You know what that great fund-raiser Isaac Newton used to say: A campaign in motion tends to remain in motion. With apologies to Brother Isaac, and to every snowball that ever rolled down a hill, it looks as if our 1990-'91 drive on behalf of Children's Hospital has become officially Newtonian.

The first five weeks of our annual eight-week drive are history, and after lagging in the early going, we have just set records all over the place.

Best single week in the 42-year history of the campaign. Best fifth-week total we've ever turned in. Largest number of group donors after five weeks. Largest percentage improvement over the previous year at this point in the campaign. All in all, it's cause for a few corks to pop.

The Week Five figures:

Grand Total to Date: $309,695.20.

Grand Total at the Same Time Last Year: $274,615.49.

Total Given This Week: $131,116.97.

Individual Donations to Date: 2,499.

Group Donations to Date: 523.

Fancy stepping, indeed. And yet . . . .

The campaign still has three weeks to go. We will no doubt need every millisecond of that time if we are to finish ahead of last year's record total of $562,000.

We particularly need contributions from those of you who have never donated before. Perhaps you're new to town, new to charitable giving or new to the wonders that Children's Hospital performs. If so, your dollars are especially welcome because our total isn't going to grow without you.

Please remember that every cent you contribute to our campaign goes where it should: to help pay the medical bills of children whose families are too poor to do so. Your money never goes for fancy gold lame carpets, conventions in Puerto Rico or further fund-raising. As I say each year, this is one check you can always feel good about writing.

So please help keep us Newtonian in 1990-'91. Please give generously to keep a Washington tradition alive and well. Please show the neediest children in our community that we care about them and want to help them get better. Many thanks.

TO CONTRIBUTE TO THE CAMPAIGN:

Make a check or money order payable to Children's Hospital and mail it to Bob Levey, The Washington Post, Washington, D.C., 20071.

THE CAMPAIGN ENDS ON JAN. 18.