Francis O. Leach of Falls Church wonders what good it does for a newspaper like The Washington Post to expose crime and corruption.

"A congressman pads his payroll and uses the money to pay his personal expenses," he writes. "A senator misappropriates thousands of dollars. Exxon makes over 115 percent profit. GSA buries thousands of dollars worth of useful equipment in a dump, and there is much more that is unreported.

"Would it not be better if we did not know this? When we are told, it is like putting salt on an open cut. There is too much dishonesty today, but the big people never go to jail. When a man robs a liquor store, he gets 10 years. When an elected official misappropriates thousands, he gets a slap on the wrist."

A somewhat similar theme appeared at the very end of yesterday's story about two vicious stabbings that took place recently in Woodbridge. Two children are being held in those cases -- a 12-year-old and a 14-year-old.

Residents of the area are deeply troubled and frightened. One told staff writer Ted Gup that she and her neighbors now think twice before opening the door for anybody. She said people are anxious to see how the courts will deal with the 12-year-old, who stabbed a woman 15 times, kicked and beat her, and left her for dead.

"We don't want him to just get a pat on the head," she said. "He's dangerous. He's 12 now. My God, what's he going to do when he's 18?"

Good question. If he is found innocent, the presumption is that he will not be a danger to society when he's 18. If he is found guilty, the worst that can happen is that he will be sentenced to the Beaumont School for Boys until he's 18, and then will be released -- anonymously -- into the community because our juvenile law forbid release of the names of little darlings who are found guilty of attempted murder.

Prince William Commonwealth's attorney Paul Ebert suggested to our reporter that media coverage of one crime can lead others to break the law.

"Publicity," he said, "breeds camaraderie among these students who try to outdo each other. It becomes a popular thing to do."

There may be some truth in Ebert's comment. Some people who see violent acts in movies or on TV, or read about them, do react by engaging in similar conduct.

But the reaction in most cases is, "Gee, I've been toying with the idea of doing something like that, but I didn't realize the police are so good at catching lawbreakers. Maybe it's not such a smart idea."

Congressmen and other public officials who have broken the law, or been tempted to, must surely quake when their colleagues are caught and subjected to public humiliation, if not always to prison sentences. The fear of being caught keeps more people honest than does respect for moral codes.

Is it better for society not to know which of its officials are stealing? Is it better not to know that police are baffled by a series of Beltway murders, or that the Son of Sam is gunning down people in parked cars? Is it better not to know that women are being stabbed in a quiet Virginia community? The answer is obviously "No." POSTSCRIPT

Exxon did not make a profit of 115 percent in its third quarter, as many people think. It made a profit of less than 6 percent on each dollar's worth of its sales. The company made $1.145 billion on $20.6 billion in sales.

For the first nine months of this year, Exxon's sales were $59 billion and its profit was $2.93 billion -- or less than 5 percent.

This year's third quarter profits were big, but not as big as they appeared to be. The company's operating profit was at least $251 million less than its $1,145 million gross profit.

Included in the gross profit was $200 million in one-time savings on British taxes, and a $51 millin swing in currency translation losses (which in 1978's third quarter cost the company $178 million but this year were "only" $127 million). We must also keep in mind that this year's third quarter profits are being compared to last year's third quarter profits, which were abnormally low for Exxon.

In short, this year's third quarter profits were more than double last year's (up 120 percent), but the firm did not make a profit of 120 percent.

I do not address myself to the question of what percentage of profit is proper. I just think we ought to be more careful in reading the numbers and assimilating what they mean.

When Exxon made a profit of $955 million in the first quarter of 1979, little was said about the matter. But when the firm made a third quarter gross profit of $1,145 million (of which $894 million was actual operating profit), Exxon profits became a cause celebre. I am not an Exxon stockholder, but I find the furor ironic.