n it you are lost.

LEARNING MECHANICAL FORMS.--Suppose, for example, we take the words
_segment_ and _sector_. Without a thorough understanding in your own
mind you are likely to confuse these terms by taking one for the other.
But let us assume you are to be called upon to explain a sector to some
one who has no idea of terms and their definitions. How would you
describe it? While it is true it is wedge-shaped, you will see by
examining the drawing that it is not like a wedge. The sector has two
sides running from a point like a wedge, but the large end of the sector
is curved.

If you were called upon to define a segment you might say it had one
straight line and one curve, but this would not define it very lucidly.
Therefore, in going over the designations given, not only fix in your
mind the particular form, but try to remember some particular manner in
which you can clearly express the form, the shape or the relation of the
parts.

For your guidance, therefore, I have given, as far as possible, simple
figures to aid you in becoming acquainted with structures and their
designations, without repeating the more simple forms which I have used
in the preceding chapters.

[Illustration: _Fig. 55.-Fig. 65._]

55. _Arcade._--A series of arches with the columns or piers which
support them, the spandrels above, and other parts.

56. _Arch._--A curved member made up, usually, of separate wedge-shaped
solids, A. K, Keystone; S, Springers; C, Chord, or span.

57. _Buttress._--A projecting mass of masonry. A, used for resisting the
thrust of an arch, or for ornamentation; B, a flying buttress.

58. _Chamfer._--The surface A formed by cutting away the arris or angle
formed by two faces, B, C, of material.

59. _Cotter or Cotter Pin._--A pin, A, either flat, square or round,
driven through a projecting tongue to hold it in position.

60. _Crenelated._--A form of molding indented or notched, either
regularly or irregularly.

61. _Crosses._--1. Latin cross, in the Church of Rome carried before
Bishops. 2. Double cross, carried before Cardinals and Bishops. 3.
Triple or Papal cross. 4. St. Andrew's and St. Peter's cross. 5. Maltese
cross. 6. St. Anthony or Egyptian cross. 7. Cross of Jerusalem. 8. A
cross patte or ferme (head or first). 9. A cross patonce (that is,
growing larger at the ends). 10. Greek cross.

62. _Curb Roof._--A roof having a double slope, or composed on each side
of two parts which have unequal inc