A rare interruption of oral arguments by a spectator at the U.S. Supreme Court on Wednesday led one professor to look up the arcane law used to enforce order in the courthouse. Just a sentence long, the law forbids setting off fireworks or shooting guns. It also warns people not to “make a harangue or oration.”

And so, what caught the attention of the law professor, Orin Kerr, on leave from George Washington University, was what was not written: any exception for the lawyers whose business it is to orate before the court.

Mr. Kerr posted the relevant sections of the law — 40 U.S.C. section 6134 — in a leading legal Web Log, the Volokh Conspiracy. He included a caveat to attorneys: “It’s illegal to make an oration in the Supreme Court building! So be careful out there, folks.”

The purpose of the law is clear, Mr. Kerr said, but it is “an example of atrocious law writing.” Still, members of the court’s bar should worry more about convincing the justices than about getting arrested mid-oration, Mr. Kerr said in a telephone interview yesterday.

Mr. Kerr’s post on the Web log came late Wednesday after a rare disturbance in the courthouse that morning while the court heard arguments on the federal ban on “partial-birth” abortions. An opponent of legalized abortion, Rives Miller Grogan, was led out of the courtroom by authorities after he shouted: “Abortion is the shedding of innocent blood!” according to news reports.

Mr. Grogan was arrested for resisting arrest and for violating the obscure law that was to spark Mr. Kerr’s interest.

The statute was enacted in 1949, and violations are punishable with up to 60 days imprisonment.

Mr. Kerr suggested that the arrestee could try to defend himself against the charges by saying the law is so vaguely written as to be unconstitutional.

Mr. Kerr’s findings sparked several amused postings on the Volokh Conspiracy Web log. One reader found in the law a new interpretation of Justice Thomas’s reputation for saying little during arguments.

“See?” the reader wrote. “Justice Thomas has just been scrupulous in his strict construction of that law, uncommonly silly though it may be.”