}{$5$}\right.$)