Practical carpentry

In offering this Work on Carpentry to the American Carpenter and Joiner, the author desires it to be understood that the work is not intended to take the place of any of the larger and more exhaustive works on the subject ; but is designed more particularly for use as a hand-book by the workman that lias not had time or opportunity to thoroughly commit to memory the principles it contains, and to occupy a small comer in the workman's tool chest, so that it may be referred to for consultation whenever circumstances require it.

It is quite true that many similar books have been written on the subject, each one of which possesses more or less merit, and the enquiring and progressive workman will make an effort to procure a copy of each kind, so that he may get at the readiest methods of performing the various operations of getting the lengths and angles of rafters, cuts and curves for circular roofs, and lines for hoppers, raking mouldings and other beveled work; but there are thousands of workmen whose limited means will not permit of their purchasing a great number of these books, and who cannot afford to buy the high-priced volumes which contain all the ordinary workman would require to know. It is for these men this manual is prepared; and the author flatters himself that he has been able to string together a greater amount of real practical matter in this little work than was ever before offered for three times its price. Another thing too, which gives the work more value, is the fact that every rule and solution contained in it can be depended upon as reliable, as an experience of many years in the supervision of workmen has given the author ample opportunities to practically test nearly every rule the book contains.

 

In order that the reader may be able to "lay out" and construct centres for arches, window and door heads, it is necessary he should have a clear conception of what an arch really is. For if a positive conclusion has not been arrived at, and if the "arch principle" is not fairly understood, he cannot be expected to design an arch, or to construct it with accuracy or intelligence, even if designed by another. Let us then state once for all, that every curved covering to an aperture is not necessarily an arch. Thus, the stone which rests on the piers shown in Fig. 53 is not an arch, being merely a stone hewn out in an arch-like shape; but at its top, the very point (A) at which strength is required, it is the weakest, and would fracture the moment any great weight were placed upon it.

It is not the province of this work to enter into a scientific disquisition on the arch, but some of its Fig. 5a properties must be known to the mechanic before he will be able to construct centres understandingly; and the general principles here laid down will help the workman materially to form correct ideas concerning the work in hand. In all cases, however, we advise the student to arm himself with a thorough knowledge of lie arch and the principles involved. Elementary works on the subject can be easily obtained, and all who would really study principles, and appreciate the exquisite refinement of the examples herein given, are strongly urged to read them.

The semi-circular arch shown at Fig. 54 is self-explanatory so far as the divisions are concerned. The under surface is called the intrados, and the outer the extrados. The supports are called the piers or abutments, though the latter term is one of more extensive application, referring more generally to the supports which bridges obtain from the shore on each side than to other arches. The term"piers" is, as a rule, supposed to imply supports which receive vertical pressure, whilst abutments are such as resist outward thrust The upper parts of the supports on which an arch rests are called the imposts. The span of an arch is the complete width between the points where the intrados meets the imposts on either side; and a line connecting these points is called the "springing" or spanning line.

The separate wedge-tike stones composing an arch are called Voussoirs, the central or uppermost one of which is called the Keystone; whilst those next to the imposts are termed "springers."

The highest point in the intrados is called the vertex or crowm, and the height of this point above the springing line is termed the "rise" of the arch. It will be evident that in a semi-circular arch, such as Fig. 54, this would be the radius with which the semi-circle is struck. The spaces between the vertex and the springing line are called the flanks or haunches.

The Semi-circular Arch was that principally used by the Romans, who employed it largely in their aqueducts and triumphal arches. The others are, however, mentioned by some writers as having been occasionally employed by the ancients. During the middle ages other forms were gradually introduced.

The Stilted Arch is an adaptation of the semi-circular arch, in which the springing line is raised above the top of the column, on a pedestal not much larger in diameter than the width of the voussoirs of the arch.

The Horse-shoe Arch, - This is almost restricted to the Arabian or Moorish style of architecture. In this form of arch the curve is carried below the line of centre or centres; for in some cases the arch is struck from one centre, and in others from two, as in Fig. 56.

Now it must not be supposed that the real bearing of the arch is at the impost A a ; for if this were really so, it must be seen that any weight or pressure on the crown of the arch would cause it to break at b, but the fact is simply that the real bearings of the arch are at b b, and the prolongation of the arch beyond these points is merely a matter of form and has no structural significancy. The Horse-shoe arch belongs especially to the Mohammedan architecture, from its having originated with that faith, and from its having been used exclusively by its followers.

The Lancet Arch. - This arch, Fig. 57, is constructed by placing the centres c c outside the span, but still on the same line with the imposts. This form of arch was first used in the Gothic, and as a rule indicates the style called "Early English."

Equilateral Arch. - Fig. 58 shows the Equilateral arch, the radius with which the arcs are struck being equal to the span of the arch, and the centres being the imposts; and thus, the crown and the imposts being united, an equilateral triangle is formed. This form was principally used in the "Decorated " period of Gothic architecture from about 1307 until about 1390, at which time the Ogee arch (Fig. 59) was also occasionally used.