WHEN IT'S HOT as you-know-what outside and the novelty of no school has worn off, think of the cool, limestone Washington Cathedral, where you can escape the heat and create your own wallhanging at the London Brass Rubbing Centre's half-price sale.

Adjacent to the cathedral's Museum Gift Shop Crypt are the small brass-rubbing rooms operated by the aptly named Richard and Ann Etches. During the school year, English-born Ann Etches helps youngsters learn a bit about the people and life of medieval Europe as she carts her brasses, paper and beeswax crayons to classrooms; after school's out, however, she knows kids grow bored. Hence the sale, through July 4. (The shop's other sale is in February, when romance-related rubbings are also half price, meaning a 3-by-3-inch rubbing is $1.25 instead of $2.50, a six-by-three-foot knight $12.50 instead of $25.)

The young artist selects one of the brass facsimiles set in epoxy resin that were taken from originals on the tombs of aristocrats and gentry, mostly English. The brass plates -- knights and ladies, kings and merchants -- pack an historical punch, illustrating the fashions and armor of the day as well as possessions and animals depicted for heraldic or personal reasons.

The brass of Sir Robert de Bures (1310) of Suffolk, for instance, shows the knight in chain mail, but Sir William Fitzralph (1323) of Essex wears the then-new plate armor. Sir William Laken (1475) of Berkshire wears the attire of a justice; Margery Paris (1427) of Cambridgeshire is shown in the veil and mantle of widow's dress. Both Margaret Peytons (1484) -- both wives of Thomas Peyton -- plucked their hairlines to accommodate the butterfly headdress popular at the time.

Megan, 12, chose the Virgin and Child, a detail from a panel for Marguerite de Scornay, abbess of Nivelles in Belgium from 1443 to 1460. The larger original features a dragon, the abbess' special symbol. There were other choices to be made: paper (black, white, beige, blue) and color of beeswax crayon (gold, silver, bronze, black). Megan selected gold beeswax with black paper -- a good choice, said Etches, since only black paper allows erasures.

With the paper taped to the resin block, Megan learned to trace around and emphasize the edges of the metal figures, then to use the flat side of the gold beeswax to do the sides and fine outside detail. Finally, she tilted the crayon and rubbed harder to emphasize certain areas, such as the faces and folds of the clothing, and ended with the quick, highlighting strokes Etches had shown her.

She worked steadily for about 20 minutes, the brass atop a table, before declaring her work finished. Etches untaped the paper, asked Megan to add her signature in gold pencil, then rolled the 1-by-2-foot rubbing carefully and added black plastic end pieces to be attached at home. The cost: $3.50 (half the regular cost of $7 to do this particular rubbing), plus 50 cents for the plastic end pieces. Part of the fee goes to royalties that help maintain historic churches in England; another portion supports the Washington Cathedral.

To delay the inevitable return outside, we visited the Pilgrim Observation Gallery for a view that stretched as fas as the Catoctin and Blue Ridge mountains to the north and west and far down the Potomac River to the south. We also checked out the grotesques and gargoyles fending off demons above and below us and discovered what's behind the wood construction fences on the cathedral grounds: more stone, awaiting placement.

Eventually, bearing Megan's art work, we departed the blessed coolness of the cathedral for the searing heat. Dante might have chuckled. RUBBING BRASSES --

Half-price sale through July 4 on rubbing supplies (paper, beeswax). Call the London Brass Rubbing Centre in the Museum Gift Shop Crypt to check whether the small facilities are crowded; large groups should make reservations (364-0030). Open 9:30 to 7:30 weekdays, till 5 weekends. Framed rubbings done by staff, booklets and other items for sale at regular prices.