In No. 3. the construction of the falling mould make a q b equal to the quadrant AB, No. 1. and draw av tangent to the arc a q b at a, and make a v equal to a v, No. 2. and the arc a q equal to the straight line AQ, No. 2. Draw the radii a z and b z, which will be a right-angle. Draw v parallel to a Make v v' equal to the breadth of the rail, as shewn by the plan, No. 1. ; and draw the concave side of the plan 71 a' q' b'. Then, having the seats of three points v,g,b', and the heights VW, QR and BX in No. 2. we may now proceed to find the face mould in the same manner as in finding the section of a cylinder, Fig. 1. and 2.; viz. draw the chord b' v', and draw v w, g r, and b' x perpendicular to by. Make v w, g r, zind b' x respectively equal to VW, QR, and BX, No. 2. Join x to and b' v, and produce them to meet each other in 8t and join x s. Draw ri parallel to x 8, and g i parallel to b' a. Join si, and produce b' v and is to meet in f, and join xf; and proceed to complete the face mould, which is the section of a cylinder, in the same manner as in Fig. 1. and 2. as is plain by the small letters instead of the large ones.
No. 4. Shews the construction of the face mould for the upper part of the circle, which is the same as in No. 3. only the heights must be taken downwards, and from the lower side of the falling mould.
No. 5. Shews the application of the face mould to the plank, according to the following description. This dia gram exhibits the two sides and the edge of time plank sketched out in one plane, as shewn for the circular sash frame in a circular wall. NV is the plane of the top of the plank, X that of the edge, and Y the under face of the same. Previous to commencing the operation of cutting
out the rail, the plank must be first bevelled on the edge. In this case, for the lower piece, it must he taken off the under edge, according to the angle b' n h, No. 3. ; that is, the two planes \V and X must form an acute angle equal to the angle b' n h. Place the face mould No. 3. upon the plane NV, and make the chord line coincident with the edge AB, that is, the extremities of the concave edge to AB. Draw cd on the edge of the plank, making the angle B c d equal to the angle f x b'. No. 3. Having drawn the lines on the top, round the face mould, apply the same point that was at c to d on the other side, and bring the chord to coincide with the edge CD. Then the plane of the mould coinciding with the plane of the plank, draw lines on the plank round the edges of the face mould as before. The lines being thus drawn, the superfluous parts that were on both sides of the mould are to be cut away, in such a manner as to form the surfaces of a cylinder on both sides. The falling mould constructed at No. 2. must be bent upon the convex side, and lines drawn upon that side round the edges of the falling mould. The super fluous wood must be cut off from the top of the piece, ac cording to the upper line, by means of a square or right angle ; but, in squaring, the edge of the stock of the square must be always placed parallel to the axis of the cylinder. The other edge will be found by guaging the piece to its thickness on both sides, which is generally about two inches.