NBC's hospital drama Mercy continued its impressive ratings rise Wednesday night as it averaged 7.8 million viewers and placed first in the 8 00 p.m. hour. One of the worst-reviewed new shows, Mercy has managed to pick up viewers steadily from the beginning of the season, when it tied for fourth place in its debut -- drawing fewer viewers than the failed Knight Rider , which preceded it last season. NBC also drew decent ratings for Law & Order SVU , which followed at 9 00 p.m., but the 8.4 million viewers who tuned in represented a 30 percent drop from last season when it aired on Tuesdays at 10 00 p.m. And the show that forced it to move, The Jay Leno Show , at 10 00 p.m. continued to draw anemic ratings, attracting only 4.8 million viewers. By contrast, Robin Roberts's interview with Janet Jackson in the same hour drew 6.6 million viewers, and CBS's CSI NY was the top-rated show of the night with 13.6 million viewers.