58

HISTORY AND PROGRESS

side of the rail, forming a tangent to the arcit would describe; the pin, in such cases,must therefore yield to the action of theweight; and, consequently, have a tendencysimilar to the rail above described ; this, how-ever, is the only imperfection it has, for theoverlap effectually prevents the distortion ofthe joinings of the rails ; whereas, in the othermodes, the ends are liable to rise and get outof the same plane.

Effectually to preserve the continuity of therail with ease and freedom, the stone shouldbe capable of moving round, or assuming anydegree of inclination, to the line of the roadthat might occur in practice, without strainingeither the pin or distorting the ends of therails: to effect this, if the pin be made thecentre of motion, the under side of the railshould be a portion of the circumference of acircle, formed from the pin as a centre ;—thebase of the chair could then be either the apexof a curve, or a circular cavity correspondingwith the exterior semi-circular surface of therail. The stone might then be depressed oneither side, without straining the pin orderanging the joints ; or w e might otherwisemake the bearance of the rail upon the chairor pedestal the centre of motion ; in such case,the pin-hole should be a circular slit or open-