  We present reverberation mapping measurements for the prominent ultraviolet
broad emission lines of the active galactic nucleus Mrk817 using 165 spectra
obtained with the Cosmic Origins Spectrograph on the Hubble Space Telescope.
Our ultraviolet observations are accompanied by X-ray, optical, and
near-infrared observations as part of the AGN Space Telescope and Optical
Reverberation Mapping Program 2 (AGN STORM 2). Using the cross-correlation lag
analysis method, we find significant correlated variations in the continuum and
emission-line light curves. We measure rest-frame delayed responses between the
far-ultraviolet continuum at 1180 A and Ly$\alpha$ $\lambda1215$ A
($10.4_{-1.4}^{+1.6}$ days), N V $\lambda1240$ A ($15.5_{-4.8}^{+1.0}$days),
SiIV + OIV] $\lambda1397$ A ($8.2_{-1.4}^{+1.4}$ days), CIV $\lambda1549$ A
($11.8_{-2.8}^{+3.0}$ days), and HeII $\lambda1640$ A ($9.0_{-1.9}^{+4.5}$
days) using segments of the emission-line profile that are unaffected by
absorption and blending, which results in sampling different velocity ranges
for each line. However, we find that the emission-line responses to continuum
variations are more complex than a simple smoothed, shifted, and scaled version
of the continuum light curve. We also measure velocity-resolved lags for the
Ly$\alpha$, and CIV emission lines. The lag profile in the blue wing of
Ly$\alpha$ is consistent with virial motion, with longer lags dominating at
lower velocities, and shorter lags at higher velocities. The CIV lag profile
shows the signature of a thick rotating disk, with the shortest lags in the
wings, local peaks at $\pm$ 1500 $\rm km\,s^{-1}$, and a local minimum at line
center. The other emission lines are dominated by broad absorption lines and
blending with adjacent emission lines. These require detailed models, and will
be presented in future work.
