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Thus, if pp be drawn from P, perdicular to the plane mn, p is the projection of p upon the plane mn. And mn is called the plane of projection. It is also evident that p is the projection of

every point of the line pp. 3. The projection of a line upon a plane is the line formed by the projections of every point of the former line.

From the different points of the curve a B, let fall perpendiculars on the plane mn; the curve line ab, formed by the feet of these perpendiculars, is the projection of the curve AB.

The perpendiculars so drawn will form a surface, called, from the parallelism of the lines which may be supposed to generate it, a cylindrical surface; ; and for this reason it is often known by the name of the projecting cylinder.

The projection ab may be considered as the intersection of the projecting cylinder with the plane mn; and it is evident that every curve, as A' B' traced on this cylinder, will have ab for its projection. 4. When ab is a plane curve, and in a plane perpen

dicular to the plane of projection, the projecting cylinder coincides with the plane of the curve; and the projection of the curve as is the straight line ab: but when the curve is in a plane parallel to the plane of projection, it is manifest that the projection is an equal curve.

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5. The projection of a straight line ab is a line ; for the perpendiculars let fall from the different points of AB are all in the same plane.

And since two points determine a line, we have to draw the projection of a , merely to M find the projections a, b, of any two points a, B; then join a, b, and the line ab, or that line produced, will be the projection required.

A plane drawn through AB I to mn is called the projecting plane of the line AB.

Hence to find the projecting plane of AB, we draw from any point A, A a, perpendicular to the plane mn; and suppose a plane to pass through the two lines ab and Aa, it will be the plane required. If AB be perpendicular to Mn, its projection is the point where AB or AB produced meets MN.

6. A point is determined in space, when its projections, upon two planes which intersect, are given.

For the point should be found upon each of the perpendiculars drawn to the planes from the given projections ; and these perpendiculars can only intersect in one point, which is the point required.

The two planes to which reference is here made, and which are generally at right angles to each other, are called the planes of projection, or co-ordinate planes.

The following proposition will determine when two points in the co-ordinate planes are the projections of the same point.

7. When two points, situated in two co-ordinate planes, are the projections of the same point in space, the per

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pendiculars drawn from these points upon the line of intersection of the planes, must meet at the same point in that line.

Let xyz, Xyu, be the two co-ordinate planes; xy the line of intersection. From any point a, draw the

perpendiculars, Aa, aa': and through a a, a d', draw the plane A a'pa, cutting xy in p, and the two planes in ap and a'p; then the line xy shall be = to the plane aapa' and .; to ap and a'p; and thus the +$, from the projections a and a' of the point

A, meet in the same point. Next, the points a and a' are always the projection of the same point in space.

The line xy being + to ap and a'p, is also to the plane apa'; and :: the planes xyz, xyu, are + to plane apa'; :. conversely apa' is + to xyz, and xyu. from a and a' perpendiculars be drawn to the planes xyz, xyu, these lines will be in the plane apa'; whence it is evident that they ought to intersect in the point a, of which the projections are a and a'.

8. A line is known, when its projections upon two planes which intersect are given.

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Let ab, a'b', be the projections of a line; through ab conceive a plane - to xyz, and through a'b', a plane 1 to XyU; the intersection as of these two planes is the line of which the projections are ab and a'b'.

If the two lines ab, a'b' are 1 to xy, but intersect it in different points m, n, they cannot be the projections of the same line, since they are obviously in parallel planes, pma and qna'.

But if the projections ab, a'b', be both - to xy, and meet xy in the same point m, then the plane ama' of these lines is - both to xyz, and xyu, , so that every line situated in ama', will have the same projections ab and a'b'.

In this case we must introduce a third plane txs, which does not pass through xy, and the line may be determined from its projection ab” upon txs.

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9. The projections of any curve upon two intersecting planes will determine the curve.

For abc, a'b'd', being the projections of a Bc upon xyz and ayu, suppose perpendiculars to these planes to be drawn from the different points of abc, a'b'c'; these will form two cylindrical surfaces, acca, a'd' ca, both of which will contain the curve ABC, and which will determine that curve by their intersection.

If the cylindrical surfaces do not intersect, the projections do not belong to the same curve.

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If Abc be a plane curve, and its plane be - to xy, the two projections are merely the intersections of the plane through a Bc, and the curve remains undetermined.

Der. The intersections of a plane with the co-ordinate planes are called the traces of the plane.

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10. When the traces of a plane are given, the plane itself is given.

We have already seen (Proposition II. of the Introduction), that two lines which intersect determine a plane ; we might, indeed, make use of the projections of three of its points upon the two co-ordinate planes, but it is more convenient to use the traces. In general, the plane meets the line of intersection of the two co-ordinate planes, and it is manifest that the point of intersection a, must belong to the two traces a a, a'a. If the plane be parallel to æy, its traces are also parallel

If it be + to xy, its traces are also I to xy. If it be parallel to one of the two planes, its trace upon the other will be parallel to xy, and will be sufficient to determine the

plane; but if the plane pass through x y, its two traces coincide with xy, and a third plane of projection must be introduced to determine its position.

Hitherto we have made no hypothesis with regard to the angle between the two planes of projection. But to render the constructions the more simple, we shall hereafter consider the two planes to be at right angles to each other, and that one of them, xyz, is horizontal, and the other, xyu, tertical. Their intersection, ay, is called the ground

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