Purposes for which they are designed. Mr. Marshal) observes,Plat poplars may stand from thirty'to fifty years; ash and elm-trees from fifty to a hundred years; but oaks should on no ac-count be felled till they have attained the age of from one hundredto two hundred years. The durability of timber, however, de-pends on the soil" and sub-soil, on which the trees have been pro-duced: thus, in loose or absorbent lands, the oak and elm decayat an earlier period than those groan in cool and more retentiveground; but, in dry hams with a rocky sub-soil, the oak rapidlydecays, after it has attained the age of two centuries. Farther,the cypress, cedar, and main gany, are supposed to be indestruc-tible by time, or the depredations of insects, in whatever exposurethey may have been rawed ; and the alder endures the action ofWater for many years longer than any other species of timber:hence it is deservedly employed for piles in constructing clams,in ordpr to secure the hanks of rivers. The season for felling oak,°r fir-timber, commences toward the end of April, though the win-ter is often preferred for that purpose, and likewise for cuttingdown ash and elm-trees, as it furnishes labour to persons whowould otherwise be unemployed. All branches or limbs, thatmay injure tlu: trees in their fall, ought previously to be lopped,and the trunk cut down close to the ground, unless it be grubbed°r rooted up ; which latter method is preferable, as well as moreprofitable ; because timber, produced from oid stocks, is of infe-rior value. After the trees are felled, it will be advisable to sea-son them, previously to working the timber; it has, therefore,been recommended to pile them, one upon another, with smallblocks of wood between each, in an open but not too exposedsituation ; so that the sun and air may penetrate every part;the surface, or uppermost trees, being plastered with cow-dung, toprevent them from cracking. Another method is that of buryingthe timber in the earth ; but the most effectual inode of seasoningit, by heat, is performed at Venice, where the trees are exposedto a strong fire, at which they are continually turned round bymeans of an engine, till they become charred, or covered with ablack crust; by this expedient the internal part of the wood is soburdened as to resist equally the effects of earth and water, for along series of years, without receiving any injury. When hoardsOr planks have been properly dried, additional care becomes neces-sary to preserve them against the depredations of worms, the effectsof air, moisture, &c. For this purpose Mr. Evelyn directs commonsulphur to he put into a glass-retort, with such a portion of aqua-fortis as will cover it, “ to the depth of three fingers ;” the wholemust be distilled to dryness, and rectified two or three times.The remaining sulphur is then to be exposed to the open air on amarble, or in a shallow glass vessel, where it will liquefy into abind of oil, with which the timber must be anointed : this mixture,be asserts, will not only infallibly prevent the attacks of worms, butalso preserve every kind of wood from decay or putrefaction, ei-ther in air or water. Timber may also be defended from the in-fluence of air or moisture, by coating it two or three times will)linseed-oil; and some builders have advised the wood-work to bepainted : the latter practice, however, ought to be adopted with8>'eat caution: because, though it may in some cases be proper,11 should always be deferred, till the planks and similar articleshave become perfectly dry. With regard to the strength of tim-ber, Mr. Muller, in his treatise on fortification, gives the followingProblem, to determine the strength ot a scantling of timber, whosemmensions are given : lie supposes that all the fibres of the wood® r e straight, and of the same strength, even in its weakest part, andthat the fibres are the same in the same sort of wood ; and although this■pay not be strictly true, yet it is sufficiently near enough in prae-*'£0, so as to cause no sensible error. Suppose the scantling A UC,y late CJJLXXX. fig. 13,) to he supported in the middle D, by|b e edge of a triangular block K, and two equal bodies, P, Q, tobe suspended at A and (J, equally distant from the middle 15, o's Uch a weight as to break the scantling, ft is evident, that the^eights P ami Q will cause the scantling to bend at first, so as tofoakeakind of curvilinear angle at 15, and then to break in thatfo ac e, in a section BD, perpendicular to either of the sides AG ;bow as the power or force of these weights is more or less, aecord-ln S as they are suspended farther from, or nearer to, the fixedPoint D ; these forces will therefore be in proportion to the pro-ducts of the weights of each multiplied bv its respective distance, r °m the section BD ; or because the weights and distances areK ‘e supposed equal, twice the product of one of the weights P,

multiplied by its distance, from the section BD, will expressthe force of these two weights. The force of the weights beingthus determined, the resistance or strength of the wood is next tobe found, which is done in the following manner; Let acb, (fig.14,) represent the section of the scantling ; it is evident that thisarea represents the sum of all the fibres to be torn or broken; andas they are all supposed equal, and of the same strength, this areawill express the sum of the strength of all the fibres: but as thepoint D, or the base ab of the section, is fixed ; and the directionsof the fibres perpendicular to the area acb ; the forte or resistanceof each fibre is equal to the product of its strength, multiplied byits distance from the base ab ; and therefore the sum of all thefibres placed in the same line, df, parallel to the base ab, multi-plied by their distance ad, from that base ab, will express theirmomentum or resistance. What Inis been proved in regard to allthe fibres placed in the line df, is equally true of all those placedin any' other line parallel to the base ab : and therefore the sum ofall these products will express the total strength or resistance ofthe wood ; but by the nature of the centre of gravity, the productof the area acb, multiplied by the" distance of its centre of gravityfrom the base ab, will express the total strength or resistance of allthe fibres, or that of the whole scantling; consequently, havingthe .strength of any. scantling of the same wood determined by ex-periment, that of any other may be found. If the scantling AC,(.fig. 13,) be supported at both ends by the triangular blocks'P, Q,atui the weight W, hanging in the middle B: then if we supposethe weight P and Q in the first figure, to represent the blocks Pand Q in this; then as each block supports half the weight W ; itis evident,' that the weight W, multiplied by the distance AB orBC, will express its momentum or force. Moreover, since theweight W is suspended in the middle betwixt the fixed points, itis evident that each block supports exactly half the weight; and asthe power or force of tills weight on the blocks P, Q, is as theproduct of half the weight multiplied by the distance AB, or BC,of its direction from the fixed point, it follows, that the wholeforce of this weight is as twice the product of half the weight W,.multiplied by A15 or BC ; or as the whole weight W multipliedby the distance AB or 15C. lienee also, if the length AC of thescantling between the fixed points A, C, be c ; the area of (hesection s ; the distance of its centre of gravity from the base d,and the weight W, w ; then will few express the force of theweight VV, and dr the strength of the scanting: therefore themomentum of the weight is to the momentum of the scantling as

few is to ds ; or as to is to * - ■ ■ ■> ami if this ratio be given, w

— From whence may be drawn several useful conse-

quences. 1. The strengths of two scantlings of the same wood,and of different dimensions ; or, which is the same, the weightsthey will bear, are to each as the products of their sections multi-plied by the distances of (lie centres of gravity from the base, di-vided by their lengths. 2. The strengths of two scantlings of thesame wood, which have the same length, are as the products oftheir sections multiplied by the distances oi their centres of gra-vity from the base. 3. The strengths of two scantlings of thesame wood, which have equal sections, are as the distances of theircentres of gravity divided by their lengths. 4. 1 he strength ofscantlings of the" same wood, whose distances of their centres ofgravity of their sections, from the base, are equal, will be to eachother as their sections divided by their lengths. What is here saidof timber, is true of any homogeneous bodies whatever sort ofmatter they are of. But’the absolute strength of any beam, le-ver, rope, &c. when drawn in direction of its length, will be asthe’section of it. For every part does in this case bear an equalstretch ; and the sum of all the parts is equal to the whole, anddial is as the section.

Prop. 1. Mr. Emerson has shewn that the strength f a tube orhollow cane AB, (fig. lit,) to that of a solid one CD ; is as thesection of the lube (exelnd ng hehollow) ‘o the section 0 : thesolid cane, and the diameter of the tube, to the diameicf of thesolid cane, nearly.

Cor. Hence the strength of different tubes are as their sections,and diameters, nearly.

Prop. 2. The same author has demonstrated, that if CD, (fig.

17 ,)