The construction of a lobster-back cowl (Fig. 221) follows somewhat similar lines to that of a quarter-bend, made up in segments, as shown in Chapter IV (Pipe Bends In Segments. Quarter-Bend For Round Pipes). In some cases where the throat part is curved the setting out of the patterns will be exactly the same as the quarter-bend; but in the present case, where the mouth and bottom pieces of the cowl meet, and are square to each other, some modification of the pattern is required.
A side elevation of the cowl, exhibiting the arrangement of the segments, is shown in Fig. 222. The curved part of the back is formed of a quadrant of a circle, and is usually, as in this case, divided into four equal segments. The mouth and bottom pieces are respectively produced in to the dotted lines a b and a c, being themselves connected along the line o a. The four back segments are thus, as it were, joined on to these.
The construction lines for the mouthpiece are obtained by describing a semicircle on the line 0 6, dividing this into six equal parts, and running lines square to the diameter and across to the dotted line a c. The pattern is set out by first marking down the girth line equal in length to twelve times one of the small arcs on the semicircle, drawing lines square across, and cutting them off equal in length to the construction lines measured on either side of 0 6 in the elevation. Thus, to show one line, the parts 4' 4° and 4° 4" on the pattern are equal to the same figured lines in the elevation.
The bottom pattern will be. marked out in the same way as the mouth pattern, and it should be noticed that the ends of both of these are cut away, so as to form the square throat, when bent and fixed in position. A small lap is allowed on to the end cuts, d et of the mouth pattern, to cover for turning inside the bottom piece and riveting, if required.
The pattern for a back strip can be marked out from the same construction lines as previously used; but here the girth line will be shorter, being equal in length to ten only of the small arcs on the semicircle; this shortness being arranged so as to avoid all the strips meeting in a point under the throat. The width of the different parts of the pattern-strip will be measured from the line 0 6 in the elevation up to the centre line of the segment. Thus the distance 2° 2" on the pattern will be the same as 2° 2" on the elevation, and so for all corresponding lines. A lap will be allowed on one side of the strip, and also on both ends, so that they may be brought round and riveted on to either the mouth or bottom piece. Allowances have also been put on mouth and bottom pieces, to cover for wiring on the outer edges.
The wind-vane can be cut out any shape to suit the individual taste, a lug being left on the bottom to turn over and rivet on to the cowl-head.
In order that the head may revolve, a spindle is rigidly fixed along the centre of the pipe-shaft, which should fit into a centre on the head. This arrangement, however, is so well known that there is no need to give further details.