These words filled Daniel with a foreboding that turned out to be fully justified: by the time the sun rose, they had fetched the dead man from the gaol, brought him back out to the cottage, and carefully cut his head off. Charles Comstock was rousted from bed and ordered to dissect the corpse, as a lesson in anatomy (and as a way of getting rid of it). Meanwhile, Hooke and Wilkins connected the head’s wind-pipe to a large set of fireplace-bellows, so that they could blow air through his voice-box. Daniel was detailed to saw off the top of the skull and get rid of the brains so that he could reach in through the back and get hold of the soft palate, tongue, and other meaty bits responsible for making sounds. With Daniel thus acting as a sort of meat puppeteer, and Hooke manipulating the lips and nostrils, and Wilkins plying the bellows, they were able to make the head speak. When his speaking-parts were squished into one configuration he made a very clear “O” sound, which Daniel (very tired now) found just a bit unsettling. Wilkins wrote down an O -shaped character, reflecting the shape of the man’s lips. This experiment went on all day, Wilkins reminding the others, when they showed signs of tiredness, that this rare head wouldn’t keep forever-as if that weren’t already obvious. They made the head utter thirty-four different sounds. For each one of them, Wilkins drew out a letter that was a sort of quick freehand sketch of the positions of lips, tongue, and other bits responsible for making that noise. Finally they turned the head over to Charles Comstock, to continue his anatomy-lesson, and Daniel went to bed for a series of rich nightmares.

Looking at Mars had put Hooke in mind of c?lestial affairs; for that reason he set out with Daniel one morning in a wagon, with a chest of equipment. This must have been important, because Hooke packed it himself, and wouldn’t let anyone else near it. Wilkins kept trying to persuade them to use the giant wheel, instead of borrowing one of John Comstock’s wagons (and further wearing out their welcome). Wilkins claimed that the giant wheel, propelled by the youthful and vigorous Daniel Waterhouse, could (in theory) traverse fields, bogs, and reasonably shallow bodies of water with equal ease, so they could simply travel in a perfectly straight line to their destination, instead of having to follow roads. Hooke declined, and chose the wagon.

They traveled for several hours to a certain well, said to be more than three hundred feet deep, bored down through solid chalk. Hooke’s mere appearance was enough to chase away the local farmers, who were only loitering and drinking anyway. He got Daniel busy constructing a solid, level platform over the mouth of the well. Hooke meanwhile took out his best scale and began to clean and calibrate it. He explained, “For the sake of argument, suppose it really is true that planets are kept in their orbits, not by vortices in the ?ther, but by the force of gravity.”

“Yes?”

“Then, if you do some mathematicks, you can see that it simply would not work unless the force of gravity got weaker, as the distance from the center of attraction increased.”

“So the weight of an object should diminish as it rises?”

“And increase as it descends,” Hooke said, nodding significantly at the well.

“Aha! So the experiment is to weigh something here at the surface, and then to…” and here Daniel stopped, horror-stricken.

Hooke twisted his bent neck around and peered at him curiously. Then, for the first time since Daniel had met him, he laughed out loud. “You’re afraid that I’m proposing to lower you, Daniel Waterhouse, three hundred feet down into the bottom of this well, with a scale in your lap, to weigh something? And that once down there the rope will break?” More laughing. “You need to think more carefully about what I said.”

“Of course-it wouldn’t work that way,” Daniel said, deeply embarrassed on more than one level.

“And why not?” Hooke asked, Socratically.

“Because the scale works by balancing weights on one pan against the object to be weighed, on the other… and if it’s true that all objects are heavier at the bottom of the well, then both the object, and the weights, will be heavier by the same amount… and so the result will be the same, and will teach us nothing.”

“Help me measure out three hundred feet of thread,” Hooke said, no longer amused.

They did it by pulling the thread off a reel, and stretching it alongside a one-fathom-long rod, and counting off fifty fathoms. One end of the thread Hooke tied to a heavy brass slug. He set the scale up on the platform that Daniel had improvised over the mouth of the well, and put the slug, along with its long bundle of thread, on the pan. He weighed the slug and thread carefully-a seemingly endless procedure disturbed over and over by light gusts of wind. To get a reliable measurement, they had to devote a couple of hours to setting up a canvas wind-screen. Then Hooke spent another half hour peering at the scale’s needle through a magnifying lens while adding or subtracting bits of gold foil, no heavier than snowflakes. Every change caused the scale to teeter back and forth for several minutes before settling into a new position. Finally, Hooke called out a weight in pounds, ounces, grains, and fractions of grains, and Daniel noted it down. Then Hooke tied the free end of the thread to a little eye he had screwed to the bottom of the pan, and he and Daniel took turns lowering the weight into the well, letting it drop a few inches at a time-if it got to swinging, and scraped against the chalky sides of the hole, it would pick up a bit of extra weight, and ruin the experiment. When all three hundred feet had been let out, Hooke went for a stroll, because the weight was swinging a little bit, and its movements would disturb the scale. Finally it settled down enough that he could go back to work with his magnifying glass and his tweezers.

Daniel, in other words, had a lot of time to think that day. Cells, spiders’ eyes, unicorns’ horns, compressed and rarefied air, dramatic cures for deafness, philosophical languages, and flying chariots were all perfectly fine subjects, but lately Hooke’s interest had been straying into matters c?lestial, and that made Daniel think about his roommate. Just as certain self-styled philosophers in minor European courts were frantic to know what Hooke and Wilkins were up to at Epsom, so Daniel wanted only to know what Isaac was doing up at Woolsthorpe.

“It weighs the same,” Hooke finally pronounced, “three hundred feet of altitude makes no measurable difference.” That was the signal to pack up all the apparatus and let the farmers draw their water again.

“This proves nothing,” Hooke said as they rode home through the dark. “The scale is not precise enough. But if one were to construct a clock, driven by a pendulum, in a sealed glass vessel, so that changes in moisture and baroscopic pressure would not affect its speed… and if one were to run that clock in the bottom of a well for a long period of time… any difference in the pendulum’s weight would be manifest as a slowing, or quickening, of the clock.”

“But how would you know that it was running slow or fast?” Daniel asked. “You’d have to compare it against another clock.”

“Or against the rotation of the earth,” Hooke said. But it seemed that Daniel’s question had thrown him into a dark mood, and he said nothing more until they had reached Epsom, after midnight.

THE TEMPERATURE AT NIGHTbegan to fall below freezing, and so it was time to calibrate thermometers. Daniel and Charles and Hooke had been making them for some weeks out of yard-long glass tubes, filled with spirits of wine, dyed with cochineal. But they had no markings on them. On cold nights they would bundle themselves up and immerse those thermometers in tubs of distilled water and then sit there for hours, giving the tubs an occasional stir, and waiting. When the water froze, if they listened carefully enough, they could hear a faint searing, splintering noise come out of the tub as flakes of ice shot across the surface-then they’d rouse themselves into action, using diamonds to make a neat scratch on each tube, marking the position of the red fluid inside.