The writer Anatole France once remarked on the majestic equality of the law, which forbids rich and poor alike from sleeping under bridges. But what if the law instead limited each citizen to a fixed number of nights of such accommodation, allowing those who rarely or never sleep under bridges to transfer their unused allotment to those in greater need? There, in schema, you have George Bush's acid rain reduction plan.

It may seem at first glance that Bush's proposal to let coal-burning plants that achieve lower sulphur dioxide emissions than required trade the difference in the form of a permit to plants that exceed mandated levels amounts to a ''license to pollute.'' Liberal environment critic Sheila Copps, who accords few issues the favor of her second glance, attacks the idea of ''pollution being a commodity that can be bought and sold.''

Well, pollution is a commodity, if people are willing to pay for it. So is clean air. Bush's market-based approach simply combines the recognition that clean air, like any other commodity, has a price, with an attempt to make that price relatively more attractive by raising the cost of its alternative: pollution.

Ideally, the costs to society of producing a good are reflected in the price negotiated between individual buyers and sellers. That's how individual actions, guided by price, work out in the social interest. But pollution is a cost of production that is shunted entirely or in part onto society as a whole - either to endure or to clean up - rather than feeding through into the price of the good.

UNIFORM CONTROLS

The economic problem, then, is how to make just those costs, and no more, part of the production calculations of firms. Simple uniform controls on pollution levels, or prescribed technological fixes such as the ''scrubber'' units some environmentalists want installed in all coal-burning plants as the solution to acid rain, are excessively costly, invite evasion and offer no incentive to firms to find the cheapest route to reductions.

The Bush plan allows individual firms to choose the means by which they will achieve emission reductions. Within the overall targets, it is left to the market to allocate the burden between plants. Those able to achieve the desired reductions at relatively little cost will have the incentive to do so; those for whom the same standard would impose relatively high costs can buy pollution credits at the market price. But the greater their demand, the greater the incentive to other plants to reduce emissions further below the required levels, in order to sell the credits thus obtained.

This need not necessarily work to Canada's advantage. Might it not simply allow plants close to the border to go on polluting, buying pollution permits from plants in Florida? No. In its first phase, until 1995, the plan requires the steepest cuts in the dirtiest plants - the ones near us - and only allows trading between plants within a state. Second, when interstate trading is allowed, it's more likely those old Ohio smokestacks will be the ones selling credits, not buying. Dirtier plants can make bigger cuts per dollar spent then those with less room for improvement, and price their credits cheaper.

Last, the trading of pollution credits need not be limited to the polluters. If possible, Canada should buy them on the open market, so raising the price of polluting and giving the plants more incentive to clean up. It might seem unfair that we should have to pay U.S. firms to stop fouling our air, but if that's our only leverage, so be it.

This might be particularly worthwhile when you take into account the accusations of some U.S. coal-company executives that Canada's acid rain lobby is simply a Trojan horse for our hydroelectricity exports. Suppose they're right. The money we invest in pollution permits is then essentially an export subsidy, the difference being that it pays dividends to the nation at large (assuming the West has an interest in cleaning up Quebec's lakes) rather than just the exporter.

The Bush plan offers such intriguing possibilities that one wonders why no one in government has suggested it might somehow be linked with the free trade agreement - a signing bonus for Canadian participation.

Sure, it's far-fetched, but no more so than the idea put about in all seriousness by anti-free trade diehards that the taxing back of old age pensions in the last budget was part of the deal. Was acid rain part of some secret side accord? I think we should be told.