  The structure of the first sharp diffraction peak (FSDP) of amorphous silicon
(${\it a}$-Si) near 2 Angstrom$^{-1}$ is addressed with particular emphasis on
the position, intensity, and width of the diffraction curve. By studying a
number of continuous random network (CRN) models of ${\it a}$-Si, it is shown
that the position and the intensity of the FSDP are primarily determined by
radial atomic correlations in the amorphous network on the length scale of 15
Angstroms. A shell-by-shell analysis of the contribution from different radial
shells reveals that the key contributions to the FSDP originate from the second
and fourth radial shells in the network, which are accompanied by a background
contribution from the first shell and small residual corrections from the
distant radial shells. The results from numerical calculations are complemented
by a phenomenological discussion of the relationship between the peaks in the
structure factor in the wavevector space and the reduced pair-correlation
function in the real space. An approximate functional relation between the
position of the FSDP and the average second-neighbor distance of Si atoms in
the amorphous network is derived, which is corroborated by numerical
calculations.
