In the new study, " 'Rally Events' and Presidential Approval," Larry Hugick, vice president of the Gallup Organization, and Alec M. Gallup, co-chairman, analyzed hundreds of Gallup polls from more than five decades -- beginning in 1938 during Roosevelt's second term -- to measure the impact of international crises, diplomatic events and military operations on Presidential popularity.

Past studies, like the book "War, Presidents and Public Opinion" (John Wiley & Sons; 1973), by John Mueller of the University of Rochester, have identified the tendency of the public to rally round their flag, troops and President during many foreign crises. Though the magnitude of such surges have been documented, the new study is the first to look closely at their duration, and it finds they do not last long.

The findings, reported at the annual meeting of the American Association for Public Opinion Research that ended on Sunday in Phoenix, showed that each of the last 10 Presidents experienced at least one such rally of opinion over a foreign event. The 42 rallies studied have occurred on average every 1.2 years.

Increases in support for previous Presidents from these events averaged eight percentage points. The smallest examined by the researchers was three percentage points, while the largest was 19 percentage points. 10 Weeks of Higher Favor

Mr. Bush's 18-percentage-point surge, registered in mid-January as the air war on Iraq began, was one of the three largest rises in Presidential popularity ever recorded by Gallup. Jimmy Carter saw a 19-percentage-point increase when American hostages were seized in Iran in November 1979, and Richard M. Nixon saw a 16-percentage point lift at the time of the Vietnam peace agreements in January 1973.