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We present new parallax measurements of 7 long-period (> 10 days) Milky WayCepheids (SS CMa, XY Car, VY Car, VX Per, WZ Sgr, X Pup and S Vul) usingastrometry from spatial scanning of WFC3 on HST. Observations were obtained at6 month intervals over 4 years. The distances are 1.7--3.6 kpc with a meanprecision of 45 microarcseconds and a best of 29 microarcseconds (SNR = 14).The accuracy of the parallaxes is demonstrated through independent analyses of>100 reference stars. This raises to 10 the number of long-period Cepheids withsignificant parallax measurements, 8 obtained from this program. We alsopresent high-precision F555W, F814W, and F160W magnitudes of these Cepheids,allowing a direct, zeropoint-independent comparison to >1800 extragalacticCepheids in the hosts of 19 SNeIa. This sample addresses two outstandingsystematic uncertainties affecting prior comparisons of Milky Way andextragalactic Cepheids used to calibrate H_0: their dissimilarity of periodsand photometric systems. Comparing the new parallaxes to their predicted valuesderived from reversing the distance ladder gives a ratio (or independent scalefor H_0) of 1.037+/-0.036, consistent with no change and inconsistent at the3.5 sigma level with a ratio of 0.91 needed to match the value predicted byPlanck+LCDM. Using these data instead to augment the Riess et al. (2016)measurement of H_0 improves the precision to 2.3%, yielding 73.48+/-1.66km/s/Mpc, and tension with Planck+LCDM increases to 3.7 sigma. The futurecombination of Gaia parallaxes and HST spatial scanning photometry of 50 MilkyWay Cepheids can support a < 1% calibration of H_0.

New Parallaxes of Galactic Cepheids from Spatially Scanning the Hubble Space Telescope: Implications for the Hubble Constant