Before the 1973-74 oil embargo, fewer than 50,000 cords of firewood were burned each New Hampshire heating season, and Bob Ferns sold heating oil.

This winter, 10 times as many New Hampshire trees -- enough to make a stack four feet high and four feet wide running from Concord through Washington to Charlotte, N.C -- will become smoke and ashes. And at Ferns' family oil business, Bob has added a new product -- firewood.

Many of Ferns customers, and a lot of other New Englanders, have been burning wood since oil became scarcer and more expensive. Ferns appears to be the first oil company in the region to offer its customers firewood.

Firewood is becoming big business. Ferns, whose wood sells for $85 a cord at his plant or $90 a cord delivered if you buy two or more, says "there is going to be a lot of competition in this business."

Ferns purchased what he said is one of 26 wood-processing machines operating in the nation. The machines, which cost from $45,000 to $100,000, according to size, are to the chain saw and splitting maul what the steam drill was to the sledge and spike. They take logs in roughly nine-foot lengths, saw them into 16-inch sections and split them so quickly and competely as to bring an envious twinge to any weekend wood splitters.

Ferns' plant is surrounded by what he guesses is about 700 cords of split firewood (a cord is four feet on a side by eight feet high). He has sold close to 250 cords since going into the business two months ago.

"We're just getting off the ground," Ferns said. His target is to sell 3,000 to 5,000 cords a year.

Even at that rate, there will be a lot of room for others. For example, there are three wood processors in Maine this year, according to Andrew Shapiro of Wood Energy Research Corp. He estimates the three handle no more than 15,000 of the roughly 500,000 cords Maine residents will burn this winter.

There is no reason to expect that the demand for firewood will decline. According to state surveys in New Hampshire, Maine and Vermont, about 60 percent of all residences burn wood.For about 20 percent of the homes in the three states, wood is the primary source of heat.

Shapiro adds that about half the households not using wood are reported to be considering switching to wood.

"Wood is a classic example of human enterprise in reaction to a problem," said Paul Bofinger, executive director of the Society for the Protection of New Hampshire Forests. "It's hard to know with firewood whether the millenium has arrived or apocalypse is just around the corner."

The millenium is possible because New England grows enough wood to sustain huge levels of firewood consumption forever, Bofinger said, but without proper controls the wrong wood can be cut and the forests could be reduced and ultimately destroyed.

It seems impossible, looking at the forests that cover more than 80 percent of the three states, but 100 years ago far less land here was forested, and New Englanders talked of a coming wood shortage. That was before cheap oil and gas transformed a log into only a recreational source of heat.

In many underdeveloped countries where cheap oil and gas were never cheap enough, wood has been burned until there are almost no trees left.

"We can have it both ways," Bofinger said. "We have the capacity to produce firewood and still keep the higher-value trees for better uses."

One cord of the best dries hardwoods (hickory, white oak, apple, black locust) produces as much heat as nine-tenths of a ton of coal of 146 gallons of heating oil or 17,400 cubic feet of natural gas or 3,800 kilowatt hours of electricity.

The worst firewood (basswood, white pine, balsam fir, cedar) have about half that heating value.

In a world in which the free lunch seems as increasingly absurd proposition, it isn't suprising that burning wood has what doctors call side effects -- bad ones.

Because wood burns much less efficiently than do oil or gas, it pollutes. You may love the smell of wood smoke, but it isn't good for you. Burning wood produces about as many particles suspected of causing cancer as does burning coal and this worries some monitors of the environment.

Some unlikely candidates may join the ranks of gritty cities if wood-burning increases and no pollution controls are developed. Berlin, N.H., for example, long had an air pollution problem as the result of a paper mill. Just as the mill pollution was conquered and the pall of smoke that sometimes rested over Berlin disappeared, a new pall from wood stoves began to take its place.

The problem is most acute -- and most visible -- in towns lying in pockets among hills. If the winds don't cleanse the air, towns such as Berlin have a problem.

A shorter-term problem are chimney fires. Some of the unburned materials produced in a fire never make it into the air, but stick to the inside of the flue. This sticky layer of creosote can catch fire in the chimney if there is a very hot fire in the stove. Creosote keeps chimney sweeps in business, and since most experts recommend that chimneys be cleaned either after every third cord of wood or once ayear, their business is picking up.

The New England switch to wood has occurred without government incentives or any of the fanfare that has surrounded other energy developments. The House of Representatives recently passed a tax credit for buying a Wood-burning stove, the first such measure to encourage the use of firewood.

While there has been a tenfold increase in wood burning since the 1973-74 oil embargo, the states have been able to make only modest increases in efforts to spread information about forest management.

State forestry experts in New Hampshire, Vermont and Maine are swamped with call from landowners seeking advice on what trees to cut. Landowners and firewood-cutters are also in need of support for building roads into the forests so that all the cutting is not near existing roads.

Ferns' heating oil costs 86.9 cents a gallon today, and he says that sales in October and November were 27 percent below those for the same period a year ago. Although the weather was unusually warm this year. Ferns said he thinks most of the reduction was due to people's determination to conserve oil. He predicts oil sales throughout the area will be 20 percent lower than last year for the whole winter.

Ferns also sees an intention to conserve in two other products he sells -- insulation and furnaces.

"For years, the furnace was that black thing in the basement and you just left it alone," Ferns said. Now people are looking for more efficient models and Ferns sells several that burn oil and wood or coal. His sales of insulation, which he first offered three years ago, are also up. "We are in the total home-heating business," Ferns said.

Next year he plans to add coal. Ferns wants to find a way of package it so that the customer simply throws a bag, including the wrapping, into the furnace without any contact with coal dust, which Americans have learned to live without.