HERE HAVE been dramatic changes in both the size and the disposition of chimney sweeps since Mayhew wrote his classic study of the art. Not only will few fit into your chimney, but you can expect even the average sweep to react strongly if you decide that lighting a fire under him or sticking a pin in his foot is the only way to set him climbing. A prearranged fee is usually sufficient.

People who look into flues every day say there are a lot of dirty chimneys out there. The recent energy church has turned people on to wood heat as a means of keeping homes cozy. Consequently, many chimneys that were never used much before are being called into service - especially, they say, in the District of Columbia, where the charms of old homes and living in the big city have lured renovators.

"People buy an old house in the District, and the first thing that happens is the chimney starts backing up," said Jeff Martin at Flood Plumbers.

"They're filthy," said Joy Farace of Farace Chimney and Roofing in Severn, Md. "The men are coming in (from jobs in the District) and saying they're doing the dirtiest chimneys they've ever done."

Jaime Widner of Metropolitan Chimney Sweeps in Silver Spring said he recently checked four D.C. chimneys in two days and "none of them were workable."

Sweeps recommend that any chimney that hasn't been used for awhile, or that hasn't been cleaned for years, be checked by a sweep before you start a fire. Many chimneys, they say, were nevery built right.

The possible hazards of flammable chimney dirt - called creosote - that collects on the inside of chimneys, especially wood burners, are well documented. In 1975 alone, reports the National Fire Protection Association, there were nearly 16,000 chimney fires nationally, resulting in more than $23 million in damages.

A chimney fire "can rip the hell out of a chimney," said John Gird, and engineer in the University of Marylnad's agricultural engineering department. "It sounds like a tornado coming through the house." Uninvited tornados make the worst guests.

Chimney sweeping was an artin its death throes not too many years ago. The 1975 Yellow Pages listed only a handful of willing sweeps. But in just two years the number has reached nearly a dozen.

"I couldn't begin to guess how many phone calls we've had," said Bill McLaughlin at the Samuel C. Boyd Co. in the District. "This year, it's just been incredible."

If you've been using a fireplace or a woodburning stove on a regular basis - say every night - your chimney should be cleaned every year. With less regular use - maybe just on weekends - probably every other year. (Furnace chimneys need cleaning too: every other year for oil and coal burners; every five years for gas.)

Calling a professional sweep in the Washington area can cost anywhere from $45 per flue, as at Metropolitan Chimney Sweeps in Silver Spring, to the $70 minimum charged by the Boyd outfit. Much depends on how many flues you've got, how high they are, whether your chimney needs other maintenance (you may have some cracked flue liners, for instance) and how far the company has to travel.

But you'd better call before. The Freese, ice and snow on rooftops being the bane of sweeps. Some places already have waiting lists.

One thing on which there is very little agreement among sweeps is how best to clean a chimney. In Portland, Me., Eva Horton, who is from Oslo, Norway, (where chimney sweeping is underwritten and enforced by the government) is the distributor of the famed Jotul Norwegian wood stove and has organized a national Chimney Sweep Guild. In "The Chimney Sweep Brush," Horton's guide for would-be do-it-yourself sweeps, the best method is described as lowering the chimney brush (a wire affair that looks as if it has been plugged into a wall circuit) on a rope, heavily weighted, from the top of the chimney; them pulling the dirt up with the brush.

There are any number of other methods.

Recently a family that lives in Northwest Washington called in Metropolitan to clean a chimney that had been collecting creosote untouched for at least six years. They have two fireplaces, one in the basement and one upstairs in the living room. The upstairs one is occupied by a wood-burning stove. The house-holders complained that smoke from the stove ended up in the basement.

Three men from Metropolitan, having performed an earlier check on the chimney system, arrived in a special $16,000 vacuum truck. They pulled into the alleyway next to the house. Sweeping is at least a two-man job. Metropolitan has two trucks with three-man crews each.

While Jaime Widner, the proprietors son, lugged the monstrous 100-foot vacuum hose into the house, where he positioned it inside the stove under the flue, Dave Girt and his assistant arranged the extension ladder to the top of the chimney - some 30 feet up.

The vacuumer, powered by a four-cylinder engine, revved into an airplane-like whine, Girt climbed the ladder with a 13-inch square brush and several extenders. While Widner kept a hearth-side vigil, Girt plunged the brush into the chimney, adding extenders as the brush descended, sending cascades of creosote bits (they lookl ike crushed cinder) into the mouth of the vacuum hose.

Widner reached behind the damper and pulled out a handfull. "Look at this!" The chimney was indeed dirty.

After a thorough cleaning, Girt and Widner shined flashlights from opposite ends of the chimney. "You see any cracks, Dave?" Seeing none, Widner lit some newspapers, which took off quite nicely in the updraft, so Girt could see where the smoke well.

Doesn't Girt get the smoke right in the face?

"Probably," replied Widner.

But Girt didn't seem to mind.

The Boyd Co. uses ropes and weights, closing the chimney opening off and vacuuming after the sweeping's done. Flood Plumbers has sand bags at the end of their ropes. Some sweeps employ a man on top and one on the bottom, pulling on a rope saw-fashion.

John Vivian, in his book "Wood Heat," suggests some things you can watch for:

The sweep's experience; you should be impressed.

The routine be uses. If it sounds fishy, forget it.

The sweep should first inspect the outer portion of the chimney for structural damage and things, such as ivy, blocking the opening.

If the brush-on-rope routine is used, fireplace openings should be sealed tight.

The insdie of the chimney should be inspected for debris before it is swept. (Dead birds, raccoons and other furry things like to hand out there.)

The damper opened or removed for sweeping.

After sweeping, the fireplace and the smoke shelf (where the flue begins inside) should be vacuumed.

The chimney should be tested for leaks.

You should get a full report on the condition of your chimney after work is done.

Whoever sweeps your chimney should have insurance against possible damages.

More gregarious fire aficionados may want to clean their own chimneys. People used to do it. Horton's guide (write Kristia Associates, 343 Forest Ave., P.O. Box 1118 CBB, Portland, ME. 04104) and "Wood Heat" are good starters.

However, anyone seeing Dave Girt perched on that chimney, 30 feet off the ground, would feel the jitters. And Connie Widner of Metropolitan said "many people write away for the booklet, then think they're experts. But they really don't have the right equipment."

Indeed, the first problem cleaning a chimney yourself will be finding the correct brush. Few places in the area have them, one of them being the ACME Stove Co., in the District and Silver Spring, where they sell for between $12 and $17 (no square hrushes). L.L. Bean, the mail-order house (Freeport, Me. 04033), sells them for $18-$29 (and has square brushes).

Horton suggests buying brushes somewhat larger thant he flue opening to get the necessary scraping action.

If you decide to hire a sweep, don't expect the top hat and tails bit. Sweeps took on that costume in the 19th century when the only thing they could get was what they begged off morticians, who shared a similar social rung. Today it is a promo gizmo. Although some will appear in costume upon request (Widner has brought the luck of the sweep to at least one wedding reception), most abandoned the habit after the hat fell once too often the way of the brush - right down the flue.

It is also said a sweep's kiss brings good fortune.But caution is urged with sweeps more than 6 feet in height.