The president gallantly responds, ''Madam, I thought it was the horse.''

Perhaps these days she would be receiving a fine from the Greater London Environmental Protection Agency, though it seems unlikely that the new Lord Mayor, Boris Johnson, who always looks as if he likes a good fart, would have anything to do with it. Boris's predecessor, Red Ken Livingstone, would, no doubt, have done the citizen's arrest himself.

The poor old horse couldn't help it, of course, but like many of our other introduced stock is a substantial methane factory. A pony, 'tis said, burps or farts about 17 cubic metres of methane a year, a saddle horse about 25 cubic metres, and a draught horse about 43. As I calculate it, though chemistry was never my strong point, a cubic metre of methane weighs about 720 grams, so the horse's contribution to the atmosphere is between 12 and 31kg of methane.

Sheep and goats seem to be about the same as ponies, while cows, it seems are good for about eight times as much, at the top end, about 180 cubic metres of methane a year. That's about 130kg weight of methane.

Another way ruminant haters put this prodigious flatulence is that a cow produces about 30 litres of methane per kilogram of milk produced. A grass-fed lamb on a butcher's slab has produced methane in its lifetime at the rate of about 1160 litres a year for every kilogram it weighs (about 60 litres if it has been artificially fed indoors), and a 40-month-old beef steer about 1040 litres for every kilogram of meat produced say 425,000 litres for a reasonable size one. (The calculation, apparently, makes some allowance for the mother's co-contribution). That's about 43 cubic metres of methane at standard temperature and pressure, weighing, say, 30kg.

I emphasise the weight, which I think is reasonably modest, because ruminant haters, vegetarians and greenhouse gas worriers tend to over-emphasise the cubic metreage, sometimes, it seems to me, raising the spectre of the whole planet suffocating in a methane haze.

Australia, for example, usually has about 100 million sheep (quite a few less at the moment, thanks to the drought), and talking of their annual contribution invites one to think of 1700 million cubic metres of methane, which appears to be quite a lot. The more so when one adds the 4.2billion cubic metres of methane from the 26 million cattle, and the output of a million other domestic animals, for say the astounding six billion cubic metres of methane a year. Hell, spread evenly around the continent, the vile gas would be 0.8 of a millimetre deep (in gaseous, not liquid form of course). The weight alone - 4.3 million tonnes - must be forcing the land to subside, as opposed to the sea falling.

Heavens, by weight, that is about the same as the total amount of soil moved around while we were building the New Parliament House. About a fifth of what we hope to be this year's wheat crop.

Yet apparently all of this fart gas amounts to 11 per cent of Australia's greenhouse gas emissions.

Two Australia ecologists, writing in an international journal Conservation Letters, have estimated that reducing the herd of agricultural stock by about a third could lower Australian greenhouse gas emissions by about 16 ''megatonnes'' by 2020. This would amount to 3 per cent of Australia's greenhouse gas emissions.

A megatonne, suggesting a million tonnes, is strictly reserved for the explosive equivalent of a million tonnes of TNT, but perhaps this is appropriate in the circumstances.

They suggest that we could achieve this by switching over to eating the national symbol. Kangaroos have negligible methane production, apparently and, Wilson and Edwards say, raising the kangaroo herd to about 175 million (perhaps multiplying the existing herd by five) would make up for the red meat taken out of production. Besides, some will say, roo meat is better for you than beef, lamb or mutton. Perhaps even better than horse and goat.

Heaven knows what Canberra's vegetarians and animal liberationists would think. They had conniptions when we slaughtered a few hundred roos in Belconnen, and, while no doubt opposed to the eating of any animal, seem to be especially upset by the eating of anything thought to be especially warm, cuddly or symbolic.

Perhaps it would be better if we shifted to eating whale meat, which I understand to be quite plentiful and nutritious.

Neither roo nor whale will do for me, I'm afraid. I do not mind the occasional piece of roo, but it is no substitute for proper red meat, say seven kilograms a week worth. If that involves the production of 7280 litres of methane a week, it's a sacrifice the planet may have to bear. My guess is that we could sustain our meat supply by reducing our use of air-conditioning by 5 per cent - a sacrifice I am willing to share in.