Atlantic Research Corp. reported an 18 percent increase in second-quarter profits yesterday.

The manufacturer of solid rocket propulsion systems and Tempest equipment for confidential computer information systems said its net income grew to $4.2 million (56 cents a share) from $3.6 million (48 cents) in the comparable period last year.

The company, which is based in Alexandria, said sales for the quarter rose to $66.5 million from $55.1 million.

For the year to date, Atlantic Research said its net income was up slightly, to $7.8 million ($1.04) from $7.7 million (also $1.04). Last year's figures included a gain of $1.1 million (15 cents) from the sale of Tri-Seal International, a subsidiary.Maryland National Corp., the holding company for Maryland National Bank, has reported second-quarter profits of $22.6 million, a 29 percent increase over net income of $17.6 million for the same period last year.

The company said the results reflected continued growth in its loan portfolio and a substantial increase in noninterest income.

For the first half of this year, the company reported earnings of $43.8 million, up 31 percent from $33.6 million for the first half of 1985.

The Baltimore company reported total assets of $8.6 billion as of June 30, up 17 percent from a year ago. Columbia First Federal Savings and Loan Association said it earned $2.04 million (83 cents a share) in its fiscal third quarter, which ended June 30, up from $777,000 a year ago.

Nine-month net income was $5.1 million, up from $530,000 in 1985.

The company said it had deposits of $970 million, total assets of $1.6 billion and a net worth of $71.8 million on June 30. Gray and Co. Public Communications International Inc. reported a 15 percent increase in revenue, but a decline in earnings of 85 percent during its first quarter, which ended May 31.

The company attributed the decline to costs associated with opening an office in Dallas and in connection with a proposed merger with JWT Group.

Gray and Co., a government affairs and public relations firm based in Washington, reported revenue of $7 million, up from $6.1 million last year. Net income was $47,000 (2 cents a share), down from $319,000 (13 cents) last year.