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thread on the end of a pm when the latter is being driven into place. (Plate II., mg. 8 •ノ 
Pin. 一 A cylindrical piece of iron used to connect bridge members. ( Plate II., Fig. B.) 
: Pitch. — The distance between centres of consecutive rivets of the same row. 
Plane of Symmetry. 一 A plane dividing a body into two equal and symmetrical P al ^ s 
similarly disposed in ret'eronco to tlio plane. 
Plant. — Tools and apparatus used in construction. 
Plat©, — A piece of flat iron wider than a bar. The common distinction between the 
is that a plate is attached to something else, and acts with it, while a bar is an independent 
member. 
Plate Girder. 一 A beam, built of plates and angles, used to span a small opening, general 
less than sixty feet. 
Pony Truss. 一 A truss so shallow as not to permit the use of overhead bracing. 
Portal. — The space between the batter braces at one end of a bridge. Sometimes the tei ，rö 
is applied to the portal bracing, though incorrectly. 
Portal Bracing. 一 The combination of struts and ties in the plane of the batter braces at a 
portal, which transfers the wind pressure from the upper lateral system to the abutment or l^ er, 
Portal Strut. 一 A strut belonging to tho portal bracing. (Plato I.) 
Positive Rotation. — Rotation in the direciion of the hands of a watch. 
Po»t. — A vertical strut. (Plate I.) 
Pi.att Truss, (called also the “ Murphy- Whipple,” or (< Quadrangular truss). — A sing】 e 
intersection trass with vertical struts and diagonal ties. 
Quadrangulär Trugs. — See Pratt truss. 
Racliet Drill. 一 A hand machine for drilling rivet holes. (Plate XII. t Fig. 4.) 
RarUus of Gyration, 一 The radius of gyration of any surface in reference to an axis is 作 し 
distance from the axis to that point of the surface in which, if tlie whole area were concentrat 0 * 
the moment of inertia in reference to the axis would be uncliangecl. It is therefore equal to 
square root of the ratio of the moment of inertia over the area. 
Ream. 一 To enlarge a rivet hole, 
Reamer. 一 A tool for enlarging rivet holes. (Plate XII., "Fig. 13.) 
Re-enforcin |； Plat©. 一 A plate used for tho purpose of providing additional pin beariD^ # 
strength to compensate for material cut away. (Plate II., Fig. 10.) 
Re-railing Apparatus. — A contrivance for returning to the track a derailed vehic ら 
(Plate Y.) 
Resolve. 一 To amde a force into its component parts. 
一 k Japanese mile equal to about 2 | Euglish miles. 
Rivet. — A short piece of round iron tightly connecting two or more thicknesses of 
and having, when in place, a head at each end. 
Roadway. 一 Tlie passage-way of a bridge for vehicles ； usually means dear roadway, (Z* v, 
11 o<l. — A piece of round iron. 
Roiled Beam. — An I-beam. (Plate II., ¥ig. 2.) 
Roller. — See expansion roller. 
Roller Frame. — « ■ A light frame of iron for holding the rollers in position (Plate ’ 
Fig. 16.) 
Roller Plate. 一 The plate upon wliicli the rollers rest, and which itself rests up£> a 
masonry (Plate II., Fig. 12.) 
Rope Sling. 一 See sling. 
liun. 一 A line, or string ； as, a run of joists. 
Set. 一 The extension or compression of a piece of material under stress. ^ 
Sliear, or Sfioaring-Strea». — The resistance which a body offers to the passage^ 01 * 
tendency to passage, of one section along the next consecutive section. … 
Siiim. — A filling piece. Here applied to a timber between the track-stingers and 1 
(Plate I and Plate II., Fig. 14.) 
