“It’s the underpinning of the whole business,” Harper insisted. “The very reason the scribe was brought into the conspiracy in the first place was that he had already impregnated the town’s madwoman; she was close to term. You see, in order for the crystal’s properties to be effective, one must apparently dissolve them in a bath of blood and water and then be baptized in the mixture once every six months. Without the constant repetition of the treatment, the fatal reaction sets in, beginning with the lines of discoloration which the Belham monk described. In the case of William and Anselm, such a reaction did set in within five years of the first experiment. Because of arithmetic. You see?”

The Morris flashed under the raised sword of King Alfred, past the twin towers of Westminster Abbey. Shot like a cannonball into the enormous roundabout of Parliament Square. The place was ablaze with racing headlights, loud with horns, streaked and foggy with the angling rain. The car plunged into the melee. The cannonball became a sewing needle: the Morris stitched its way neatly through the chaos, Bernard rapidly steering, rapidly shifting gears. They were through—and speeding away into the broad canyons between the granite monoliths of Whitehall.

“There is a clear inference—so clear that the monk didn’t even think to spell it out: in order for the stone to work, the blood must come from one’s own child,” Harper continued calmly. “That was the mathematical problem Iago was trying to solve when he gathered his cult around him. It takes, as you may have heard, nine months to produce a baby. The amount of blood required for the bath is clearly enough to kill the infant. Given the time of gestation—not to mention the occasional female resistance to any of various stages in the proceeding—how does one produce enough of the priceless stuff to bathe in it every six months?”

“You know, I think I had that problem on my last maths exam.”

Harper, with rigid dignity, ignored this. Went on, “But what if mad Annies’s child was not the scribe’s first? What if he also had a child already grown? It might be possible to take the necessary amount of blood from an adult without killing him—often enough, at least, to fill in the gap between new births.”

Bernard pondered these matters in silence as two huge double-decker buses converged on either side of him, threatening to crush the Morris to cinders. He downshifted nimbly, causing his car to spurt forward. The buses lumbered after him like two elephants in the slipstream of a jet.