Union Lt. Gen. Ulysses S. Grant appears in a portrait, circa 1864, taken by famed portrait photographer Mathew Brady. Credit: Handout | MCT

Amid President Donald Trump’s mounting legal problems, TV talking heads and bar stool philosophers from Boise to Britain have been pondering one of the great mysteries of the U.S. Constitution.

Can the president of the United States actually be indicted? Arrested, handcuffed, the whole deal. Possible?

The prevailing answer is this: Nobody is sure.

But that’s not entirely true.

President Ulysses S. Grant knows — err, knew.

In 1872, while president, Grant was arrested on a street corner in Washington. This was not a high crime, but it was — at least theoretically speaking — a misdemeanor.

The man who led the North to victory in the Civil War was busted for speeding on his horse.

The story of his arrest — confirmed a few years ago by then-District Police Chief Cathy Lanier — is told in a remarkable but obviously forgotten story in the Sept. 27, 1908, edition of the Washington Evening Star under the headline: “Only Policeman Who Ever Arrested a President.”

That policeman was William H. West, a black man who had fought in the Civil War.

“Since his retirement,” the story said, “he has decided to let the public know the true story of the arrest.”

It begins with Grant’s love of fast horses.

“Gen. Grant was an ardent admirer of a good horse and loved nothing better than to sit behind a pair of spirited animals,” the Star story said. “He was a good driver, and sometimes ‘let them out’ to try their mettle.”

And that’s where Grant, as president, rode into the law.

The police had been receiving complaints of speeding horse carriages. After a mother and child had been run over and badly injured, officer West was dispatched to investigate. As West spoke to witnesses, another group of speeding horse carriages headed toward him — including one driven by the president of the United States.

“Policeman West held up his hand for them to stop,” the story said. “Grant was driving a pair of fast steppers and he had some difficulty in halting them, but this he managed to do.”

Grant was a bit testy.

“Well, officer,” he said, “what do you want with me?”

West replied: “I want to inform you, Mr. President, that you are violating the law by speeding along this street. Your fast driving, sir, has set the example for a lot of other gentlemen.”

The president apologized, promised it wouldn’t happen again, and galloped away.

But Grant could not curb his need for speed.

The next evening, West was patrolling when he came barreling through again, this time speeding so fast that it took him an entire block to stop.

Now Grant was cocky with a “smile on his face,” the Star article said, that made him look like” a schoolboy who had been caught in a guilty act by a teacher.

He said, “Do you think, officer, that I was violating the speed laws?”

“I do, Mr. President,” West said.

Grant had an excuse for his speeding, not unlike one being given somewhere right now: He had no idea he had been going so fast.

West was sympathetic but firm.

“I am very sorry, Mr. President, to have to do it,” he said, “for you are the chief of the nation, and I am nothing but a policeman, but duty is duty, sir, and I will have to place you under arrest.”

It’s worth pointing out here that standards of journalism, particularly with quotations, were not as rigorous back then as today, so it’s nearly impossible to know if this is the whole truth and nothing but the truth so help us hot type.

However, Lanier did confirm the arrest, and there are other historical references to it.

Anyway, Grant and several of his speeding buddies also arrested went with West to the police station. The president of the United States was ordered to put up 20 bucks as collateral. A trial was held the next day.

“Thirty-two ladies of the most refined character and surroundings voluntarily came into the court and testified against the drivers,” the Star story said. “The cases were contested bitterly.”

The judge imposed “heavy fines” and a “scathing rebuke” to the speeding drivers, which didn’t include the president.

He didn’t show up for court.