Ford Motor Co. reported profits of $512 million in the second quarter of this year - a poorer showing than the comparable 1978 period and the first quarter of this year.

Slumping car sales and higher marketing costs were blamed for the decline in earnings.

Ford's foreign operations showed earning increases for the period.

"In the United States, the earnings decline resulted from lower unti volumes, the shift to smaller cars and higher marketing costs," a company statement said.

"In view of the slowdown in the U.S. economy, high dealer stocks and energy-related problems, industry car and truck sales in the U.S. market are expected to be below last year's levels in the third and fourth quarters," Ford said.

The profits of $512 million ($4.28 a share) were $5.2 percent below earnings of $540 million ($4.55) in the second quarter of 1978 and 14 percent less than the $595 million reported in the first quarter of this year.

Ford reported worldwide sales of $11.9 billion for the quarter, the same as a year ago. Sales in the U.S. were $6.8 billion, 12 percent below year-ago levels of $7.7 billion. Foreign dollar sales were $5.1 billion in the second quarter compared with $4.2 billion last year.

Worldwide sales of Ford cars, trucks and tractors in the second quarter totaled 1.703 million units, while unit sales in the U.S. were down 221,000 units from last year.

Ford's first-half profits amounted to $1.1 billion ($9.25), still ahead of the $1 billion ($8.48) reported in the first six months of last year. First-half sales rose to $24.49 billion from $21.86 billion.

An industrywide big-car sales slump, spurred by fears of gasoline shortages and high prices, has fallen heavily on Ford since March. The company's U.S. car sales so far this year are 16.7 percent below last year.

Like other automakers, Ford has been unable to retool quickly enough to meet the public demand for smaller vehicles. Some industry executives believe the slump hit bottom in June.

Suffering from flagging camera sales and increased costs. Polaroid Corp reported second-quarter earnings nearly half those of 1978 levels.

The self-proclaimed leader in the instant camera field showed earnings of $13.3 million on sales of $321.8 million in the second quarter compared with earnings of $26.2 million on sales of $319.7 million for the second quarter of 1978.

Earnings per share dropped to 40 cents from the 1978 level of 80 cents.

First-half earnings were $30.4 million (92 cents a share) compared with $40.7 million ($1.25) as sales dipped to $560.4 million from $586.7 million.

Polaroid President William J. McCune Jr., blamed the decline on lower-than-expected sales and increases in overhead and research and development expenses.

Increased service and rental revenue helped boost Xerox Corp.'s second-quarter earnings by 20 percent, the film announced yesterday.

The diversified copier and computer equipment manufacturer reported net income of $152.8 million ($1.90 a share) for the three months ending June 30. Last year's second-quarter net was $127.7 million ($1.59).

Xerox reported revenues of $1.71 billion for the quarter, up from $1.47 billion a year earlier.

First-half earnings were $286.6 million ($3.56), up 23 percent from $233.3 million ($2.90) for the similar period last year.

Six-month revenues reached $3.28 billion compared with $2.83 billion in 1978.

Earnings and revenues for both the quarter and half were records, the firm said.

Lockheed Corp. second-quarter earnings dropped sharply to $9.3 million from $22.4 million a year earlier, but Chairman Roy A. Anderson said results from continuing operations for the full year would compare favorably with those 1978.

Anderson said the company's longterm operating cycles, varying delivery schedules and mix of sales create fluctuations in quarterly earnings.

Second-quarter earnings dropped from $1.33 per share fully diluted to 52 cents, and sales rose from $894 million to $982 million.

First-half earnings also declined from $27.8 million ($1.62 a share fully diluted) to $2.17 million ($1.24) as sales rose from $1.6 billion to $2 billion.

TriStar program losses rose to $32.4 million in the latest period from $26.2 million in the 1978 second quarter and to $68.3 million in the 1979 first half from $55.9 million a year earlier.

The TriStar losses were caused by higher costs connected with increasing the production rate and introducing the Dash 500 model, lockheed said.

Textron Inc. earning rose by 5 percent in the second quarter despite the loss of Iranian contracts for its Bell Helicopter unit and adverse foreign currency changes.

Net income was $43.2 million ($1.15 a share fully diluted) compared with $41.2 million ($1.10) last year. Textron reached $859.6 million in sales compared with $801.5 million the year before.

For the first half, Textron earned $85 million ($2.26), also up 5 percent from last year's first-half earnings of $81.1 million ($2.16).Sales rose 12 percent to $1.7 billion from $1.53 billion.

A continued boom in passengers spurred Eastern Airlines second-quarter earnings to a record high, but the carrier said first-half earnings remained below last year's levels.

The airline carried 11.3 million passengers during the quarter, an increase of 18 percent, Chairman and President Framk Borman said at the Firm's monthly meeting in Philadelphia. It was the first time Eastern topped the 10-million-passenger mark in one quarter, he added.

Rising fuel costs, however, kept Eastern from matching the passenger gains with higher earnings, Borman said.

Although Eastern's profits for the quarter rose 8 percent to $26.1 million from $24.2 million a year earlier, its per-share earnings fell to 99 cents from $1.15. Eastern said it had about 25 million shares outstanding during the quarter just ended compared with 20 million in the 1978 period.

Eastern's net for the first six months fell to $39.3 million ($1.47) from $43.7 million ($2.06) in the first half of 1978.

Second-quarter results included net income of $850,000 from equipment sales compared with $7.3 million from similar sales a year ago, the Miami-based carrier said.

Revenues reached $726.2 million for the second period, up from$600 million. For the first half, Eastern had revenues of $1.39 billion, up from $1.18 billion.

Buoyed by its No. 1 position in the television network rating race, American Broadcasting Cos. Inc. reported a 36 percent increase in net income for the second quarter.

ABC's earnings reaffirmed its strong financial health in relation to its network competitors, CBS and NBC, a subsidiary of RCA.

ABC had net income of $57 million ($2.03 a share) in the second quarter, up from $41.8 million ($1.51) in the year-earlier period. First-half earnings rose to $82.4 million ($2.94) compared with year-earlier net income of $64.7 million ($2.35).

All per-share earnings figures reflect a three-for-two stock split last September.

ABC reported revenues from continuing operations of $501.8 million in this year's second quarter compared with $451.9 million. First-half revenues reached $953.4 million compared with $867.1 million.

Chairman Leonard H. Goldenson and President Elton H. Rule said the earnings were helped by increased profits by ABC-owned television stations and by its publishing unit.