Bill Henry Jr. of Glen Echo is $20 poorer this morning and thinks you might find it useful to know why.

A week ago, Bill and his wife decided to take advantage of some beautiful fall weather by hiking along the C&O Canal towpath.

They drove to Lock 7, where they found the parking area full. Several autos were parked on an adjoining grass plot, so the Henrys left their car on the grass, too.

When they got back, there was a ticket on their car. The United States Park Police told Bill his ticket would cost $20, which he thinks is a rather stiff parking fee for a supposedly free public facility.

If you, too, occasionally venture into park lands, it might be a good idea to remember the old enjoinder, "Keep off the grass." These days, that advice is usually given to young people who risk arrest by smoking pot, but it also retains its earlier meaning. POSTCRIPT

Mention of smoking pot reminds me of the good work that Drugfair has been doing with its recent public service commercials, especially those that deal with drug abuse.

This is advertising at its best, and I think Drugfair merits praise for taking this tack. I'm told that the company will spend about $25,000 on this advertising program during the final four months of this year. That may not be much for the Bell System's ad budget or Exxon's or IBM's, but it is a rather substantial sum for a local company to devote to public service ads.

If drug abuse affects you directly because you've become hooked, or if it touches your life indirectly because you are the parent, child, friend or relative of a person who uses drugs, be advised that useful pamphlets are available free at every Drugfair prescription counter. The pamphlets are being distributed in cooperation with the National Institute on Drug Abuse. b

The titles of some of these pamphlets are, "What Parents Should Know About Drugs," Drug Abuse: How to Spot It, What to Do About It," "Preventing Drug Abuse," "Drug Abuse: Why Worry?" and "Drug Jargon." Four other pamplets tell the reader where to go for help in the D.C. metro area, in Maryland, Virginia, Pennsylvania, Delaware and West Virginia. If this information saves even one boy or girl from getting hooked on drugs, Drugfair will have spent its money usefully. "AN HISTORIC"

Robert Bransom of Clarksville, Md., has clipped two stories from recent editions of our paper and forwarded them to me.

One contains the phrase "an Hispanic law" and the other refers to "an historic" event. Bob wants to know whether I am prepared to defend our usage.

I am not. More important, I know of no recognized authority who would. In "American Usage and Style: The Consensus," Roy H. Copperud has this to say:

"'a' rather than 'an' should be used before certain words beginning with 'h' (notably hilarious, history, hotel, humable, hysterical, habitual, hallucination). The test is whether the initial 'h' is sounded in pronouncing the word: a (not an) historical, hotel, humble, etc. Similarly, 'a' is required with unique, utopia, eulogy, etc., which begin with a consonant sound."

We say "an hour" and "an heir" because in words of this kind the h is not pronounced. Usages like an hotel and an historic trace back to times and places in which the initial hs were not sounded. Copperud says pronunciation should also be our guide in phrases like "an $800 salary" and "an RCA contract."

I don't know why the use of an before words like historic, hotel and habitual annoys me so much. Perhaps it is because it implies that I pronounce these words as a cockney would.

Incidentally, each time I mention Copperud's valuable compendium, readers want to know where they can buy it. The publisher is Van Nostrand Reinhold Co., 135 W. 50th St., New York, N.y. 10020. IS CHICKEN A VEGETALBE?

As I turned into the driveway of a fast food restaurant yesterday, I noticed that the car ahead of me carried a message on its rear bumper: "Love Animals, Don't Eat Them."

Two teen-aged girls got out of the car, went into the shop and, I was perplexed to note, ordered chicken.