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A right line is perpendicular to a plain, when it makes right
angles with all the lines that touch it, and are drawn in the
same plain.
IV.
A plain is perpendicular to a plain, when all the right lines in
one plain, drawn at right angles to the common section of the
two plains, are at right angryto the other plain.

The inclination of a right line to a plain, is the acute angle
contained under that line, and another right one drawn in
the plain, from that end of the inclining line, which is in
the plain, to the point where a right line falls from the other
end of the inclining line, perpendicular to the plain,

- - wo

Book XI. - VI. -

Jryov-The inclination of a plain to a plain, is the acute angle contained by the right lines drawn in both plains, to the same point of their common section, and making right angles with

1t. VII. Plains are inclined similarly, when their angles of inclination are equal. VIII.

Parallel plains are such, which being produced, never meet.
IX. -

Similar solid figures are such as are contained under an equal number of fimilar plains.

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A solid angle is the inclination of more than two right lines that meet in one point, but are not in the same superficies. XI

A pyramid is a solid figure, contained by more than two \ plains set upon one plain, and meeting at one point in the

Vertex, -
XIII.

A prism is a solid figure contained by plains, whereof the two opposite are equal, fimilar, and parallel; and the other pa

rallelograms. -
XIV.

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tion. XV The axis of a sphere is that fixed right line about which the

femicircle revolves.

XVI.
The centre of a sphere is the same with that of the femi-
circle.
XVII.

The diameter of a sphere is a right line drawn through the centre, and terminated on either side by the superficies of the

sphere.

XVIII,

A cone is a solid figure described by a right angled triangle revolving about one of the fides, containing the right angle, remaining fixed. If the fixed right line be equal to the other side containing the right angle, then it is a rečtangular cone;

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