THE TEMPERATURE AT NIGHT began 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.