Kirby Corp.'s net earnings dropped by more than a third in the first quarter from the same period a year earlier, as difficulties in the oil and gas market continued to take a toll on its business transporting petrochemicals and providing equipment for the industry, the Houston company said Thursday. The company's business in repairing and selling diesel engines used for pumping oil and gas has dropped off during the downturn, with an $800,000 operating loss in diesel engine services in the first quarter.