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When the end of the board Z is turned up (Fig. 49), it has merely the longitudinal parallel lines B. The bevel square may now be used in the same manner as on the side of the board Y, and the fitting angles will then be accurately true.

This is shown in Fig. 50, in which, also, two of the cutaway parts are removed.

TOOLS USED IN LAYING OUT TENONS AND MORTISES.--A sharp-pointed knife must always be used for making all marks. Never employ an awl for this work, as the fiber of the wood will be torn up by it. A small try square should always be used (not the large iron square), and this with a sharp-pointed compa.s.s and bevel square will enable you to turn out a satisfactory piece of work.

The foregoing examples, carefully studied, will enable you to gather the principles involved in laying off any work. If you can once make a presentable box joint, so that all the dovetails will accurately fit together, you will have accomplished one of the most difficult phases of the work, and it is an exercise which will amply repay you, because you will learn to appreciate what accuracy means.

CHAPTER VI

THE USES OF THE COMPa.s.s AND THE SQUARE

THE SQUARE.--The square is, probably, the oldest of all tools, and that, together with the compa.s.s, or dividers, with which the square is always a.s.sociated, has const.i.tuted the craftsman's emblem from the earliest historical times. So far as we now know, the plain flat form, which has at least one right angle and two or more straight edges, was the only form of square used by the workman. But modern uses, and the development of joinery and cabinet making, as well as the more advanced forms of machinery practice, necessitated new structural forms in the square, so that the bevel square, in which there is an adjustable blade set in a handle, was found necessary.

THE TRY SQUARE.--In the use of the ordinary large metal square it is necessary to lay the short limb of the square on the face of the work, and the long limb must, therefore, rest against the work side or edge of the timber, so that the scribing edge of the short limb does not rest flat against the work. As such a tool is defective in work requiring accuracy, it brought into existence what is called the try square, which has a rectangular handle, usually of wood, into which is fitted at one end a metal blade, which is at right angles to the edge of the handle. The handle, therefore, always serves as a guide for the blade in scribing work, because it lies flat down on the work.

THE T-SQUARE is another modification of the try square, its princ.i.p.al use being for draughting purposes.

THE COMPa.s.s.--The compa.s.s is one of the original carpenter's tools. The difference between _compa.s.s_ and _dividers_ is that compa.s.ses have adjustable pen or pencil points, whereas dividers are without adjustable points. Modern work has brought refinements in the character of the compa.s.s and dividers, so that we now have the bow-compa.s.s, which is, usually, a small tool, one leg of which carries a pen or pencil point, the two legs being secured together, usually, by a spring bow, or by a hinged joint with a spring attachment.

PROPORTIONAL DIVIDERS.--A useful tool is called the proportional dividers, the legs of which are hinged together intermediate the ends, so that the pivotal joint is adjustable. By means of this tool the scale of work may be changed, although its widest field of usefulness is work laid off on a scale which you intend to reduce or enlarge proportionally.

DETERMINING ANGLES.--Now, in order to lay out work the boy should know quickly and accurately how to determine various angles used or required in his work. The quickest way in which to learn this is to become familiar with the degree in its various relations.

[Ill.u.s.tration: _Fig. 51._]

DEFINITION OF DEGREE.--A degree is not a measure, as we would designate a foot or a pound to determine distance or quant.i.ty. It is used to denote a division, s.p.a.ce, interval or position. To ill.u.s.trate, look at the circle, Fig. 51. The four cardinal points are formed by the cross lines (A, B), and in each one of the quadrants thus formed the circle is divided into 90 degrees. Look at the radial lines (C, D), and you will find that the distance between these lines is different along the curved line (E) than along the curved line (F). The degree is, therefore, to indicate only the s.p.a.ce, division or interval in the circle.

THE MOST IMPORTANT ANGLE.--Most important for one to know at a glance is that of 45 degrees, because the one can the more readily calculate the other degrees, approximately, by having 45 degrees once fixed in the mind, and impressed on the visual image. With a square and a compa.s.s it is a comparatively easy matter accurately to step off 45 degrees, as it is the line C, midway between A and B, and the other degrees may be calculated from the line C and the cardinal lines A or B.

DEGREES WITHOUT A COMPa.s.s.--But in the absence of a compa.s.s and when you do not wish to step off a circle, you will in such case lay down the square, and mark off at the outer margin of the limbs two equal dimensions. Suppose we take 2 inches on each limb of the square. The angle thus formed by the angle square blade is 45 degrees. To find 30 degrees allow the blade of the angle square to run from 2 inches on one limb to 3-1/2 inches on the other limb, and it will be found that for 15 degrees the blade runs from 2 inches on one limb to 7-1/2 inches on the other limb. It would be well to fix firmly these three points, at least, in your mind, as they will be of the utmost value to you. It is a comparatively easy matter now to find 10 degrees or 25 degrees, or any intermediate line.

WHAT DEGREES ARE CALCULATED FROM.--The question that now arises is what line one may use from which to calculate degrees, or at what point in the circle zero is placed. Degrees may be calculated either from the horizontal or from the vertical line. Examine Fig. 53. The working margin indicated by the cross mark is your base line, and in specifying an angle you calculate it from the work edge. Thus, the line A indicates an angle of 30 degrees. The dotted line is 45 degrees.

[Ill.u.s.tration: _Fig. 52._]

[Ill.u.s.tration: _Fig. 53._]

[Ill.u.s.tration: _Fig. 54._]

THE DIVIDERS.--The dividers are used not only for scribing circles, but also for stepping and dividing s.p.a.ces equally. There is a knack in the use of the dividers, where accuracy is wanted, and where the surface is of wood. Unless the utmost care is observed, the s.p.a.ces will be unequal, for the reason that the point of the dividers will sink more deeply into the wood at some places than at others, due to the uneven texture of the wood grain. It will be better to make a line lengthwise, and a cross line (A) for starting (see Fig. 54). You may then insert one point of the dividers at the initial mark (B), and describe a small arc (C). Then move the dividers over to the intersection of the arc (C) on the line, and make the next mark, and so on.

Some useful hints along this same line will be found under the chapter on Drawing, which should be carefully studied.

CHAPTER VII

HOW THE DIFFERENT STRUCTURAL PARTS ARE DESIGNATED

THE RIGHT NAME FOR EVERYTHING.--Always make it a point to apply the right term to each article or portion of a structure. Your explanation, to those who do know the proper technical terms, will render much easier a thorough understanding; and to those who do not know, your language will be in the nature of an education.

PROPER DESIGNATIONS.--Every part in mechanism, every point, curve and angle has its peculiar designation. A knowledge of terms is an indication of thoroughness in education, and, as heretofore stated, becomes really the basis of art, as well as of the sciences. When you wish to impart information to another you must do it in terms understood by both.

Furthermore, and for this very reason, you should study to find out how to explain or to define the terms. You may have a mental picture of the structure in your mind, but when asked to explain 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 a.s.sume 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.

[Ill.u.s.tration: _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. _b.u.t.tress._--A projecting ma.s.s of masonry. A, used for resisting the thrust of an arch, or for ornamentation; B, a flying b.u.t.tress.

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 inclinations; a gambrel roof.

63. _Cupola._--So called on account of its resemblance to a cup. A roof having a rounded form. When on a large scale it is called a dome.

_Crown Post._--See _King Post_.

64. _Console._--A bracket with a projection not more than half its height.

65. _Corbels._--A ma.s.s of brackets to support a shelf or structure.

Largely employed in Gothic architecture.

[Ill.u.s.tration: _Fig. 66.-Fig. 79._]