$12\mathrm{.}6\mathrm{.}2$ Given that the pipe in Fig. $\backslash (12\mathrm{.}5$ a $\backslash )$ is $300$ m long, that water is initially flowing through it at $\backslash (0\mathrm{.}8\backslash$mathrm$\{$~m$\}/\backslash$mathrm$\{$s$\}\backslash ),$ and that pressure waves travel along it at $\backslash (1000\backslash$mathrm$\{$~m$\}/\backslash$mathrm$\{$s$\}\backslash ),$ find the maximum water hammer pressure at the valve, and at points $100$ m and $200$ m from the valve, $($a$)$ if the valve is completely closed instantaneously, and $($b$)$ if the valve is partially closed instantaneously, reducing the flow velocity to $\backslash (0\mathrm{.}2\backslash$mathrm$\{$~m$\}/\backslash$mathrm$\{$s$\}\backslash ).$