Chap. XXII.
TIMBER AND ITS PROPERTIES.
1201
The oak varies in its specific gravity according to the soil which produces it; itsstrength is in proportion to its density, and that timber is the most durable which hasthis quality in the liighest degree : density is mainly owing to the length of time occupiedin the production of the wood ; that which grows fast, as it will do on light soils, is not soheavy, or so hard and compact, as that produced on cold soils, and of consequence slowergrowth.
From all the experiments made upon the strength of oak timber it has been found thatthis is in proportion to its weight and density, and invariably the heaviest is the strongest:in the trunk of a tree the most dense wood is found in the lower parts ; this quality de-creases in those branches and portions farther removed from the base. Trees which aresuffered to complete their growth have their heartwood throughout of the same weightand strength, whilst those cut down prematurely are found to possess these requisites onlyin their centre wood, which is considerably harder than that formed by the outer concentricrings; it may be said to decrease in hardness in arithmetical proportion as it approachesthe sap-wood.
When a tree, however, is suffered to stand too long, and its decay has commenced, theouter wood is in many instances the hardest: the specific gravity of the oak when cut downvaries from 1000 to 1054, and its weight per cube foot from 70 to 74 lbs. ; this weight byseasoning is reduced to GO or G3 lbs., and it has been shown that the greatest degree of dry-ness that can be acquired is when it has lost about one-thirdof its original weight; but in this state it is not so durable,and more apt to break when exposed to a strain, or when aweight is imposed upon it. That preferred for carpentryis when only one-sixth of its weight has been lost, as acertain portion of sap is necessary for the fibres of thewood to preserve their union; in a perfectly dry statethey have less adherence, lose their stiffness, and splitand break more readily; oak seasoned so as to reduce itto the weight of about GO Jbs. per cube foot is the mostpreferable.
The fir in common use is imported generally from the portswhich surround the Baltic Sea, or from the coast of Norway or Sweden , and the best deals come chiefly from Christianaor other ports of Sweden , Prussia, or Petersburgh. Thelightness and stiffness of this timber render it preferable forgirders and framing in general; it is less expensive to convert,and, although not quite so durable as oak, has the ad-vantage of cheapness. The timber of the best quality has itsannular rings much thinner than that of the inferior kinds ;and when the saw is put in it does not cut so as to leave arough or woolly surface, nor is it in the least of a spongyquality.
White deal is obtained from the spruce fir, the best fromtimber of 70 or SO years’ growth ; in seasoning it will loseabout one-seventieth part: deals are usually 3 inches inthickness, and 9 inches wide, and are sold by the hundred,which consists of 120; whatever may be their thicknessthey are reduced to a standard; one of inches in thickness,
11 inches wide, and 12 feet long, contains 1 foot inchescube.
The Teredo navalis is most destructive to timber used inthe supports of a bridge or other purposes by the water-side : this pile-ivorm y as it is sometimes called, is particu-larly active in destroying timber in contact with the mudof rivers like the Thames; and wherever the water isbrackish its ravages are certain. On the left of the figurethe worm is represented on its belly, about half its naturalsize, and to the right on its back ; A a, at the top of thefigure, is the shell; A B C D E is the form of the hinge Fig. 2054.
by which the shell is united; AC, B C, the two defences of the tail ; ABD and ADCare its moving fibres ; A A, the circular membrane covering the head ; and the bottomfigure, A A A A, is the same membrane turned back: this animal, so well describedin the 41st volume of the Philosophical Transactions , No. 455., is very small when firstexuded from the egg, but soon acquires a considerable size ; its head is provided witha hard substance which resembles an auger, by which it penetrates the hardest wood; thefir and alder are the woods it most readily destroys.
The carpenter, before he commences any work, should thoroughly comprehend that im-portant proposition in mechanics, which shows the composition and resolution of forces i