§ I. 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. § II. 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. The lateral forces which walls have to sustain are of three distinct kinds: dead weight, as of masonry or still water; moving weight, as of wind or running water; and sudden concussion, as of earthquakes, explosions, &c. Clearly, dead weight can only be resisted by the buttress acting as a prop; for a buttress on the side of, or towards the weight, would only add to its effect. This, then, forms the first great class of buttressed architecture; lateral thrusts, of roofing or arches, being met by props of masonry outside--the thrust from within, the prop without; or the crushing force of water on a ship's side met by its cross timbers--the thrust here from without the wall, the prop within. Moving weight may, of course, be resisted by the prop on the lee side of the wall, but is often more effectually met, on the side which is attacked, by buttresses of peculiar forms, cunning buttresses, which do not attempt to sustain the weight, but _parry_ it, and throw it off in directions clear of the wall. Thirdly: concussions and vibratory motion, though in reality only supported by the prop buttress, must be provided for by buttresses on both sides of the wall, as their direction cannot be foreseen, and is continually changing. We shall briefly glance at these three systems of buttressing; but the two latter being of small importance to our present purpose, may as well be dismissed first. § III. 1. Buttresses for guard against moving weight and set towards the weight they resist.