nitrogen, he might have found by a scrutiny much more accurate still yet another gas, helium, which we now know to exist in extremely minute quantities in the air. But meantime this discovery was made in another way. [Sidenote: Still something to be explained.] [Sidenote: Probably nutation.] [Sidenote: His nineteen years' campaign.] When Bradley had extracted all the aberration from his observations he found that there was something left, another problem to be solved and some more thinking to be done to solve it. But he was now able to profit by his previous labours, and the second step was made more easily than the first. The residuum was not the parallax of which he had originally been in search, for it did not complete a cycle within the year; it was rather a progressive change from year to year. But there was an important clue of another kind. He saw that the apparent movements of all stars were in this case the same; and he knew that a movement of this kind can be referred, not to the stars themselves, but to the plumb-line from which their directions are measured. He had thought out the possible causes of such a movement of the plumb-line or of the earth itself, and had realised that there might be a _nutation_ which would go through a cycle in about nineteen years, the period in which the moon's nodes revolve. He was not mathematician enough to work out the cause completely, but he saw clearly that to trace the whole effect he must continue the observations for nineteen years; and accordingly he entered on this long campaign without any hesitation. His instrument was still that in his aunt's house at Wansted, where he continued to live and make the observations for a few years, but in 1732 he removed to Oxford, as we shall see, and he must have made many journeys between Wansted and Oxford in the course of the remaining fifteen years during which he continued to trace out the effects of nutation. His aunt too left Wansted to accompany Bradley to Oxford, and the house passed into other hands. It is to the lasting credit of the new occupant, Mrs. Elizabeth Williams, that the great astronomer was allowed to go on and complete the valuable series of observations which he had commenced. Bradley was not lodged in her house; he stayed with a friend close by on his visits to Wansted, but came freely in and out of his aunt's old home to make his observations. How many of us are there who would cheerfully allow an astronomer to enter our house at any hour of the night to make observations in the coal-cellar! It says much, not only for Bradley's fame, but for his personal attractiveness, that he should have secured this permission, and that there should be no record of any friction during these fifteen years. At the end of the whole series of nineteen years his conclusions were abundantly verified, and his second great discovery of nutation was established. Honours were showered upon him, and no doubt the gentle heart of Mrs.