Our present method of dating by a continuous era, giving each year its number in the series as its ordinary name, has this great convenience that we can calculate the space of years between two dated events by a simple subtraction. But if we find, to take an actual example from our oldest chronicle, that a certain event is dated in the ninth year of the emperor Theodosius II, and another event in the second year of the emperor Phocas, then in order to calculate the distance of years between, we must first know the length of each imperial reign from Theodosius to Phocas. The old chroniclers were constantly at the trouble of making calculations of this kind, calculations to which certain errors were incidental. Small errors accumulating become great errors, and so as a safeguard and corrective, here and there in the chronicle, at the record of some important event, we find these summaries of years. In the year 664, a very destructive plague broke out in Ireland. To the record of the event, the chronicler adds: "From the death of Patrick, 203 years." So the seventh-century chronicler knew 461 as the year of Patrick's death. There are various things that indicate that professed paganism continued to exist in Ireland in the second half of the sixth century, _i.e._ for a century at least after Saint Patrick's death. By that time, however, as I have shown in the sixth lecture, a blending of the old native culture and the newly introduced Christian learning had taken place. And just as two elements in the chemical sense unite to form something that seems to have a nature and virtue all its own and not derived from the quality of either component, so this blending of two traditions in Ireland brought forth almost a new nation, with a character and an individuality that gave it distinction in that age and in the after ages. Mr. Romilly Allen, in his book on "Celtic Art," has something to the purpose. "The great difficulty," he writes, "in understanding the evolution of Celtic art lies in the fact that, although the Celts never seem to have invented any new ideas, they professed an extraordinary aptitude for picking up ideas from the different peoples with whom war or commerce brought them into contact. And once the Celt had borrowed an idea from his neighbour, he was able to give it such a strong Celtic tinge that it soon became something so different from what it was originally as to be almost unrecognisable." There is a mixture of truth and error in this statement that is characteristic of a great deal of modern scientific comment. For the explanation of a fact, something is offered which, upon close examination, is seen to be no more than the unexplained thing stated again in different terms. Why do masses of matter tend to approach each other? Because of the law of gravity. What do we mean by the law of gravity? We mean that masses of matter tend to approach each other.