GO AHEAD, TAKE AN EXTRA HELPING. Today is the birthday of William Howard Taft, our last 300-pounds-plus president, and tomorrow morning you can do what Taft used to do:

"Lie flat on the back. Raise chin to chest and then back to floor pushing the arms along

the side of the body as you raise the chin. 8 to 15 times."

That was the first exercise in a litany of 15 presidential exercises that if marketed today would probably be known as The President Taft Workout Book, a program promulgated by Dr. Charles E. Barker, Taft's "physical instructor." Actually, the program has already been marketed. The discreet doctor ran it off on a mimeograph machine when Taft left the White House, but no one paid much attention to it then. Today it's forgotten, nestled in the Taft papers at the Library of Congress.

Nestled nearby is the documentary evidence that fleshes out why our 27th president needed an exercise program -- some selected measurements for Taft's pajamas:

"Neck 19 inches

Sleeve 341/2 inches from center of shoulder to wrist bone

Chest 53 inches

Waist 54 inches

Hips 58 inches

Inseam 311/2 inches

Taft had no need to tent up in his PJs, though, in order catch a little sleep. That came easy, as Taft's White House military aide Archie Butt explained in a letter to his sister-in-law:

"I went to the Green Room at 3 and found the little artist there, really lovely and quite naive in her enthusiasm. The President did not come over until 5 and by that time I was well acquainted and had been induced to put on a coat of the President's and pose for his clothes. But, unfortunately, the coat hung like a bag from my shoulders and helped on the portrait very little. The President tried to pose for a little while, but he was very tired and very weak. He fell asleep twice while standing up, and sat in a chair for a minute and was sound asleep."

A week later Taft fell asleep in public during a funeral for a powerful congressman's wife. The call went out for Barker, a former Baptist minister who turned to "physical culture" after an exercise regimen restored him from very bad to robust health. He had come to Washington to help a "tired" railroad executive, and met Taft, then secretary of war.

Taft, it seems, was sleeping through too much of his tenure at the War Department. And there were other embarrassments: a busted sedan chair in Hong Kong and a bathtub on the ship to Cuba that wouldn't let him go. (After the tub- gripping incident, the can-do Navy procured Taft a 41-inch-wide model that followed him all the way to the Executive Mansion.)

Under Barker's supervision the secretary trimmed down to 260 pounds.

COULD BARKER work his magic on President Taft? He had Taft up at 7 a.m. for one hour of: "Stand with the weight on the right fot. Left toe back, hands at the side. Raise arms in front and inhale, lower arms and exhale. 10 to 20 times."

There was some rougher stuff too, sit-ups and what Barker called "gymnastics," but there was also a 10- minute rest period.

The exercise helped, Taft reported to his wife, who had left with the family for vacation: "I am in much better condition. I am harder and able to go through the exercises and have lost that tendency to sleepiness which makes me think of the fat boy in Pickwick. My color is much better and my ability to work is greater."

Then the Taft diet began. The presidential menu is not in the Taft papers, but Barker apparently had no trouble finding food to drop from daily consumption. Butt filed this report:

"I could not help laughing at him one evening at dinner. He had finished dinner and we were sitting around the table, smoking. There was a large bonbon dish of candied fruit before him, and every nowand then he would take a piece, apparently unconscious that he was doing it, and before we arose from the table he had eaten every piece there was in the dish. He never smokes and in consequence has to do something, I suppose, to break the monotony of sitting still."

Four days into the diet, in August, Taft wrote to his wife:

"By curtailing some of my food I have reduced myself from 314 to an ounce or two under 310 this morning. I suppose my loss will not continue at this ratio, but I shall expect to get rid of half a pound a day on the average."

He dreamed of slimming down to 275 pounds by Sept. 15, his 52nd birthday and the day he began a tour of the western states. But after his initial optimism, his letters stop mentioning pounds. And once on the road, well, in Texas he saw the largest turkey he had ever seen -- a succulent 40 pounds.

THE PEOPLE liked roly-poly President Taft, even if Congress didn't. Here was a man who wouldn't lead them on a charge up San Juan Hill. He made people relax and smile, not enough to win reelection, of course, but the people paid him back in other ways.

A New York bakers' association baked him a 50-pound mince pie for Thanksgiving. But fate intervened on the side of dieting. Between New York and the Taft belly, somebody stole the pie. So they baked him a 92- pound pie and armed guards got it to the Taft Christmas table.

Once out of office, Taft finally trimmed down to nearer the average size. In 1929, a year before he died at age 72, Chief Justice Taft tilted the scales at 244 pounds.

So empty the bonbon bowl and kill the cake. And in the morning get flat on your back and bob your chin while saying, "Happy birthday, Will."