Sometimes a ventilator base follows the design shown in Fig. 209, which it is not difficult to imagine represents the intersection of a round pipe and cone for the top, and a cone and square pipe for the bottom.

The patterns can be struck out in the way shown in Fig. 210. The half-elevation and part plan are drawn as in the last case. The arc 0 3 is divided into three equal parts, and the division points joined to d. Lines d 1', d 2', and d 3' are turned around on to the base line, and perpendiculars run up to meet the line 0 c in points 3", 2", etc. The pattern for the conical part is obtained by fixing the compasses to the length c 0 describing the arc as shown, and setting along it twelve lengths each equal to the length of one of the corresponding arcs in the plan. After the radial lines are drawn in, the compasses are set respectively to the lengths c 0", c l", etc., in the elevation, and the arcs on the pattern drawn. Where these cut, the same numbered line will give points on the curve, which can be joined up as shown. The inner curve of the pattern is, of course, marked out by using a radius equal to c 1 from the elevation.

Fig. 210.

The pattern for one side of the square base is shown set at the top of Fig. 210. Here the line 3' 3' is made twice the length of 0' 3' from the plan, the division points being the same. Lines are drawn square through each point, and cut off respectively equal to 0' 0", 1° 1", 2° 2", and 3° 3" from the elevation. The resulting curve is (to those who understand geometry) a hyperbola, and may be set out by other methods common to that curve. None, however, are simpler than the one shown.

Very many different kinds of bases for ventilators are made; but sufficient has perhaps been shown to explain the general principles involved in the marking out of the patterns for flat-bottomed bases. The above can easily be modified to cover the setting out for bases resting on the ridge and sides of a roof.