SINCE history began, people have always felt they lived in "modern" times - whether they saw their own era as an improvement on previous ones or a sorry decline from some ancient Golden Age. From our own late-20th-century perspective, the past 70 years - the 1920s and after - strike most of us as indubitably modern, and many of us would extend the term further back.Fixing a date for the birth of the modern is an engaging historical game. If modern means machine age, we might start with the invention of the steam engine. If modern implies the kind of thinking that led to the Industrial Revolution, we might locate its beginnings as early as the Renaissance or as late as the Enlightenment. If modern means the questioning of traditional beliefs and institutions, we could look to the Protestant Reformation - or the French Revolution. In his earlier book "Modern Times" British journalist and historian Paul Johnson examined "The World from the Twenties to the Eighties" with a focus on the ideologies and practices that made this century a time when totalitarian nightmares came true. A former socialist turned Tory, Johnson deftly illustrated the ways in which the totalitarian regimes of the right and the left learned new techniques of coercion and control from each other. Subsequently, Johnson turned his brilliant, ambitious, sometimes overly tendentious pen to "A History of the English People" and "A History of the Jews." And most recently, he wrote a much shorter book called "Intellectuals a scathing group portrait of left-wing thinkers like Rousseau and Marx, all caricatured by Johnson as a veritable rogues' gallery of hypocrites who talked about human rights while abusing their friends, wives, and children. Considering that Johnson traces the political evils of our century to the ideologies formulated by the romantics and revolutionaries of the previous century, one is a little surprised to find him nominating 1815 to 1830 as the focal period for "The Birth of the Modern." Johnson himself seems surprised: We might have expected him to choose the 1780s, as he remarks in his introduction. But instead, he has skipped over both the American and the French Revolutions and chosen to concentrate on a period of reaction, retrenchment, stabilization, material growth, and renewed attempts at reform. The period opens with the seeming triumph of the forces of reaction but ends with the return of what Johnson calls the "Demos": the age of Andrew Jackson in the United States, the Refor m Bill in England, and mass meetings in Ireland. Although Johnson's strongly held views are still in evidence, "The Birth of the Modern" is less strident - and finally more persuasive - than some of his more recent work. True, he still takes an excessive, unseemly relish in magnifying the character flaws of 19th-century cultural heroes like Beethoven, Byron, Shelley, and Hegel - not to mention the heroic arch-villain Napoleon. But he ranges with style and energy over an impressive variety of subjects - the "special relationship" between Britain and the United States; the discoveries and inventions of men like Humphry Davy, Michael Faraday, Robert Fulton, and Charles Babbage, who worked with Byron's daughter Ada on plans for the first computer; the political ferment in Russia; the nationalist movements that would transform China and Japan; the evolution of a new kind of society in Brazil; the dreadful state of education in Brit ain that led many of the best and brightest parents - Robert Browning's, John Ruskin's, John Stuart Mill's - to teach their children at home. The vividness of his writing, the clarity of his exposition, and the lucidity of his arguments make this the kind of history that all but the most recalcitrant and incurious of readers will find hard to resist dipping into. Johnson's history is admittedly Anglocentric: He rightly reminds us that Britain was indeed one of the world's paramount nations in those years. But he also provides a comprehensive and fascinating tour of developments all over the world - from the American frontier, where settlers and Indian-Americans alike harbored dreams of genocide, to West Africa, where Europeans, Americans, Arabs, and African chieftains continued to profit from the disgraceful slave trade. Much attention is also paid to the status of women. The 15 years Johnson surveys so broadly, yet in such detail, were by and large a period of consolidation as Europe recovered from the shocks of the French Revolution and Napoleonic conquests. But it was also a period in which dynamic forces were stirring - and a period in which philosophers like Hegel, artists like Beethoven, and scientists like Davy, Faraday, and the geologist Lyell perceived dynamism as a guiding principle of natural science, natural history, and human history. Dynamism explained - and indeed mandated - progress. But dynamism also threatened traditional ideas of order and of morality. At the outset of the period, Johnson notes, there was still a great deal of interchange between the arts and sciences: Poets like Goethe, Shelley, and Coleridge speculated fruitfully about science, and engineers sometimes started out as artists and vice versa. By 1830, however, the split between the "two cultures" was beginning to develop as specialization increased. The confident polymath, the "Renaissance Man," was starting to yield to the specter of "Modern Man," harried, overextended, confused, unabl e to assimilate the information explosion. "Future shock" was beginning to make itself felt. "The Birth of the Modern" does a remarkable job of conveying the dangers that loomed ahead, while maintaining a firm and realistic optimism about the value of many, perhaps most, aspects of progress and civilization.