BUILDING. 



is one which rests upon a plan in the form 

 of the letter L : a tee roof upon a plan in 

 the form of the letter T; and an aitch 

 roof upon a plan formed like the letter 

 H ; but when two common roofs, having 

 their ridges parallel to each other, and a 

 side of the one either joins one of the 

 other, or these two sides intersect each 

 other, and thereby leave a gutter above 

 the roof; then the roof which is thus com- 

 pounded of the two simple roofs is call- 

 ed an em roof, as the vertical section is in 

 the form of the letter M : or rather an in- 

 verted W as M : this is an instance where 

 the roof is denominated by the vertical 

 section, and not by tbe plan. All roofs 

 whatever are said to be truncated, whe- 

 ther they terminate in a plane or raised 

 platform, or have a void at the top, bound- 

 ed by a level curb. 



When the side of a roof is a plane sur- 

 face, except at the eaves, at which place 

 it is concave, the roof is said to have a 

 bell cast at that place. 



The general names of the timbers are, 

 straining pieces, tie pieces, and bearers ; 

 under straining pieces are included, prin- 

 cipal rafters, camber beams, hip and val- 

 ley rafters, collar beams, or straining 

 beams, straining sills, struts, auxiliary raf- 

 ters, or principal braces and studs. 



Under the pieces are included, tie 

 beams, diagonal ties, and truss posts; and 

 under bearers are included, plates, pur- 

 lins, common rafters, small rafters, ridge 

 pieces, boarding and dragon beams. 



The sloping sides of roofs are of two 

 kinds, single and double, or plain and 

 carcase: single roofs are those which have 

 one row of rafters upon the same side ; 

 double* o* carcase roofs are those which 

 have two ties of rafters; the lower tie sup- 

 porting the upper by the intervention of 

 transverse pieces called purlins. 



Stone liridges. A stone bridge is a thick 

 wall built across a hollow, with one, two, 

 three, or a series of apertures, formed in- 

 to arcades, which either serve to lighten 

 the masonry, or to give passage to a 

 stream of water, or both. 



"When a stone bridge is resolved upon, 

 the first consideration is its place : in this 

 several particulars should be taken into 

 consideration, and the advantages com- 

 pared to the disadvantages. As the height 

 of the bridge depends on the banks of 

 the river, the expense will be increased 

 according to their height: therefore acon- 

 venient situation should be chosen, where 

 the banks will be adequate in height to 

 that necessary for the bridge, though the 

 expense will be increased by the length 

 of the bridge. In most cases, where the 



river runs in a valley, a wide part of 

 the stream must be preferred to a narrow 

 part, as the water at this narrower part 

 has not only a greater degree of velocity 

 of itself, but the velocity would also be 

 increased by the piers of the bridge ; in 

 times of heavy floods it would be liable 

 to be thrown down, and in a navigable 

 river the navigation would be impeded. 

 As the expense depends on the bed of 

 the river, it must also be taken into the 

 account. 



These being settled, the form and 

 height of the arches come next under 

 consideration; the height of the arches, 

 which determine that of the bridge, de- 

 pends on the rise of the water in time of 

 floods ; and whether there is to be a na- 

 vigation, and what kind of vessels there 

 are to pass. 



Stone bridges ought to be constructed 

 with as few arches as possible, which will 

 not only give greater beauty, but will re- 

 quire fewer foundations, piers, and cen- 

 terings, and also easier passage for craft. 

 The piers ought to be so proportioned as 

 to enable them to withstand the thrust of 

 the adjacent arches, though the rest were 

 thrown down. The number of arches 

 ought to be odd, in order that one may 

 stand in the middle, where the stream 

 has its greatest velocity. 



When the passage-way along the top 

 of the bridge is a convex curve, the arch- 

 es should diminish from the middle to- 

 wards each extreme, so as to be similar 

 to the middle one ; this will allow a more 

 free passage to the water, the velocity 

 being greatest in the middle. With re- 

 spect to the choice of arches, the ellipti- 

 cal, cycloidal, and equilibrated arches, 

 are not only convenient, in allowing more 

 room for the passage of ships at the 

 hanches, but they require fewer materials 

 than most other curves of the same di- 

 mensions. 



When the extrados is convex, and the 

 height of the arch small in proportion to 

 the span, a segment of a circle may be 

 used with success : in this case the arch 

 should not exceed 60 degrees. 



These particulars being fixed, the prac- 

 tice is as follows: When the foundation 

 of a stone bridge is to be laid in a river 

 which is not very deep, a single or dou- 

 ble inclosure of wood is formed, and the 

 intervening space is rammed well with 

 clay or chalk, to prevent the water from 

 coming in. These inclosures are either 

 made with piles driven closely together, 

 and dovetailed at their jointings, or by 

 piles driven at certain distances from one 

 another, and grooved on the sides oppo- 



