The other way is, to make directly over the void spaces Vaulted Arches with Stones cut corner-ways and tending to one Center. For the Walls be so strengthned by the means of these easements, that part of the Wall which is below will not sink at all being easied of the load of the part that is above, and if some defect should happen by tract of time, it may be mended without propping that which is above. The second way of easing, is, for Walls that are made to support the Earth; for, besides the extraordinary thickness which they ought to have, they should have likewise Buttresses on that side next the Earth, so far distant one from another as is the breadth of the Wall; they ought likewise to have an _Emparement_ or large Foundation which must be equal to the height of the Wall, so that they go diminishing by degrees from the bottom to the top, where they come to equal the height of the Wall. [Sidenote: _Lib. 1. Chap. 5._] The effect of these _Buttresses_ is not only to support the Earth by their Resistance, but likewise to lessen its Efforts when it swells, in dividing it into many parts. [Sidenote: _Lib. 6. Chap. 11._] And if it be judg'd that these _Buttresses_ be not sufficient, the Wall also which supports the Earth must be strengthned with other _Buttresses_ within. ART. V. _Of Flooring and Ceiling._ There are four sorts of Flooring, some are upon the _Superficies_ of the Ground, others between two Stories, others make the Roof of the House in Plat-form, and the last is _Plat-Fond_. To make those Floors that are upon the Ground, you must first make the Earth smooth and plain, if it be firm and solid, if not, it must be beaten with a Rammer with which they ram down their Piles; and after having cover'd the Earth with the first _Lay_ or _Bed_, call'd _Statumen_ by the Ancients, which was of Flinty Stones about the bigness of ones Fist, among which was mixed Mortar made of Lime and Sand. Then they laid the second _Bed_, which they call'd _Rudus_, which was made of lesser Stones, of which there were three Parts for one of Stone if they were new, for if they were taken out of old Buildings, five parts of Stones or Pibbles would be required for two of Lime. [Sidenote: _Lib. 7. Cap. 4._] The _Greeks_ had a way of making their Floors in those low places where cold and humidity ordinarily reign, which freed them from these Inconveniences. They digged the Earth two Foot deep, and after having beaten it well, they laid a Bed of Mortar or Cement a little sloping from either side to the Channel, which convey'd the Water under Ground; they laid a Bed of Coal upon the first Mortar, and having beaten them well, they cover'd them with another Cement or Mortar made of Lime, Sand and Ashes, which they made smooth when it was dry with a Polishing-Stone. These Floors presently drank up the Water that fell upon them, that one might walk barefoot without being incommoded by the Cold.