Observing Washington.

Two Moons, by Thomas Mallon (Pantheon, 303 pp., $24)

TOWARD the end of Thomas Mallon's 1994 novel Henry and Clara, the heroine Clara Rathbone reflects on a phenomenon of post-Civil War Washington, D.C.: the influx of female clerks who toil in government offices. For the beleaguered Clara, whose husband is sliding toward insanity, these women lead seductive lives; she envies their "impoverishment and freedom and [imagines] herself as one of them."

Just such a woman is the central character of Two Moons, Mallon's absorbing new work of historical fiction. At 35, Cynthia May has been on her own for years, living precariously in Washington boardinghouses. As the story opens, in the spring of 1877, she seeks to better her current wages by landing a position at the U.S. Naval Observatory.

As Cynthia fills out her application forms, her life's tragedies unfold: a husband killed at Chickamauga, a child who succumbed to diphtheria. The damage wrought by the Civil War feels irreparable to Cynthia, who considers "Recon struction," and the fact that President Ruther ford B. Hayes has just de clared it completed, a laughable irony. She can neither fathom nor emulate the lightheartedness of those who missed the cataclysm.

Thanks to a marked facility with numbers, Cynthia easily wins the coveted post of "computer"-one who translates the telescopic observations of the astronomers into mathematical notation. The work, at which she is un erringly accurate, inspires an unwonted sense of hope. Perhaps she might gain "some imaginative perspective from which she could regard herself as the denizen of some faraway star instead of the overheated little District of Colum bia."

That perspective is soon provided by one of the observatory's astronomers. Hugh Allison is handsome, carelessly charming, and too young to have suffered in the war. His superiors regard him as somewhat unsound, and he is clearly bored by the nightly work at the observatory: tracking comets, observing double stars, even making discoveries (such as that of the two moons of Mars) that will earn the astronomers public attention and congressional largesse. Hugh's true desire is to send a signal to other life that may exist in the universe. The notion, familiar enough to modern readers, would be outlandish for Hugh's 19th-century fellows. He confides only in Cynthia, with whom he gradually begins an affair.

Given the difference in their ages, both the scientists at the observatory and Cynthia's fellow boarders regard the couple as mismatched; even Cyn thia thinks of herself as too old for Hugh, or for anyone. Yet she retains enough of her beauty to attract the attentions of Roscoe Conkling, a powerful senator from New York, whose path she crosses in an astrologer's studio.

While Cynthia and Hugh are fictional characters, Conkling is drawn from life. He was a crony of Ulysses S. Grant's, a leader of the "Stalwart" faction of the Republican party, and the ruler of the New York Custom House, a font of money and patronage for the Republican machine. In Mallon's imaginative recreation, Conk ling alternatively besieges Cynthia and plots his next move in a high-stakes political battle.

On both fronts, romantic and political, he faces stiff opposition. Although Conkling helped put Hayes in the White House, the ungrateful new president has taken two unpalatable steps: He has moved to reform the Custom House, source of Conkling's power, and to lift sanctions against the defeated South.

As for Cynthia, she ignores Conk ling's amorous missives until Hugh falls seriously ill with malaria, an affliction attributed to the "miasmic" night air around the observatory. The sicker Hugh becomes, the more fervently he pursues his vision of projecting his image into space, using a powerful machine he must import from France. Desperate to help him, Cynthia finally turns to Conkling, who has the power to cancel the insurmountable duty his Custom House will levy on Hugh's device.

The story culminates with the un avoidable collision of Conkling, Cyn thia, and Hugh, and with the playing out of the two men's dreams of immortality. In a sense, Hugh's desire for a sort of eternal life and Conkling's lust for endless power are two sides of the same coin. Cynthia's longings, by contrast, are far more modest. She wants marriage to Hugh, another child, and a bank account that draws 8 percent interest-goals that prove the most illusory of all.

Creating a character who wants to shine a light through the cosmos is one of several risks Mallon takes in this novel-and pulls off. He also succeeds in enlivening an episode that textbooks sometimes scurry past: the contested Hayes-Tilden election of 1876 and its aftermath. And then there is Roscoe Conkling.

As portrayed here, the so-called War God of the Norsemen is choleric and manipulative, but he is also faithful to his convictions, in his own way. He cheats on his wife, but, as a leader in the party that devised Reconstruction, he is the first to shake hands with a black senator from Mississippi. Each year he reads all of Shakespeare, "not just so he might quote him, but to find himself in the pages." They don't make politicians like Roscoe Conkling anymore, and Mallon understandably revels in this historical found object, whose only flaw as a character is his tendency to overwhelm the other actors in the story.

Those who know Mallon's previous books (he is the author of four other novels and four works of nonfiction) will find familiar pleasures here, while new readers will be happy to discover his unfailingly graceful prose, his gift for melding historical research seamlessly into a story, and, as in Henry and Clara, his ability to write convincingly from a woman's point of view. In Two Moons, Mallon also captures a Washington that is still a muddy southern town, full of unfinished monuments but as suffused with gossip as it is today.

Two Moons may not be the last novel to include a search for extraterrestrial life, but it will certainly be one of the most interesting.