The number of casks being pumped is minute, given an American beer market still dominated by big corporate brewers. But throughout the country, growth in the beer market has been almost entirely in the craft brewing segment, and that has been especially true in New York.

While the American market for craft brews and specialty imports has risen 13 percent this year, Mr. Hodson said, in New York the rise in 2007 has been much higher, partly because the beer culture in New York has lagged behind that in the rest of the country and is now catching up.

It’s the rare bar in New York today that doesn’t offer alternatives to the insipid mass-market brews. Craft breweries have brought to American consumers every conceivable European beer genre and more than a few that Europeans never imagined. Now, with cask-conditioned ale, Americans have the opportunity to taste beers of unmatched subtlety and authenticity.

Image WORTH THE EFFORT Bartenders work a pump, not a standard tap, to pour cask-conditioned ale, since its kegs arent charged with carbon dioxide. Credit... Hiroko Masuike for The New York Times

Because cask ales are naturally carbonated and best served at cellar temperature — about 55 degrees — they have often been described as warm and flat. But as you get to know them, it can become hard to imagine drinking beer any other way. The softness of the bubbles and the gently cool temperature permit nuances that would otherwise be undetectable.

Cask ale is made the same way as other good beers, until it is left to mature in tanks. Mass-market beer is filtered and pasteurized for a stable shelf life.

Cask beer is treated differently. It goes, naturally, into casks, or firkins, if you want the British word for a container of 9 imperial gallons (around 11 U.S. gallons). Firkins used to be wooden, but now are generally made of metal. A small dose of sugar is added to produce a secondary fermentation, just as Champagne or certain other beers are refermented in bottles. Brewers may also add more hops and a fining material, like isinglass, to help settle the yeast and clarify the beer.