Very sensitive H \scriptstyle { I } 21cm observations have been made in 860 directions at \delta \geq - 43 \arcdeg in search of weak , Galactic , high-velocity H \scriptstyle { I } emission lines at moderate and high Galactic latitudes .
One-third of the observations were made toward extragalactic objects that are visible at optical and UV wavelengths .
The median rms noise in the survey spectra is 3.4 mK , resulting in a median 4 \sigma detection level of N _ { HI } = 8 \times 10 ^ { 17 } cm ^ { -2 } averaged over the 21 \arcmin beam of the telescope .
High-velocity H \scriptstyle { I } emission is detected in 37 % of the directions ; about half of the lines could not have been detected in previous surveys .
The median FWHM of detected lines is 30.3 km s ^ { -1 } .
High-velocity H \scriptstyle { I } lines are seen down to the sensitivity limit of the survey implying that there are likely lines at still lower values of N _ { HI } .
The weakest lines have a kinematics and distribution on the sky similar to that of the strong lines , and thus do not appear to be a new population .
Most of the emission originates from objects which are extended over several degrees ; only a few appear to be compact sources .
At least 75 % , and possibly as many as 90 % , of the lines are associated with one of the major high-velocity complexes .
With the increased sensitivity of this survey , the Magellanic Stream is seen to extend at least 10 \arcdeg to higher Galactic latitude than previously thought and to be more extended in longitude as well .
Wright ’ s Cloud near M33 has an extended low- N _ { HI } component in the direction of the Magellanic Stream .
The bright H \scriptstyle { I } features which have dominated most surveys may be mere clumps within larger structures , and not independent objects .
Although there are many lines with low column density , their numbers do not increase as rapidly as N _ { HI } ^ { -1 } , so most of the H \scriptstyle { I } mass in the high-velocity cloud phenomenon likely resides in the more prominent clouds .