The Washington Post Co. reported a 25 percent gain yesterday in second-quarter earnings and a 44 percent jump in profits for the first half of the year and said it "continues to anticipate that 1982 will be an excellent year."

The Post Co. earned $16.6 million ($1.17 a share) on revenues of $207.2 million for the quarter, up from a profit of $13.2 million (94 cents) on revenues of $194.3 million in the same period a year ago.

Profits for the 26 weeks ended July 4 climbed to $21.7 million ($1.53) from $15.1 million ($1.07), while revenues grew 8.4 percent to $390.4 million from $360.1 million.

"Strong performances by the newspaper and broadcasting divisions contributed to the revenue and earnings gains," the company reported.

The earnings report said revenues were up 17.2 percent for the quarter and 18.8 percent for the year so far at the newspaper division, "reflecting gains in advertising and significantly larger circulation" at The Washington Post. The division also publishes the Everett (Wash.) Herald.

The Post Co. sold the Trenton (N.J.) Times and Inside Sports magazine late last year, and that improved earnings because both publications "had been operating at a loss," the report noted.

Revenues of the company's radio and television stations are running about 13 percent ahead of last year, but Newsweek magazine is "behind the second quarter and first half of last year as a result of a decrease in advertising pages," the company said.

Also adding to first-half earnings was a gain of $1.1 million (8 cents) from the sale of a minority interest in a photocomposition company.

United Services Life Insurance Co. reported that second-quarter operating profits increased slightly to $3.90 million (69 cents a share) from $3.85 million (68 cents) in 1981. Operating profits for the half dipped slightly to $6.9 million ($1.22) from $7 million ($1.23) in 1981.

United Services's after-tax earnings for the second quarter were down to $3.9 million (69 cents) from $4.7 million (83 cents) last year. For the half, after-tax earnings were $7.2 million ($1.27), down from $7.9 million ($1.39) a year earlier.

Total income was $31.5 million for the second quarter compared with 39.2 million a year ago. For the half, United Life's total income was $62 million compared with $68.6 million last year.

NS&T Bankshares Inc., parent company of NS&T Bank N.A., increased its second-quarter operating profits to $2.9 million ($1.88 a share) from $1.8 million ($1.00) a year earlier. Net income rose to $1.6 million ($1.14) from $1.3 million (98 cents) in 1981.

For the half, NS&T posted pretax earnings of $3.9 million ($2.95), up from $3.3 million ($1.85) last year. Net income was $3 million ($2.21), up from $2.5 million ($1.83).

The half-year boost reflects nonrecurring profits from NS&T's sale of the Securities Building at 729 15th St. NW.

NS&T increased its loan volume to $406.2 million in the half from $330.9 million last year. The 22.8 percent increase in loans--mostly to Washington-area businesses--"contributed significantly to our profit margin," said Joseph H. Riley, chairman of the holding company and the bank.

C3 Inc., a Reston-based custom integrator and manufacturer of microcomputer products, reported a 17 percent increase in earnings yesterday, from $1.8 million (21 cents a share) last year to $2.1 million (25 cents) in its first quarter ended June 30. Revenues climbed to $12.3 million from $10.7 million.