  We study the non-thermal velocities in the quiet-sun using various high
spatial, temporal, and spectral resolution observations from the Interface
Region Imaging Spectrograph (IRIS). We focus our analysis on the transition
region using the optically thin line (Si IV 1393.7 \AA), and select line
profiles that are nearly Gaussian. We find evidence of a centre-to-limb
variation using different observations having different exposure times, ranging
from 5-30 s. The distribution of non-thermal velocities close to the limb are
observed to peak around 20 km s$^{-1}$ while the disc observations show a peak
around 15 km s$^{-1}$. The distributions are also different. The overall
variation in the non-thermal velocities are correlated with the intensity of
the line, as found previously. The on-disc velocities are smaller than most
previous observations. In general, we find that the non-thermal velocities are
independent of the selected exposure times. The Si IV lines didn't seem to
exhibit any significant opacity effects. We conclude that these Doppler motions
are mostly transverse to the radial direction. The possibility of
swaying/torsional motions leading to such variations are validated from these
IRIS observations.
