It's often good fun and sometimes revealing to divide American history into distinct periods of uniform length. In working on my forthcoming book on American migrations, internal and immigrant, it occurred to me that you could do this using the American-sounding interval of 76 years, just a few years more than the biblical lifespan of three score and ten.

It was 76 years from Washington's First Inaugural in 1789 to Lincoln's Second Inaugural in 1865. It was 76 years from the surrender at Appomattox Courthouse in 1865 to the attack at Pearl Harbor in 1941.

Going backward, it was 76 years from the First Inaugural in 1789 to the Treaty of Utrecht in 1713, which settled one of the British-French colonial wars. And going 76 years back from Utrecht takes you to 1637, when the Virginia and Massachusetts Bay colonies were just getting organized.

As for our times, we are now 71 years away from Pearl Harbor. The current 76-year interval ends in December 2017.

Each of these 76-year periods can be depicted as a distinct unit. In the Colonial years up to 1713, very small numbers of colonists established separate cultures that have persisted to our times.

The story is brilliantly told in David Hackett Fischer's "Albion's Seed." For a more downbeat version, read the recent "The Barbarous Years" by the nonagenarian Bernard Bailyn.

From 1713 to 1789, the colonies were peopled by much larger numbers of motley and often involuntary settlers -- slaves, indentured servants, the unruly Scots-Irish on the Appalachian frontier.

For how this society became dissatisfied with the colonial status quo, read Bailyn's "The Ideological Origins of the American Revolution."

From 1789 to 1865, Americans sought their manifest destiny by expanding across the continent. They made great technological advances but were faced with the irreconcilable issue of slavery in the territories.

For dueling accounts of the period, read the pro-Andrew Jackson Democrat Sean Wilentz's "The Rise of American Democracy" and the pro-Henry Clay Whig Daniel Walker Howe's "What Hath God Wrought." Both are sparklingly written and full of offbeat insights and brilliant apercus.

The 1865-to-1941 period saw a vast efflorescence of market capitalism, European immigration and rising standards of living. For descriptions of how economic change reshaped the nation and its government, read Morton Keller's "Affairs of State" and "Regulating a New Society."

The 70-plus years since 1941 have seen a vast increase in the welfare safety net and governance by cooperation among big units -- big government, big business, big labor -- that began in the New Deal and gained steam in and after World War II. I immodestly offer my own "Our Country: The Shaping of America from Roosevelt to Reagan."

The original arrangements in each 76-year period became unworkable and unraveled toward its end. Eighteenth-century Americans rejected the colonial status quo and launched a revolution, then established a constitutional republic.

Nineteenth-century Americans went to war over expansion of slavery. Early-20th-century Americans grappled with the collapse of the private-sector economy in the Depression of the 1930s.

We are seeing something like this again today. The welfare state arrangements that once seemed solid are on the path to unsustainability.

Entitlement programs -- Social Security, Medicare, Medicaid -- are threatening to gobble up the whole government and much of the private sector, as well.

Lifetime employment by one big company represented by one big union is a thing of the past. People who counted on corporate or public-sector pensions are seeing them default.

Looking back, we are as far away in time today from victory in World War II in 1945 as Americans were at the time of the Dred Scott decision from the First Inaugural.

We are as far away in time today from passage of the Social Security in 1935 as Americans then were from the launching of post-Civil War Reconstruction.

Nevertheless our current president and most politicians of his party seem determined to continue the current welfare state arrangements -- historian Walter Russell Mead calls this the blue-state model -- into the indefinite future.

Some leaders of the other party are advancing ideas for adapting a system that worked reasonably well in an industrial age dominated by seemingly eternal big units into something that can prove workable in an information age experiencing continual change and upheaval wrought by innovations in the market economy.

The current 76-year period is nearing its end. What will come next?

Michael Barone,The Examiner's senior political analyst, can be contacted at mbarone@washingtonexaminer.com. His column appears Wednesday and Sunday, and his stories and blog posts appear on washingtonexaminer.com.