The number of rigs drilling for oil and natural gas in the United States has fallen to the lowest level since at least the 1940s, the oil-field services company Baker Hughes said on Friday. Combined rigs in United States oil and gas fields fell by nine this week to 480, overwriting a previous record low of 488 in April 1999. Oil rigs alone fell for a 12th week in a row, and gas rigs fell to the lowest level since at least 1987. Also on Friday, the International Energy Agency, which represents the major oil-consuming nations, said signs were emerging that the oil market had “bottomed out.” In its monthly energy market report, the agency said market forces “are working their magic and higher-cost producers are cutting output.”