Adding a layer of technology to manual searches of bags and packages taken into the New York City subway, police officials said yesterday that they would begin using mechanized devices that check for conventional explosives. At various stations beginning next week, riders will encounter officials using either hand-held devices that resemble portable vacuum cleaners or larger tabletop ones that look like fax machines. Both work the same way, the police said, by analyzing a cloth swab that is passed over a bag or package to test for the presence of such things as ammonium nitrate or hydrogen peroxide, which was...