A very common form of base for a ventilator or cowl is that shown in Fig. 200, and known as a "tall-boy" base.

Fig. 200.

It is either square or rectangular at the bottom, ana circular at the top. On examining the plan in Fig. 201, it will be seen that the curved part of the article t 0 3 will be exactly a quarter of an oblique cone whose apex may be considered to be at t. Four of these equal segments will, of course, make up the curved portion of the surface; the remaining parts being flat triangles.

In making the pattern, a half-elevation and quarter-plan is first drawn (Fig. 201). The quarter-circle in plan is divided into three equal parts, and the division points joined up to t. The point b is now swung around d as centre on to the base line, and connected up to e. To get the true lengths of the lines, of which 0 t, 1 t, and 2 t are the plans, their lengths are set along the base line from 0 and joined up to e; the respective true lengths, therefore, being 0' e, 1' e, and 2' e. The middle line B 3 of the pattern will be made equal in length to the line so named in the elevation; then T T, drawn at right angles, and the lines B T cut off equal to b t from the plan. With centre T and radii respectively equal to e 2', e 1', and e 0', arcs of circles are described. Then opening the compasses to the length of one of the arcs (say 3 to 2) in the quarter-plan, and commencing at point 3 on the pattern, the points 2, 1, and 0 are obtained by cutting the first drawn arcs. The compasses are now set to the length of the end line on the elevation, and with 0 on the pattern as centre, an arc drawn (shown passing through F): this being cut by another arc which is described from point T, with radius equal to t f; and so the point F is obtained. An even curve joining up the points 0, 1, 2, etc., is drawn, and allowances put on the sides for grooving, and on the bottom for the flange, and the pattern is complete.

Fig. 201.

After having obtained one-half the pattern, it will be quite accurate enough for practical work to produce the lines B 3 and F 0 until they meet in A, and use this as a centre in a similar way to that explained in connection with Fig. 202. If the tallboy base is square at the bottom, then a portion of the pattern for one-eighth of the surface will be all that is required to obtain the centre A; the line down the middle of the corner meeting the line along the centre of a side.

Fig. 202.

After the base has been formed into shape, grooved-up, and the bottom flange bent over, corner plates are riveted on to the flanges as shown in Fig. 200.

Another method for marking out the pattern, which is quite good enough for ordinary practice, is shown in Fig. 202. The line d t in the plan is swung around d on to the base line, the point t1 then being joined up to e, and produced to meet the centre line in c. The compasses are set to c e on the elevation, and taking a point C on a line like C T in the pattern, the arc 0 to 2 is drawn. The lengths 1 to 0 and 1 to 2 are now made equal to the lengths of the correspondingly numbered arcs in the quarter-plan. The line C T is measured off equal to the line c tl in the elevation. The compasses are next fixed respectively to the radii t 2 and t f, and the arcs at B and F described. Lines are then drawn touching these arcs, and passing through the points 0 and 2 to meet in A. This gives what may be called an approximate centre for the describing of the pattern. Taking A as centre and A T as radius, the part circle is drawn; then commencing at T the sides and ends are stepped along as shown. The end lines of the pattern are best obtained by marking full end lengths on the arc, bisecting them, and then joining up to A as shown in the figure. The inside curve of pattern is marked out by taking A as centre and A H as radius (this being the average between A 0 and A 2), and running around to meet the seam lines. Allowances as required must, of course, be put on as in the last method. It will be observed that in this case the pattern (Fig. 202) is developed for the whole surface.