The close of the eleventh century--The “new manner of building”--Conditions necessary to an _arcuated_, as distinguished from a _trabeated_, style--First principles of Grecian and Roman architecture--Rationale of the _arcuated_ style--Its developments--Cloisters of St. Paul without the Walls and St. John Lateran, Rome--Doorways--Windows--Vaulting over spaces enclosed by walls or ranges of piers--Simplest elements defined--Barrel-vaults--Hemispherical vaults or _domes_--Groined vaults. When I delivered my last lectures in this Academy, it was my intention to give a practical sketch of the history and development of architecture in this country from the earliest rise of civilisation among the races of which our nation is composed, down, perhaps, to the period of the revival of Classic architecture. As, however, such continuous history has been disturbed by the omission of my lectures last season, and as few now present heard, and fewer, probably, now remember, those lectures, it is not my intention to continue my former course, but, adopting as my stand-point the stage at which I had then arrived, to digress into an inquiry into some of the practical and artistic principles of the class of architecture of whose development I was then treating. The chronological point which I had reached was the close of the eleventh century,--a point well fitted to be chosen as one for leaving the beaten track for the purpose of inquiring into principles. It was the very stage at which the great round-arched style, which had just developed itself into a strong and sturdy luxuriance, was in the condition best suited to receive the refinements of art. It was, too, the very eve of that wonderful politico-religious movement which was to bring the nations of the West into contact with the East;--thus preparing the way for a vast influx of new ideas and of fresh artistic elements; and, so far as our own country was concerned, it was just the moment when the simple and unambitious architecture of the Anglo-Saxon race had given place to the more colossal edifices and the more systematic style of the Norman invaders; and when the newly-imported architecture, having taken firm root in our soil, was ready to become naturalised as our own, and to be pressed forward in all zeal and earnestness by the united races which--now neither Saxon nor Norman--were becoming, to all intents and purposes, _English_.