Dow Chemical Co. had lower earnings for the third quarter and first nine months despite substantial sales gains.

Net income for the quarter was $145 million (78 cents a share) on sale of $1.6 billion compared with $167 million (90 cents) a year ago on sales of $1.4 billion.

Nine-month net income was $452 million (2.44) on sales of $4.7 billion against $464.2 billion ($2.50) a year ago on sales of $4.197 billion.

Occidental Petrolium Corp. became the first of the big oil companies to announce third-quarter results yesterday when it announced third reported earnings of $59 million, 57 per cent more than the $37.6 million earned in the comparable 1976 period.

Primary per-share earnings for the quarter were 75 cents compared with 55 cents in the third quarter of 1976, when there were fewer shares outstanding. Fully diluted earnings per share were 69 cents versus 47 cents.

Net sales in the third quarter were $1.5 billion compared with $1.4 billion in the third quarter last year.

For the full nine months, net sales were $4.5 billion compared with $3.9 billion in the first nine months of 1976. Earnings for the nine months were $142.3 million, a 27 per cent increase over the previous year.

Primary per-share earnings for the nine-month period were $1.91 ($1.72 fully diluted) compared with $1.63 ($1.40 fully diluted) in the first nine months of 1976.

he company said the gains resulted from rising gas production in the Piper Field in the North Sea, from improved operations of the Hooker Chemical Division and a general global sales increase. But these were offset partly by a weak market for Libyan crude oil.

Xerox Corp., a leading manufacturer of copying machines and other equipment, reported increased third-quarter earnings of $104.5 million ($1.32 a share) compared with $95 million ($1.19) a year earlier.

Gross revenues rose to $1.25 billion from $1.13 billion in the third quarter of 1976.

For the nine months ending Sept. 30, the firm reported earnings of $309.3 million ($3.89) compared with $279.7 million ($3.51) for the same period last year. Revenues rose to $3.7 billion from $3.26 billion.

Chairman C. Peter McColough said growth was slower in the third quarter than in the first half as Xerox customer took advantage of price restructuring and selected less expensive leasing plans.

Xerox also indicated that its fourth-quarter earnings may be less than $1.15 a share, or somewhat lower than expectations of some analysts.

General Telephone & Electronics Corp. reported record results for the third quarter and first nine months of 1977.

In the third quarter, the company earned $24.01 million (99 cents a share), up 21 per cent from the $13.58 million (82 cents) recorded in the year-ago period. Revenues rose to $1.92 billion from $1.69 billion.

For the first nine months, net income came to $61.00 million ($2.88), a gain of 29 per cent from $45.43 million ($2.23) in the corresponding 1976 period. Revenues totaled $5.57 billion versus $4.90 billion.

American Airlines said it had record profits in the third quarter and first nine months.

Third-period profits rose to $55.4 million ($1.83) from $39.6 million ($1.38) a year earlier. Revenues climbed to $628.2 million from $561.1 million.

Nine-month net income increased to $91.1 million ($2.96) from a year-earlier's $55.5 million ($1.94). Revenues advanced to $1.7 billion from $1.5 billion.

The company said profit figures for boeth periods were aided by a provision in the Tax Reform Act of 1976 that allows airlines to offset 1977 federal income tax liability with investment tax credits.

Polaroid Corp., the instant-camera and No. 1 film producer concern, reported earnings for the third quarter ended Sept. 25 rose to 61 cents a share from 57 cents in the year-ago period. The company earned $20.04 million on sales of 254.22 million, up from $18.73. million on sales of $236.88 million a year earlier.

For the first nine months, net income came to $54.54 million ($1.66 a share), up from $47.79 million ($1.45). Sales rose to $695.33 million from $647.01 million.

TRW, Inc., reported higher sales and profits for its third quarter and first nine months, and predicted a strong fourth quarter.

The manufacturing conglomerate said third-quarter net income was $36.4 million ($1.12 a share) on sales of $797.3 million compared with 1976 third-quarter net income of $32.4 million (98 cents) on sales of $708.2 million.

Nine-month net income rose from $96.7 million ($2.93) in 1976 to $110.6 million ($3.41) as sales increased from $2.17 billion to $2.4 billion.

Northrop Corp. said that sales, net income and earnings per share for the third quarter were the highest in the company's history.

Northrop, a diversified aerospace concern, had sales of $408.3 million in the third quarter, a 27 per cent increase over the $321.1 million recorded in the same period last year.

Net income was $16.9 million compared with $2.9 million in the third quarter of 1976. Last year's third-quarter results were depressed, however, by an $11.4 million after-tax charge arising from investment losses in an affiliate, and would have been $14.3 million without the extraoridinary charge.

Primary per-share earnings for the quarter were $1.22 compared with 23 cents in the same period last year after accounting for the unusual charge. Fully diluted earnings were $1.21, versus 22 cents a year ago.

For the first nine months, Northrop's sales were $1.149 billion, up 26 per cent from the $913.3 million reported for the first nine months of last year. Net income was $48.3 million compared with $21. million for the comparable period last year.