CYLINDROID, (from taAtvopog, cylinder, and etdoc,form) a solid of such property, that all sections parallel to either end, are equal and similar ellipses, and that a straight line, called the axis, will pass through the centre of the ellipses.
All the axal sections of a cylindroid, and every section parallel to an axal section, are parallelograms or rectangles.
All parallel sections of a cylindroid are equiangular, and of equal length to the axis.
If an oblique cylinder be cut by a plane perpendicular to the axis, near to each end, it will be cut into three parts, of which the middle portion will be a cylindroid.
The cylindroid is frequently employed in vaulting, instead of a segmental cylinder, less than the half, where the height would not admit of a semi•cylinder. It is freqnently em ployed in the composition of groins, and where the transverse openings vary in their horizontal dimensions, and where it is required to keep the angles of the groin straight, one of the simple vaults is necessarily a semi-cylindroid.
The solidity of a cylindroid is found, as in the prism or cylinder, by multiplying the area of one of the ends by the distance between the two.
The superficial content of the curved surface is found by multiplying the girt by the length of the axis, as in the cylinder.
The method of finding the envelope is the same as that of a cylinder with an edge, so that when the envelope is lapped round the solid, its edge may coincide with a plane passing through three given points.
The method of finding the envelope's of cylinders and cylin droids, is one of the most useful parts of Stereography, not only in forming the coverings of bodies, but in forming the angles of all parts of work, where the surf:Ices of two different solids meet each other.