No. 226. Method of obtaining a Segment of one-sixth of Vase.
As the diameter is about one-third of the circumference the elevational drawing of the cylinder gives one-third of the area and half a diameter gives one-sixth.
In the truncated cone shape the widths are similarly determined, but it will be found that the height, when measured on the centre line, is less than the lengths of the profile lines which constitute the actual height.
When the profiles are curved, the procedure is more complicated. As in the case of the cone shape, there is naturally some discrepancy between the height of the elevation and the profile, the actual dimension of which is affected by perspective (as also in plan curvature).
To obtain the actual height of the area the profile must be measured vertically with some flexible material, such as thin lead wire, which will readily embrace the curvature.
If a division of a third or a sixth is required the diameter or half diameter can be taken, but the segment of the area should be set off on a fresh centre line quite independent of the elevational drawing.
In order to obtain the true shape of the segment the elevation should be divided by horizontal lines drawn at the points of marked change in curvature, and these can be lettered or numbered for identification. The distance between each of these lines should be measured and set off on the new centre line, and then these can be used for drawing the parallels through.
The various diameters can be determined from the corresponding lines on the elevation. Lines drawn through the points thus obtained will give the required segment or area on the flat.
