The burdensome restrictions imposed on their Natural-Philosophic researches by the host made it necessary to convene a very long Symposium in the Kitchen, where Hooke and Wilkins dictated a list (written by Daniel in a loosening hand) of experiments that were neither noisy nor smelly, but (as the evening wore on) increasingly fanciful. Hooke put Daniel to work mending his Condensing Engine, which was a piston-and-cylinder arrangement for compressing or rarefying air. He was convinced that air contained some kind of spirit that sustained both fire and life, which, when it was used up, caused both to be extinguished. So there was a whole range of experiments along the lines of: sealing up a candle and a mouse in a glass vessel, and watching to see what happened (mouse died before candle). They fixed up a huge bladder so that it was leak-proof, and put it to their mouths, and took turns breathing the same air over and over again, to see what would happen. Hooke used his engine to produce a vacuum in a large glass jar, then set a pendulum swinging in the vacuum, and set Charles there to count its swings. On the first really clear night at the onset of winter, Hooke had gone outside with a telescope and peered at Mars: he had found some light and dark patches on its surface, and ever since had been tracking their movements so that he could figure out how long it took that planet to rotate on its axis. He put Charles and Daniel to work grinding better and better lenses, or else bought them from Spinoza in Amsterdam, and they took turns looking at smaller and smaller structures on the moon. But here again, Hooke saw things Daniel didn't. 'The moon must have gravity, like the earth,' he said.