Part 10

=SPHERE.=--A solid bounded by a curved surface every point of which is equally distant from a point within called the center.

=SURFACE.=--The boundary of a solid. It has but two dimensions--length and breadth. Surfaces are plane or curved.

A Plane Surface is one upon which a straight line can be drawn in any direction.

A Curved Surface is one no part of which is plane.

The surface of the sphere is curved in every direction, while the curved surfaces of the cylinder and cone are straight in one direction.

The surface of a solid is no part of the solid, but is simply the boundary of the solid. It has two dimensions only, and any number of surfaces put together will give no thickness.

=SYMMETRY.=--Design. A proper adjustment or adaptation of parts to one another and to the whole.

=TRISECT.=--To divide into three equal parts.

=TRIANGLE.=--A triangle is a polygon having three sides and three angles. Tri is a Latin prefix signifying three; hence a Triangle is literally a figure containing three angles.

A Scalene Triangle is one in which no two sides are equal. See fig. 50.

An Isosceles Triangle is one in which two of the sides are equal. See fig. 51.

An Equilateral Triangle is one in which the three sides are equal. An Equiangular Triangle is one having its three angles equal. An Acute-Angled Triangle is one in which each angle is acute.

A Right-Angled Triangle is one which has one of the angles a right angle. See fig. 53.

An Obtuse-Angled Triangle is one having an obtuse angle. Fig. 54.

Equiangular triangles are also equal sided, and vice versa.

=VERTICAL.=--Upright or perpendicular. Vertical and perpendicular are not synonymous terms.

=VERTEX.=--See Angle, Quadrilateral, Triangle. The vertex of a solid is the point in which its axis intersects the lateral surface.

=VIEW.=--See Elevation. Views are called front, top, right or left side, back, or bottom, according as they are made on the different planes of projection. They are also sometimes named according to the part of the object shown, as edge view, end view, or face view.

=WORKING DRAWING.=--One which gives all the information necessary to enable the workman to construct the object.

Free-Hand Drawing.

A free-hand drawing is executed with the unaided hand and eye, without guiding instruments or other artificial help. It is necessary to be known that all drawing required cannot possibly be done by rule and compass, but that some portions must be drawn “free-hand,” trusting to the eye alone.

Hence, it is important that the student should be able to sketch at sight from objects he may see, or to draw roughly, with a piece of chalk or a pencil, pieces of mechanism required to be represented.

Practice in free-hand should go along with mechanical drawing as progress is made, and thus cultivating both branches equally.

“A simple sketch will often,” as has been rather roughly said, “express more than yards of talk.”

Even a slight sketch refreshes the memory, and in the case of the preparation of a complete set of drawings, with a view to the making of a thoroughly finished mechanical drawing, the proper course to pursue is, to make a general sketch, letter the various parts for reference, and then prepare a series of detailed sketches, similarly lettered, and diffuse with dimensions.

Everyone, whatever his specialty, feels to-day that the ability to sketch rapidly and clearly is among the absolute necessities for correct and prompt transactions of business, in giving and executing orders and doing business with persons outside his profession.

Mistakes and misunderstandings may be averted by means of rough sketches taken at the time and shown for confirmation; this also saves assistants from getting into trouble, especially if they pin the sketch to the order, for reference, in case of the arising of any dispute. These are a few of the advantages of knowing how to sketch quickly and correctly.

In “free-hand” any sort of pencil is better than none, but there is a considerable advantage in having a good serviceable article--a pencil not too soft nor too hard, and one which will retain its point for some little time.