SUMMER BOX OFFICE: A TEST OF STAYING POWER

It takes more than a blockbuster opening to make a hit. That's the toughest lesson to be learned from the summer's movies so far. As the week-by-week chart below shows, many of the biggest openers quickly dive-bombed off the charts, while slow and steady entries like Disney's little-heralded Sister Act proved money-makers. After a dismal spring, Hollywood looked to the big summer releases to get back on track. And things got off to a bang-up start with the $33 million opening of Lethal Weapon 3 on May 15. Then Memorial Day weekend saw Alien3 debut with a four-day take of $23 million; two weekends later, Patriot Games shot up the screen with an $18.5 million debut. Spirits hit a high on June 19 when Batman Returns opened, amassing a record first-weekend gross of $45.7 million. And then, faster than you could say ''revival,'' it all went south. Plagued by bad word of mouth, Batman's returns plummeted by more than 40 percent from week to week. ''When the first Batman came out in 1989, we weren't in a recession. This one's just not getting the repeat business,'' noted one rival studio head. And that is the trouble with much of the class of the summer of '92: no ''legs.'' Alien3 did more than 40 percent of its business in its first four days, ultimately scurrying from theaters with a disappointing $54.9 million. In contrast, though Sister Act and A League of Their Own opened quietly, good word of mouth has kept the money coming for weeks. With a gross of $116.6 million and counting and a budget of just $20 million, the humble Sister Act may well become the summer's most profitable film. A strong $15 million opening by Clint Eastwood's Unforgiven Aug. 7-9 gave the box office a late boost. Still, total summer receipts are just 1.5 percent ahead of '91-not a disaster, but not enough to make up for a slow spring either.