Page:Cyclopaedia, Chambers - Supplement, Volume 1.djvu/764

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but left it to become more weak and defective for a fucceflion of years ; during which the feveral annual circles of fuperve- ning btea all became found and perfect wood in their natural courfe.

It appears very plain, that the blea muft be that part of the wood which fuffers moft, becaufe it is the outer wood of all, and itfelf makes a covering for the inner part; and add to this, that it is of itfelf of a much lefs compact texture, and contains more juices for the fro/1 to act upon. It was obferved in France the year after this fevere _/r£/?, that the young trees had fuffered much lefs by it than the old ones. But it it is not improbable, that the reafon of this might be, that the fap- veflels being very tender and flexile in thefe, and hard and ri- gid in the old trees, the fame force which only dilated them in the young ones, might burft them to pieces in the old ones. It was fuppofed by fome, that the blea of all the old trees was abfolutely killed in this fevere winter. But that feems not to have been the cafe. Experiments made it appear, that it Hill maintained a courfe of fap for the fervice of the remaining part of the tree, though it had fuffered fuch a rupture in its veflels, as not to be convertible, according to the due courfe of na- ture, into fohd wood ; and, in fine, that it got an incurable difeafe, though not abfolute death.

In the accurate examinations made after the year 1709, the falfe blea was not found in the fame ftate in all the trees that were examined, but appeared to have fuffered by the fro/1 in very different degrees. Some trees afforded much thicker on one fide than on the other, which is a ftate wholly unnatural to It; and others had it fo thin all round, as plainly to prove that the whole blea had not fuffered, but that the inner part of it had efcaped, and had abfolutely been fince turned into wood according to the due courfe of nature, while only the outer parts had fuffered thi diftemperature, and remained, though much damaged, yet not at all changed into wood. The fat (e bleas of the feveral trees were not at ail alike in co- lour, nor had fuffered the diftemperature in an equal degree; and in thole which had fuffered it in ever fo great a degree, on digging down towards their roots, that part of the blea which was under ground was found to have efcaped unhurt, and to have been fince regularly converted into hard wood ; the co- ver of earth having proved an abfolute defence againft the

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The injuries done by fevere frofls to old trees are therefore ve- ry great, and it is not one or two particular fpecies that are fubject to them ; but it is hard to name any one that is exempt from them. Timber may always be difcovered to have this defect by the eye, when viewed in the whole tree ; and where there is feen a bed of falfe blea in it, its ftrength is according- ly to be accounted the lefs, in proportion to the thicknefs and unfoundnefs of this blea.

Another defect occafioned in trees by the fevere fro/Is of that year, and probably in a greater or leffer degree by thofe of many others, is, that the blea, and with it the bark, has in part perifhed, and the reft has remained either wholly unhurt, or, at the worft, in a much better condition than that. This is difcovered when a number of years afterwards the trees are felled for timber. In fa wing thefe horizontally through the body, there is found a piece of dead blea covered with a dry and decayed bark, which is buried as it were in the wood of the tree, being covered by feveral circles of living wood. This decayed matter generally occupies about a fourth part of the circumference of the body of the tree, and is fometimes much browner, and fometimes much whiter than the reft of the wood ; and it is a general obfervation, that thefe decayed parts are ufually found on that fide of the tree which has faced the fouth. The great number of trees that were examined after felling in the different parts of France, were found to have this dead blea and bark all covered under an equal num- ber of annual circles of found wood, which being counted were found to be the fame number with the years that had paf- fed fince the year 1709 ; and confequently all muft have been the effect of the terrible fro/Is of that winter. A part of the bark and blea of the trees having perifhed by the fro/l y and the reft remaining alive, that part had finally been grown over and covered by the fucceeding increafe of the other parts of the tree. The reafon of its being ufually found to the fouth feems to be, that the frozen juices being then thawed every day by the fun, fuffered the violence of a new freezing every night. This defect was feldom found to run the whole length of the trunk, but ufually left part quite free, and fcemingly found.

Finally, another accident attending trees from very fevere frojh is, that they are fometimes fplit by them according to the direction of their fibres, and this often with a very loud crack,

It is not uncommon in large forefts to meet with trees that have been thus fplit, and which having outlived the winter that occafioned it, are found to have a long rib marking the place where the crack was, and forming a fort of cicatrix or fear over the wound. This cicatrix or rib is however only fu- perficial ; and where-ever it is, the crack is ftill found re- maining under it in the tree ; for though the bark and blea

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can in this manner consolidate and form a coveiino- for a wound, the fibres of the folid wood when once broken or fe- parated never clofe again. It is the general opinion of the workmen that all thefe cracks in timber are the effects of frofts ; and it is very eafy to conceive, that it may be natu- ral for frofl to effea it, as it is well known that all frozen liquors take up more room, then they did while in a fluid ftate. It is not certain however, that all the cracks in timber are from this caufe, many of them may have been occafioned merely by a redundance of fap.

On examining the trees felled in France in the year 1736 great numbers of them were found to be thus cracked, and that not in any particular part, as is the cafe in the other dif- temperatures occafioned by frofl, which are ufually to the South, but in all expofitions, and in trees growing in all foils They were found to bemoft frequent, however, in the trees that had grown on moift foils, and in the North and Weft expo- fitions. This feems to have happened from the frofls having afled more violently at north and weft; and becaufe the text ture of the fibres of wood is much more lax and rare in thofe trees which ffand on marfhy ground than in others and their fap being more plentiful and watery, the effefls of the frofl will be the greater, as the quantity of juices for it to aft upon is greater, and as the fibres are capable of lefs refif- tance. That the watery nature of the juices of trees is what expofes them to thefe injuries from fro/Is appears very probable from hence, that the pine and fir, and other trees whofe juices are of a refinous nature, are not found to be fubjea to thefe injuries. We well know that oils and refinous juices are not fo fubjea to congeal by frofls as water, and that when they do, they contraa, not increafe in their bulk by the congelation ; and therefore the injuries from frofl which depend on the fwelling of the fap from freezing, can by no means affea thefe.

In the trees which have been fplit by frofl, there is always found in the crack under the cicatrix a remainder of decayed wood or of fap ; and the appearance is no way different from thofe defeas which arife from a decay of the woody fibres by mere redundance or diftemperature of the fap, only that thefe laft are never covered with any cicatrix. It is poffible that the frofls in fevere winters may do many other damages to trees belide thefe ; but thefe are very certain- ly known to be wholly owing to it. Nor are the frofls of the fpr.ng feafon, tho' lefs violent, to be fuppofed to do us any lefs injury ; for they attack vegetables, at a time they can lefs bear them, and what they want in violence, they have in frequency.

The opinions of authors about the expofitions of trees to the different quarters, have been very different, and moft of them grounded on no rational foundation. Many are of opinion that theeffeas of frofl are moft violently felt on thofe trees which are expofed to the north ; and others think, the fouth or the weft the moft ftrongly affeaed by them. There is no doubt, but the north expofure is fubjea to the greateft cold It is denied the benefit of the fun, whofe heat alone can at- temperate the rigour of fevere frofls ; and belide this, it is expofed to the north, north-eaft and north-weft winds,which are ever the coldeft of all others, as is proved abundantly, not on- ly by our own fenfations, but by the thermometer ; and the earth on the north fide of a hedge is often found perfeflly fro- zen, while that on the fouth is Toft enough to bear working The fnow found on the north fide of mountains, when there is none elfewhere, is another proof ; and from the whole, it is certain, that the north expofure is of all others the cold'elf and the froS there the ftrongeft. It does not however follow from this, that the injury muft be always greateft on the trees expofed to the north in fro/Is ; on the contrary, there are abundant proofs among the preceeding obfervations, that it is on the fouth fide that trees are generally more injured by frofl ; and it is plain from repeated experiments, that there are particular accidents, under which a more moderate frofl may do more injury to vegetables, than the moft fevere cine which happens to them under more favourable circumftances. It is plain from the preceeding accounts of the injuries trees received by the frofls in 1709, that the greateft of all were owing to repeated falfe thaws, fucceeded by repeated new frofls. But the frofls of the fpring feafon furnifli abundantly more numerous examples of this truth; and fome experiments made by Mr. deBuffon, at large in his own words, prove in- conteftably, that it is not the fevereft cold or moft fixed frofl, that is the greateft injury to vegetables.

This gentleman caufed kven or eight acres of coppice wood ■which flood in an open plain, and in a dry foil, furrounded every way with cultivated land, to be cut down in the courfe of the winter, in the year 1734. He ordered feveral fquare clutters iof trees to be left in the place ; thefe were left fquare, and were fo contrived, that their four faces direaiy pointed to the four expofures, north, fouth, eaft and weft ; the reft of the wood being all cut clean down, he obferved in the fpring what would be the manner of mooting of the old ftoeks, and of the trees of thefe clufters. On the twentieth of April he found that there were ftrong (hoots on the fouth fide of every

clutter,