USAir yesterday reported a slight earnings decrease during August. Net income for the month was $7.3 million (58 cents a share) compared with $7.6 million (61 cents) for the same month last year. Revenues, however, increased from $69.5 million to $86.3 million.

For the eight months ended Aug. 31, the company had earnings of $39 million ($3.05), up from 22.1 million (1.79) a year earlier. Revenues were $625.7 million, up from $461.5 million.

USAir's figures for August 1979 include a gain of $3.4 million on disposition of flight equipment, the company said. The figure for the eight months this year includes a gain of $1.2 million. A gain of $3.6 million was also reported in 1979 for flight equipment.

Disposition of equipment figures are before taxes, the company said.

Potomac Electric Power Co. yesterday reported increased earnings for the first eight months of the year. Earnings rose to $57.7 million ($1.37 a share) from $41.9 million ($1.02) a year earlier. Operating revenue for the period was $545.3 million compared with $490.9 million last year.

McCormick & co. Inc. of Baltimore yesterday reported increased earnings and revenues for the third quarter ended Aug. 31. Net income was $2.8 million (25 cents a share), up from $2.6 million (22 cents). Revenues were $129.3 million compared with $107.6 million.

For the nine months ended Aug. 31, net income was $9.6 million (84 cents) compared with $8.7 million (76 cents). Revenues were $370.6 million, up from $304.9 million.

"Interest expense has had a substantial impact on this year's performance, running approximately $3.5 million higher than for the same period in the prior year," said McCormick Chairman Harry K. Wells. "However, sales and operating profit are up generally in line with expectations."