CAR

96

CAR

It is singular that in the foregoing table, the oak scantlingsare greater than those of fir. Oak is more cohesive than fir,but fir is less compressible by forces acting in the directionof the fibres; oak is therefore more fit for ties, and fir forstruts, or straining pieces. But Mr. Price, in this table,inconsiderately and indiscriminately makes the oak scantlingslarger than those of fir.

The following observations, in the introduction to Price’swork, are very judicious, and worthy of transcription.

“ Nevertheless, it may not be improper, in this place, tomention some general observations. There is a moisture inall timber; therefore all bearing timber ought to have amoderate camber, or roundness: for till that moisture is insome sort dried out, the said timber will sag with its ownweight; and that chiefly is the reason girders are trussed whenused, as in its place will be shown. But here observe, thatgirders are best trussed when they are first sawn out, for bytheir drying and shrinking, it tightens the trusses in themyet more.

“ Observe also, that all beams, or ties, be cut, or forced inframing, to a camber, or roundness, such as an inch in thelength of eighteen feet; and that principal rafters be alsocut, or forced up to a camber, or roundness, as before: thereason of this is, all trusses, though ever so well framed, bythe shrinking of the timber, and weight of the covering, willsag, and sometimes so much as to offend the eye of thebeholder; so that by this preparation your truss will everappear well.

“ Also observe, that all case-bays, either in floors or roofs,do not exceed twelve feet if possible; that is, do not letyour joists in floors, your purlins in roofs, &c. exceed twelvefeet in their length, or bearing ; but rather let the bearing beeight, nine, or ten feet; which should be observed in forminga plan.

“Also, in bridging-floors, do not place your binding orstrong joists above three, four, or five feet apart; and thatyour bridgings or common joists are not above ten or twelveinches apart, that is, between one joist and the other.

“ Here also observe, never to make double tenants or tenonsfor bearing uses, such as binding-joists, common joists, orpurlins; for, in the first place, it weakens very much what-ever you frame it into; and, in the second place, it is a rarityto have a draught in both tenons, that is, to draw your jointclose by the pin; for the said pin, by passing through bothtenons, (if there is a draught to each,) must bend so much,that without the pin be as tough as wire, it must needs breakj in driving, and consequently do more hurt than good.”

We are now come to Mr. Batty Langley , in whose nume-rous publications are to be found many particulars relatingto Carpentry. In his Builder’s Complete Assistant, published1738, page 147, the fourth edition, he has the followingobservations:—

“When partitions have solid bearings throughout theirwhole extent, they have no need to be trussed; but whenthey can be supported but in some particular places, thenthey require to be trussed in such a manner, that the wholeweight shall rest perpendicularly upon the places appointed- for their support, and nowhere else. Partitions are made ofdifferent heights, to carry one, two, or more floors, as thekinds of buildings require.

“ The first things to be considered in works of this kindare—the weight that is to be supported, the goodness andkind of timber that is to be employed, and proper scantlingsnecessary for that purpose.”

So far his observations are tolerable; but his subsequent rea-sonings are drawn rather from his own caprice, than from theprinciples of science, as will be seen in the following quotations:

“ The strength of timber in general is always in propor-tion to the quantity of solid matter it contains. The quan-tity of solid matter in timber is always more or less, as thetimber is more or less heavy; hence it is, that all heavywoods, as oak, box, mahogany, lignum-vit®, &c., are strongerthan elder, deal, sycamore, &c., which are lighter or (rather)less heavy; and, indeed, for the same reason, iron is not sostrong as steel, which is heavier than iron ; and steel is notso strong as brass or copper, which are both heavier thansteel. To prove this, make two equal cubes of any tvV °kinds of timber, suppose the one of fir, the other of oak>weigh them singly, and note their respective weights; thisdone, prepare two pieces of the same timbers, of equ*lengths, suppose each five feet in length, and let each betried up as nearly square as can be, but to such scantling 9 ’that the weight of a piece of oak may be to the weight oa piece of fir, as the cube of oak is to the cube of fir; thenthose two pieces being laid horizontally hollow, with eq u "bearings, and being loaded in their middles with increaseequal weights, it will be seen that they will bend or sagequally, which is a demonstration that their strengths are t°each other as the quantity of solid matter contained •"them.”

This is reasoning only from conjecture, and therefore^"consequence must be erroneous. The relation between weig 11and strength is not general. In some instances the vevreverse takes place to what this author asserts.

“ As the whole weight on partitions is supported by * ^principal post, their scantlings must be first considered, ""this should be done in two different manners, viz., n 1 " 3 !when the quarters, commonly called studs, are to be m 1 ®with brickwork, and rendered thereon; and, lastly, whenbe lathed and plastered on both sides. , .

“ When the quarters are to be filled between with bi' lCwork, the thickness of the principal posts should be as m" c

less than the breadth of a brick, as twice the thickness

of*

lath; so that when these posts are lathed to hold on

the re"'

dering, the laths on both sides may be flush with the sun"of the brickwork. And to give these posts a sulfie 1 ®^strength, their breadth must be increased at discretion!when the quarters are to be lathed on both sides, or "wainscoting is to be placed against the partitioning, t * ien q'[ ) ethickness of the posts may be made greater at pleasure. .usual scantlings for the principal posts of fir, of 8 i eeheight, is 4 or 5 inches square; of 10 feet in height, 5 0 ^inches square; of 12 feet in height, 6 or 7 inches squa| 6 ’

14 feet in height, 7 or 8 eight inches square; of 16 .height, from 9 to 10 inches square. But these last, m ^opinion, are full large, where no very great weight issupported. As oak is much stronger than fir, the scanof oak-posts need not be so large as those of fir; and t 1fore the scantlings assigned by Mr. Price, in his 'l reat heCarpentry, are absurd, as being much larger than tohas assigned for fir-roofs. To find the just scantling of 0,1 t3)posts that shall have the same strength of any given fi"'P

of

end

ube

this is the rule:

“ As the weight of a cube of fir is to the weight of " coak of the same magnitude, so is the area of the squ" reof any fir-post to the area of the end of an oaken P° s ’ oS twhose square root is equal to the side of the oakenrequired.”

ieht

He might as well have asserted, that as the weigcube of steel is to the weight of a cube of lead, soarea of the square end of any steel bar to the area ot

of*

•a

IS Ie""

of any leaden bar; which proposition would have led t0 ,'fest falsehood. In the rule he has not mentioned the I**#which, if taken into the consideration, would bring

ill' j