  Using Laplace transform techniques, we describe the determination of the
longitudinal structure function $F_{L}(x,Q^2)$, at the leading-order
approximation in momentum space, from the structure function $F_{2}(x,Q^2)$ and
its derivative with respect to ${\ln}Q^2$ in a kinematical region of low values
of the Bjorken variable $x$. Since the $x$ dependence of $F_2(x,Q^2)$ and its
evolution with $Q^2$ are determined much better by the data than $F_L(x,Q^2)$,
this method provides both a direct check on $F_L(x,Q^2)$ where measured, and a
way of extending $F_L(x,Q^2)$ into regions of $x$ and $Q^2$ where there are
currently no data. In our calculations, we ultilize the Block-Durand-Ha
parametrization for the structure function $F_{2}(x,Q^2)$ [M. M. Block, L.
Durand and P. Ha, Phys.Rev.D {\bf89}, 094027 (2014)]. We find that the Laplace
transform method in momentum space provides correct behaviors of the extracted
longitudinal structure function $F_{L}(x,Q^2)$ and that our obtained results
are in line with data from the H1 Collaboration and other results for
$F_{L}(x,Q^2)$ obtained using Mellin transform method.
