Scientific American goes cheesecake! And violent! There it is, right on the latest covers. In January, a shapely pink leg. This month, a frightful death.

Okay, relax, seriousness fan; just pulling your tibia. The January cover illustrates an article on surgical replacements of the human knee joint. And on the February cover, it's a hover fly (Syrphus ribesii ) about to be consumed by a carnivorous plant, illustrating an article on that subject.

A pink leg is about as sexy as the magazine gets. Farrah Fawcett-Majors must remain without the remotest hope of attaining that bright, pornless Scientific American cover, doomed to eternal humiliation by such rivals as the Lagoon Nebula of the constellation Sagittarious, a field of lettuce growing under drip irrigation and the compound eye of the female horsefly (Tabanus Lineola ).

For these are the visions that make the Scientific American reader's synapses crackle with intellectual excitement. And the reader population is growing. Circulation has steadily crept upwards to 650,000 copies a month, higher than Fortune or Harper's. There are four foreign-language editions, a fifth on the way, and the Soviet Union makes English reprints. Advertising space has more than doubled in the last four years; the magazine is fat with exhortations to buy expensive internal combustion vehicles or to introduce alcohol into the circulatory system.

And for some reason (there is no serious promotion effort) newsstand sales are rising. Scientific American is moving more street copies at $1.50 a pop (over 130,000 each month) than Esquire.

"It really puzzles me," says the editor, Dennis Flanagan. "There must be more interest in this material than you would think." He laughs then, as he says: "Though it amazes and delights me, the only policy change it would lead to is putting out more copies."

At Scientific American, you see, there is no identity crisis. Flanagan and Gerard Piel, the publisher, knew exactly what they wanted when they revived a tired and ignored science journal 30 years ago: to cover the most significant work in science - as reported by the scientists doing the work. They've never deviated from that vision, even by a milliquark.

The revolutionary new theories on continental drift . . . human evolution . . . DNA . . . the nature of matter . . . the living cell . . . the micro-electronic explosion - all these and more have been faithfully explained through the years, in enormous, unblinking, uncompromising detail. With all the charts, maps and graphs necessary.

No apologies are heard from the editors. The way they see it, they're providing top-grade goods, elsewhere unavailable and inherently difficult. If you want them, you can damn well pay the price. The price is work. But first they work. "The struggle our readers find with some of our articles comes after a long struggle we have with an author," says Piel.

The scientist-writer is apt to neglect such standard magazine components as the lead - the part at the beginning that tells you what the article is about. The magazine has nine editors who work with the authors, waging the struggle on behalf of readability. "They're mostly just plain old writers," Flanagan says of the nine. Several have come from non-science magazines. Five have bachelor degrees in science subjects, but, says Flanagan, "not a one of 'em is an expoit ."

Besides, it isn't as though all the articles re difficult, Piel insists. "Look at this issue," he says. "This piece on the weaver ant is an enchanting little story. That's easy as pie." Ahead of His Time

What kind of man reads Scientific American? A man like Rufus Porter. Inventor, landscape painter, sign painter, peddler, technological prophet, drum teacher, farmer, shoemaker's apprentice, wanderer. Porter made outstanding contributions to art, science and journalism.

So how come nobody's ever heard of him?

Maybe he was too far ahead of his time. In his home century, the 19th, Porter did abstract paintings and said that someday people would travel from New York to California in only three days. By air. He was regarded as a crackpot.

He founded Scientific American in 1845 on an investment of $100. It was a four-page weekly paper bringing news of inventions, scientific essays, and "useful information and instruction in various Arts and Trades: Curious Philosophical Experiments; Miscellaneous Intelligence, Music and Poetry."

Porter was a restless man. He left Scientific American after two years, selling it for $800 to a patent lawyer, Orson D. Munn, Porter died at 93 in 1884. Hardly anyone noticed.

His journal outlived him. It became, in fact, the oldest continuing general circulation magazine in the country. It covered some big stories: the Industrial revolution for one. But different editors brought to it differing quality. By the 1940s, Scientific American had sadly declined, limping by with 30,000 subscribers, fleshed out with publicity handouts from industry.In short, dying.

Recovery was at hand. Just about 100 years after its inception, up pops a spiritual heir to Rufus Porter. What This Country Needs . . .

What kind of man reads Scientific American? A man like Gerard Piel: journalist, traveler, cigar-smoker, overseer of Harvard University, publisher, flunker of prep school physics, author of two books on science and society, liberal, trustee of the American Museum of Natural History, businessman, suspender-wearer.

Piel and Flanagan have certain similarities. In their office habitats, both are likely to be encountered jacketless but in tie and vest and smoking cigars. They are four years apart, Piel the senior at 62. Neither has much hair. Flanagan seems more relaxed, Piel intense. Both were liberal arts majors in college. "One of the basic principles of this peculiar publication," says Flanagan, "is that people like Piel and me do exist, English majors who are very interested in knowing what's happening in science."

They both found themselves at Life magazine in the '40s. Piel had become science editor - much to his surprise. He was a Harvard history major with "a certificate of illiteracy in science," and he had wanted to be a foreign correspondent. But he dove in, boned up on college science textbooks and . . . Eureka! "I found myself in the thick of the most breathtaking, exciting, headspinning developments." Piel discovered relativity, and the quantum theory and DNA and electronics. He covered the isolation of the first virus. He got hooked.

But he had a problem: Henry Luce. In '44, Luce was running what Piel considered a vicious campaign against FDR, whom Piel voted for four times. "Luce used the magazine like he was Hearst," says Piel.

He and Flanagan had been talking about starting a publication of their own Flanagan says it was an obvious idea to both of them, but that he happened to first articulate it - in the men's room at Life one day. "What the country needs is a good science magazine," he says to Piel. Piel had already noticed that the scientists and engineers of the country eagerly ate up the science stories in Life and wanted more.

The idea began to take on urgency when Piel, working his beat, learned a secret.

"I decided I had to get off Mr. Luce's payroll when I came into possession of the knowledge that we were gonna have an atom bomb. I couldn't have my conscience hostage to Henry Luce when this kind of power was available to national states. Suddenly my job became terribly important. And the need to get a good magazine started became clear."

Piel quit. He went to see the industrialist Henry Kaiser. He'd written a hero-mongering Life profile of Kaiser, who had said: Come work for me. Piel did, and learned one more subject - money. "The year I spent with Henry Kaiser was better than three years at the Harvard Business School."

Thus armed, he went out and raised $450,000. With it, he, Flanagan (who'd replaced him as Life's science editor) and Donald Miller (another partner who's still with them bought Scientific American from the Munn company.

Piel sits in his office pointing to a photo on the wall. "That shows three frightened young men who, as they saw their first issue come off the press, were realizing they had to raise another half a million dollars to make it go." He laughs heartily, knowing that he did make it go.

"The first thing we had to do was persuade the scientific community that we were in dead earnest," Piel says. Scientists were needed not just to read but write. They had to be shown the trivial , insipid Scientific American had mulated into a new species. It didn't take along.

The symbol of that acceptance is on Piel's wall - a letter that arrived after two years of publishing. "I admire your magazine very much," wrote Albert Einstein. "It fills a real need very satisfactorily." Piel had requested an article and Einstein enclosed it. But, he wrote, he wouldn't be surprised if it were rejected. "The article is somewhat long and not quite easy to grasp."

Silly Albert. Just what they were looking for. Images of the H-Bomb

What kind of government burns Scientific American? A government like the U.S. government of the '50s, democratic, yet very nervous. And you thought it was an uncontroversial, head-in-the-clouds magazine. Not in April 1950 it wasn't. Not with physicist Hans Bethe inside stewing about a theoretical possibility known as the H-Bomb.

The presses were rolling when the censors from the Atomic Energy Commission ordered the deletion of three passages in Bethe's article. The editors squawked but to no avail. "It was delicately made known to us," Flanagan recalls, not smiling, "that this was the only federal law that carried the death penalty." The presses stopped.The article was edited. About 3,000 printed copies were actually burned under the AEC's supervision.

Piel never bought the stated reason - protecting military secrets. He believes that the AEC's real motivation was to prevent insiders like Bethe from making public the debate within the government over the ethical and political questions that H-Bomb-building raised.

The H-Bomb story was no fluke. In its quiet, incredibly complicated fasheon. Scientific American reports some of the most important issues of the times, the ones where science and technology spill over the breaker into policies and human behavior. "No magazine has covered the race as we have covered it," says Piel. "The heart-breaking efforts at disarmament - you go back through the 30 years we have been publishing, that's the one place where there is a record of that story." Similarly, there has been coverage on overpopulation, the job problem, drugs.

All this seems to reflect Piel's philosophy that science and the humanities should get to know each other better. Also, that we'd all be better off running things as scientists do. It's plan that he's in love with them. The scientific community, governed by "openness, reason and tolerance" is the model that world societies should emulate, Piel says. "The people who are most concerned about the fate of mankind are the scientists I know," he says "A self-governing democracy of intellectual peers," he calls them.

And there he is keeping this elite community in touch, making sure the biophysicists know what the astronomers are up to and the geologists are down to. In these ambivalent times, it's nice to find a man ecstatic about his work. "We're talking to the cerebral cortex of a fabulous audience." Piel is went went to proclaim when you ask him who read the magazine.

Piel's unbounded pride and pleasure in his magazine and the scientific world it reflects are pulsating out of every word he utters as he opens the December issue to Edwin H. Land's cover opus. "The Retinex Theory of Color Vision." Here is "a glorious story," Piel gushes, written by "an authentic genius" who would probably win a Nobel for the work.

Land, who of course invested the Polaroid camera, has long been fascinated by the question of how the eye distinguishes colors. In the article (which a Scientific American editor spent three months helping Land prepare) he describes experiments he did to trace the actions of eye and brain (retinex is a word he made up combining retina and cerebral cortex) to capture color.

Piel's retinex system caresses the story. His vocabulary croons to it. He is head-over-heels mad for the thing. Scientific objectivity is reeling. "This article by Land will be read by everybody in the world who is interested in this subject," he declares. "It will take them to the absolute frontier of understanding."