For every action, there is an equal and opposite reaction.

That generally is taken to apply to Newtonian physics, but it can apply to social and even governmental issues as well. Case in point: Crime is increasing on New York City's subways. The NYPD is not providing that equal and opposite reaction, so that gap is once more being filled by a group that has been contending with crime in New York City since 1979: The Guardian Angels.

The Guardian Angels are resuming their patrols of the Big Apple’s subways as if it were crime-riddled Gotham in 1979, after the horrifying arson murder of a sleeping straphanger on a train last week, founder Curtis Sliwa said Sunday.

The red-beret-wearing volunteer vigilante squad is beefing up its ranks to its level 45 years ago, Sliwa said.

“We’re going to have to increase our numbers, increase the training and increase our presence as we did back in 1979,” Sliwa said at the Stillwell Avenue-Coney Island station in Brooklyn where the woman was killed.

Too many in the press toss around the word "vigilante" like it's a bad thing. A good argument can be made that the presence of groups like the Guardian Angels in New York is not only a good thing, but a necessary thing, and applies the word "vigilante" in the original sense. The term comes from the Latin Vigiles Urbani, (City Watchmen) who were the watchmen and firefighters of Rome. They kept order in the streets of the Roman Republic and into the years of Empire, much like the Guardian Angels - although the Angels generally employ fewer swords and more water bottles.

Still, they are meeting an essential need, just as they did in 1979--and starting immediately:

“We went from 13 to 1,000 [members] back then within a period of a year,” [Sliwa] said. “Because the need was there. The need is here now once again. We’re going to step up. We’re going to make sure we have a visual presence just like we had in the ’70s, 80’s and ’90s.”

Ever since last week’s shocking slaying, “hundreds of citizens” have requested the Guardian Angels return to patrol the subway cars, Sliwa claimed.

“We’re covering the actual trains from front to back, walking through the trains and making sure that everything is okay,” he told The Post on Sunday. “We’re doing this constantly now. Starting today. that’s going to be our complete focus because the subways are out of control.”

There's a lesson here for city officials all across the fruited plain.

See Related: Illegal Immigrant Accused of Burning NY Woman Faces Murder Charges - but NYC Will Ignore an ICE Detainer

Subway Savage Who Lit Woman on Fire, Watched Her Burn to Death Should Not Have Been in the United States

If city officials can't handle problems with crime, with addicts, with lunatics wandering the streets and the public transport, someone else will. If the police can't maintain a presence on the subways, someone else will. If the judicial system can't address rampant crime on the streets, someone else will. In New York, that someone else is Curtis Sliwa and the Guardian Angels.

For every action, there is an equal and opposite reaction.

New Yorkers should be (and I'm willing to bet are) happy to see the return of the red-bereted Guardian Angels. The streets and subways will be a little safer with the latter-day Vigiles Urbani standing guard. The city is better off for their presence. It's a shame that it's necessary - but it would be a greater shame if they weren't there.