DEAR PROFESSOR: I clipped the enclosed tale about an exploding outhouse from a newspaper more than 50 years ago and have had many a chuckle rereading it.

I guess this was the inspiration for the modern jokes about toilets blowing up. - RUTH BRYERS, LIVERPOOL, N.Y.DEAR RUTH: There's no doubt that this is an old rural gag that evolved into an urban legend. Thanks for sending me the clipping to add to my collection.

The column that Ruth sent is undated, but a reference to President Franklin D. Roosevelt proves it must have been published sometime between 1933 and 1945 - the years FDR held office.

Under the headline "The Tale of a `Johnny' That Blew Up," the article tells about a wife who poured some naphtha left over from spring housecleaning down a hole in the family outhouse.

Her husband, while using the privy, lit his pipe and tossed the match down the same hole.

The resulting explosion blew the man 50 feet away and into a manure pile. Neighbors rescued him and asked how the accident happened.

"I dunno," he replied, "it musta been something I et."

This gem of rustic humor still has the power to amuse, but now it appears in the form of a modern legend about a wife who squirts insecticide or hairspray into a toilet. When her husband uses the bathroom and tosses a smoldering cigarette into the "Johnny," it blows up beneath him.

Current versions of the story always conclude with paramedics being summoned. When they hear how the accident happened, they laugh so hard they drop the man from the stretcher.

In the 1930s, many Americans still used outhouses, or at least remembered using them. The idea of disposing of a waste product by pouring it down through a hole in the privy must have seemed logical.

Whether flammable liquid would remain volatile enough to cause a large explosion under an outhouse seems doubtful, but that questionable detail didn't prevent the tale from spreading.

There's even a versified version of the story composed by the popular Canadian poet Robert Service. It's titled "The Three Bares," referring to a family's three-hole outhouse. The poem begins:Ma tried to wash her garden slacks but couldn't get 'em clean

And so she thought she'd soak 'em in a bucket o' benzine.

It worked all right. She wrung 'em out then wondered what she'd do

With all that bucket load of high explosive residue.Ma's solution to the problem is to dump "the liquid menace" into the center hole of the family's three-hole outhouse.

Enter Grandpa, who takes his customary seat at the end of the row, lights his pipe and drops the glowing match "clean through the middle seat."

There's a huge explosion, which makes a "dreffel roar," as his grandchild, Rosyleen, describes it. When the smoke clears, they find Grandpa squatting in the duck pond, his whiskers singed and the toilet seat hanging around his neck.

The last stanza of Service's poem is:He cried: `Say, folks, oh, did ye hear the big blow-out I made?

It scared me stiff - I hope you-uns was not too much afraid?

But now I best be crawlin' out o' this dog-gasted wet . . .

For what I aim to figger out is - WHAT THE HECK I ET?Robert Service published "The Three Bares" in 1949, but by that time I suspect that the exploding outhouse story was losing popularity, since indoor plumbing had become commonplace.

The poem was reprinted in Service's 1963 book, "More Collected Verse," and by then I'll bet that "The Exploding Toilet" legend had nearly replaced its rural prototype.

Nowadays, one seldom hears of anyone pouring leftover cleaning fluids into a toilet. Instead, the current legend usually mentions a spray can with a clogged nozzle that supposedly set up the accident.

And - since modern legends are told as true - the punchline "musta been somethin' I et" was left behind, so to speak, when the joke became an urban legend.