i !
Figure 8, “ exhibits an arch boarded over, wherein theseveral figures 1, 2, 3, &c., represent so many ribs, or jack-rafters, set upon the circular body of the arch, in order foranother arch to intersect it, where those hoarded over thegroins are formed.” We learn nothing from this, but the man-ner of placing the jack-ribs on the body of the arch. Pricedescribes the method of placing these low ribs upon theboarding, and calls them small centres; but his diagram isdifferent, and not near so clear.
In the boarding of domes with the joints in horizontalplanes, Swan has shown the first ideas of the subject:Figure 9, “ represents a circular body. To find the curveof any lath or margin to he bent round this body, parallel toits base.
“ Let the points b and c represent the margin which youintend to bend round; then draw a right line through thesepoints, to meet the perpendicular or diameter produced, as ina, and it gives the length a b the shorter, and a c the longerradius for striking the curve required.”
This author has nothing more of novelty in the art ofcarpentry.
Our next author is Mr. William Pain. In his BritishPalladio, lie shows the methods of hip-bracketing for coves,and plaster cornices, as follows : Plate XII. Figure 1, “ Dis an angle-bracket for an internal angle, which are (is) tracedby ordinates.” Figure 2, “ e is an angle-bracket for a plastercornice, at an internal angle ; f, an external angle, allowingone inch for lath and plaster.” The formation of angle-bracketsis so easy, that a very little reflection, on inspecting the figure,will show the method adopted, without description : but stillsomething more might have been said on the practical part.
From The Builders Golden Rule, the following diagrams,with their descriptions, are taken. “ The backing of curve-line hips, and tracing them. — Figure 3, (No. 1) is the rib ofa dome, and (No. 2) is the hip traced from it. Divide thegiven rib (No. 1) into five parts, on the base line, and drawthe ordinates 1, 1, 2, 2, 3, 3, 4, 4, 5, 5 ; then divide the baseline of the hip into the same number of parts ; take themfrom (No. 1) and set them on (No. 2) ; then tack in nailsat the points 1, 2, 3, 4, 5 ; bend a thin slip round, and markas that curve directs, which gives the hip-mould. To backthe hip, take from (No. 3) the plan of the hip, 1, 2, and setit on the hip at the bottom 1, 2; then shift the hip-mouldto 2, and out to the top: mark it by, and that will be thewood to come off for the backing of the hip.” The practiceof dividing the base of the given rib, and the base of therequired rib into equal parts, was first shown by Halfpenny,and now by our present author, Pain ; and though the prin-ciple is true, the practice is bad, as it leaves so great a por-tion of the curve to be traced by the eye, where it rises fromthe base; and though it is not necessary that each base shouldbe divided into equal parts, or into any series of parts whichshall have a given proportion to each other, yet it would bebetter to divide the curve of the given rib into equal parts,then divide the base of the required rib in the same propor-tion ; and the arcs of the required rib terminated by theupper extremities of each two ordinates, will be very nearly,if not quite, proportional ; that is, the distance between thetracing points will be nearer where the curve is quickest,and where the greater number of points are most required.
Figure 4, shows “ the backing for a straight hip. Youare to observe that the piece of wood be of the same thick-ness as the hips, and form of the curve, for the little part youwant; then cut it to the pitch of the hip at foot, set it onthe plan, and mark it by that, which will give the backingexactly ; and so for any other. Or, if you draw a lineparallel with the base line, and take off 1, 2, on the plan
(No. 2) and set it on the said lines 1, 2, all the way u P gand mark by the mould, it will give the backing in any casrequired, straight or curved.”
This general and correct principle was first noticed by **} 'Figure 5. “ The method of coving the angles, when “ ie .is a circle or oval in the centre of the ceiling .—Dra'V
ib e<l
centre part, touching the sides and ends ; then draw anoto the extreme of the angles, parallel with the centre; ‘
xo xne exxreme oi xne angles, parallel wixn xne centre , . ,draw the semicircular arch a, and from that trace the s ^arches b, b, and the rib c, c, c, C, which is a mould toall the brackets for the angles ; as is plain to inspectionthe. lines on the plan.” .
This principle is erroneous, and the descriptionThere is no respect paid to the elliptic base; but the brackare traced upon the principle of angle-brackets. The sectlof any body must depend upon the construction of that bo Por upon its properties; thus, if a body is intended to Bthis property, that all parallel sections are to be si®figures, and if the method of forming it is not founded °Pthis principle, the body is not what it was intended t0but something else ; the construction will therefore be f . „neous. All sections of a body must be found by desor jDthe seats of the curves of as many parallel sections asbe thought sufficient, on one of the largest of them; 1 ^having a given or transverse section, that will cut allparallel sections of the body, all other sections whatever ^be found : but our author, Mr. Pain, forms the section 0bodies like those of prisms, without attending to the P , gperties of the body required, as is the case in the examPbefore us. j,e
In the first edition of his Practical House Carpent er ’^has presented the diagram of an interior circular dome, l° r ^into pendentives by bracketing the spandrels above, ^traced according to the same erroneous principle : hut ^the publication of the Carpenter's New Guide, by the a “of this Work, the error was corrected, in the second e jof the said Practical House Carpenter, so as to corre®Pwith the legitimate principle, first published in theNew Guide : his description, which is very short, is P^t,contained in the text, and partly on the plate : in th e johe says, Figure 6, “ is a conical skylight, showing ® u llbracket the angles of the ceiling under the kirb, the bip'^ oll iJg at the angle is traced from the rib b, and that mould ^ ndo to cut all the ribs at the angles, as shown at the angl . oI1 ;
What is here said, refers to the diagrams in the first e . yghbut the text stands in the sixth edition, as in the first,the diagram is altered in the second and succeeding
mi • *1 T 1.1 .1 • —-J
There is no rib b, nor angle a, shown on the improved d®^ 0 oOn the plate he writes thus : “ f dome with a skjPSthe top. g, g, moulds for the ribs of the dome ; a > ’kirb of the light.” This latter description refers
diagram as it now stands.
The skylight on the top is exhibited in a very e rr ° $manner, being inconsistent with the principles of arl ^£ tb eof projection that we are acquainted with. The plaj 1 „, n b/
ill*
bracketing, and the ribs of the domical part, are sho jsa common ichnographical projection, while the skyr'gexhibited in a kind of false perspective, and being f jjtiSany connection with the kirb on which it is placed, jjithe appearance of being raised upon its edge, restingtwo points in the kirb.
Figure 7, “ is an ogee roof, whose plan is a pentag 0shows the method of drawing the polygon figure to any ^ 6’side ; make a radius of that side, and draw the arehe ^divide one of these arches into six parts, and turn t #<:•'the centre line, as shown by the letters and figures 5 d,the centre c will draw a circle to receive the side