We present a proper motion mini-survey of 35 fields in the vicinity of Baade window , ( l,b ) = ( 1 ^ { \circ } , -4 ^ { \circ } ) , sampling roughly a 5 \times 2.5 deg region of the Galactic bar .
Our second epoch observations collected with the ACS/HRC instrument on board the Hubble Space Telescope were combined with the archival WFPC2/PC images .
The resulting time baselines are in the range of 4–8 years .
Precise proper motions of 15,863 stars were determined in the reference frame defined by the mean motion of stars with magnitudes between I _ { F 814 W } = 16.5 - 21.5 along the line of sight .
We clearly detect small gradients in proper motion dispersions ( \sigma _ { l } , \sigma _ { b } ) \sim ( 3.0 , 2.5 ) { mas yr } ^ { -1 } , and in the amount of anisotropy ( \sigma _ { l } / \sigma _ { b } \sim 1.2 ) .
Both the longitude dispersion \sigma _ { l } and its ratio to the vertical motion \sigma _ { b } increase toward the Galactic plane .
The decline of the anisotropy ratio \sigma _ { l } / \sigma _ { b } toward the minor axis of the bulge is mostly due to increasing \sigma _ { b } .
We also find , for the first time , a significant negative covariance term in the transverse velocity field \sigma _ { lb } / ( \sigma _ { l } \sigma _ { b } ) \simeq - 0.10 .
Our results extend by a factor of \sim 15 the number of the Galactic bar fields with good proper motion dispersions .