By John S. McClenahen U.S. small businesses continue to warm up to the economy, claims the Washington, D.C.-based National Federation of Independent Business (NFIB). Its Index of Small Business Optimism was 105.8 in January, the fourth highest quarterly reading in the survey's 30-year history. "Not since the beginning of a long economic expansion in the 1980s have we seen numbers like this," says William Dunkelberg, NFIB's chief economist. "The January figures have been exceeded only by those of the last two quarters of 1983 and the first quarter of 1984." The index suggests new jobs are about to be created, with 17% of small employers planning to increase employment, a percentage that is up by four points from October of last year.