§ 1. We have hitherto supposed ourselves concerned with the support of vertical pressure only; and the arch and roof have been considered as forms of abstract strength, without reference to the means by which their lateral pressure was to be resisted. Few readers will need now to be reminded, that every arch or gable not tied at its base by beams or bars, exercises a lateral pressure upon the walls which sustain it,—pressure which may, indeed, be met and sustained by increasing the thickness of the wall or vertical piers, and which is in reality thus met in most Italian buildings, but may, with less expenditure of material, and with (perhaps) more graceful effect, be met by some particular application of the provisions against lateral pressure called Buttresses. These, therefore, we are next to examine.

§ 2. Buttresses are of many kinds, according to the character and direction of the lateral forces they are intended to resist. But their first broad division is into buttresses which meet and break the force before it arrives at the wall, and buttresses which stand on the lee side of the wall and prop it against the force.