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The tangent at any one of these points, as for example m,m', is easily obtained. The tangent plane to the cylinder at this point is vertical, and has for its horizontal trace the tangent mn, to the base abc....

Now the tangent required is the intersection of this tangent-plane with the given one; the projections therefore of this tangent are mn and qp'.

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2°. To determine the true magnitude and form of the section let us suppose the plane pap turned down on one of the planes of projection by rotating on the trace qp'. The point m, m' is situated in space on a perpendicular to qp' at a distance from m' equal to mm": hence by drawing mm at right 28 to qp' and making it equal to mm", m will be one point of the curve sought. By the same construction as many points may be found as may be deemed neces

sary to allow the curve abc ... being drawn. To ļraw the tangent to this curve, at the point m, it is only necessary to observe, that in the rotation of the plane the line an will fall in qn, perpendicularly to qp', so that by drawing MN, we obtain the tangent at the point m.

If the plane pap' be turned round on its horizontal trace pq, the curve of the section may also be drawn of its true magnitude in the horizontal plane of projection. Draw mr perpendicular to pq; if we then conceive a line in space between r and the point m, m', this line will be perpendicular to pq and will be projected of its true magnitude in qm': if then m'r, in mr, produced, be made equal to qm', the point m' will be one in the curve of the section, when turned down on the plane of projection, and on the same principles any requisite number of other points may be obtained; nm will obviously be the tangent to the curve in min' when this curve is turned down.

There is yet another construction for this purpose, which may be occasionally advantageously employed. Let the plane pap be supposed to turn round on a line perpendicular to the vertical plane, as on dd", d' till it assume a position parallel to the horizontal plane; the curve, lying in this plane, will then be projected on the horizontal plane of its true form and magnitude: to obtain points in this projection, it is only necessary to make du” equal to d'm', as will be readily understood from the figure by what has been already explained. The point of contact of the tangent is now in m"; to draw the tangent at that point it is only necessary to deduce a second point in it, as n”, on the same principle as that by which m" is obtained, the line nn" being drawn parallel to xy for the horizontal trace of the plane, in which the trace of the tangent lies during the rotation of the plane pop on dd", d'.

N" will conse

quently lie in nn" thus drawn. It will also be observed that the point o in which the tangent nm, qm' cuts the axis of rotation dd", d', not being changed during the rotation of the plane, the tangent nom" will pass through o.

3°. If a cylinder be considered as a prism with an infinite number of faces, it is clear that if this surface be developed, or unrolled, every section perpendicular to the generatrixes will become a right line on the developement at right angles to the edges, or arrises, of the prism when developed ; and the length of these arrises, comprised between the oblique and perpendicular sections will not be

altered. If therefore the curve abc. be divided into any number of arcs ab, bc, ed, &c., and a line be drawn, and made equal to the curve abc by setting off along it the equi-distances , Bry, y8 ... respectively equal in number and magnitude to the arcs ab, bc, cd, ... then by setting the true lengthsof the respective arrises

along perpendiculars, aa', BB', р i

WW, 88... to the line a ..., 8

the curve a'B'y drawn through these points will be the develope

ment of the curve of the obliquè B

section. The true lengths of the á

arrises are furnished by their projections a'a", b'B.... on the ver


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tical plane.

Let the point m, m' be represented by u' on the developed curve; then, since the elements of the original curve

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do not change their angle of inclination to the generatrixes, or arrises, by being developed, the tangents, which are these elements produced, preserve their inclination to the arrises when developed. Now the tangent at the point mm' passes through n, and the angle' made by this tangent with the arris of the cylinder is an angle of the right-angled triangle, of which mn is the base, and mom the altitude: if, therefore, u v be made equal to mn, and u'y be joined, this line will be the tangent to the curve.

Scholium. In the figure the curve abo... is a circle, but the construction above described is not the less general : whatever may be the curve abc.. it will always be found that the tangents in m and m’, to the curves abc ... A BC..., cut the trace pq in the same point n, or, which is the same, the tangents in m and m' to the curves abo... A BC..., cut the right line dd" in the same point 0; hence this remarkable

Theorem. If any, whatever, cylinder be cut by any number of planes, passing through a right line perpendicular to its generatrixes, and all the sections are brought into one plane by being rotated on this common line, the tangents to the different curves of the sections, at points in them, situated in one line perpendicular to the common one above-mentioned, will all meet this common line in the same point.

This property is analogous to the well-known one of ellipses described on a common axis; and is true in this case only inasmuch as these ellipses are the intersections of the same cylinder by planes fulfilling the above-stated conditions.


Section of a cylinder by a plane perpendicular to its

generatrixes. 48. Let abc be the horizontal trace of the cylinder, and let the outlines of the cylinder be determined according to the construction to Proh. 1, p. 53. The cutting plane being perpendicular to the generatrixes, its traces must be per

pendicular to the projections of these lines (Prob. 14, p. 41); let qp and qp' be these traces. The first step is to find the intersections of the generatrixes with the plane pop: let a vertical plane be conceived to pass through one of these, ae a'é for example. The horizontal projection of the intersection of this plane with pop' is ae, and one point in the vertical

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