Daniel was going for days without hearing any other sounds but these. All possible subjects of conversation could be divided into two categories: (1) ones that would cause Drake to unleash a rant, previously heard so many times that Daniel could recite it from memory, and (2) ones that might actually lead to original conversation. Daniel avoided Category 1 topics. All Category 2 topics had already been exhausted. For example, Daniel could not ask, 'How is Praise-God doing in Boston?' * because he had asked this on the first day, and Drake had answered it, and since then few letters had arrived because the letter-carriers were dead or running away from London as fast as they could go. Sometimes private couriers would come with letters, mostly pertaining to Drake's business matters but sometimes addressed to Daniel. This would provoke a flurry of conversation stretching out as long as half an hour (not counting rants), but mostly what Daniel heard, day after day, was corpse-collectors' bells, and their creaking carts; the frightful Clock; cows; Drake reading the Books of Daniel and of Revelation aloud, or playing the virginal; and the gnawing of Daniel's own quill across the pages of his notebook as he worked his way through Euclid, Copernicus, Galileo, Descartes, Huygens. He actually learned an appalling amount. In fact, he was fairly certain he'd caught up with where Isaac had been several months previously – but Isaac was up at home in Woolsthorpe, a hundred miles away, and no doubt years ahead of him by this point.