Positive corporate earnings trends boosted the Dow Jones Industrial average strongly upward Friday. At 2 p.m., the Dow was up 40.43 points to 4,805.34. If this levels holds, the Dow would set a new record. On the New York Stock Exchange, the composite index gained 1.94 to 314.31. On the Big Board, advancers led decliners 1,494 to 751, with 743 unchanged. Volume was 258.73 million shares traded. The Nasdaq composite had risen 5.83 to 1,021.46. The Standard and Poor's 500 stocks were up 3.39 to 586.49. The American Stock Exchange market value index gained 2.88 to 534.68. In London, gold was off $0.10 to $384.65.