460. The durability of the framed timbers of buildings is also very considerable. The trusses of the old part of the roof of the Basilica of St. Paul, at Rome, were framed in 816, and were sound and good in 1814, a space of nearly a thousand years. These trusses are of fir. §

The timber-work of the external domes of the Church of St. Mark, at Venice, is more than 840 years old, and is still in a good state. || And Alberti observed the gates of cypress to the Church of St. Peter, at Rome, to be whole and sound after being up nearly 600 years.¶

* ' Quarterly Review,' vol. xix., p. 422; or ' Ency. Brit.,' art. Dry Rot.

† King's 'Observations on Ancient Castles,' p. 99.

‡ ' History of Windsor,' p. 329.

§ Rondelet, ' L'Art de Batir,' tome iv., p. 168. || Ibid., p. 259.

¶ 'Alberti,' book ii., chap. vi.

The inner roof of the Chapel of St. Nicholas, King's Lynn, Norfolk, is of oak, and was constructed about 500 years ago.*

Daviller states, as an instance of the durability of fir, that the large dormitory of the Jacobins' Convent, at Paris, had been executed in fir, and lasted 400 years.†

The timber roof of Crosby Hall, in London, removed in 1869, was executed about 400 years ago; ‡ and the roof of Westminster Hall, which is of oak, is now above 340 years old.

The rich carvings in oak which ornamented the ceiling of the king's room in Stirling Castle, are many of them still in good preservation. It is nearly 350 years since they were executed, and they remained in their original situation till a part of the roof gave way in 1777, when the whole was removed, and has since been dispersed among the collectors of curious relics of old times. §

Moreton Hall, in Cheshire, where "the staircase winds round the trunk of an immense oak tree," and the building itself is chiefly constructed of wood, has now existed nearly 300 years. ||

And Mr. Britton describes an old house at Islington, constructed chiefly of wood, which he has ascertained to be about 240 years old.¶

461. Other notices of extraordinary durability will bo found in the descriptions of the different kinds of wood. But enough already has been collected to show that timber is very durable where nothing more than ordinary means have been used to render it so; that is, nothing more than judicious selection and good seasoning.

* Britton's 'Archit. Antiq.,' vol.iii., p. 58.

† Daviller, ' Cours d'Architecture,' tome ii., art. Bois.

‡ Ibid., vol. iv,, p. 137.

§ ' Lacunar Strevelinense,' p. 4. This work is a collection of engravings of the carvings; and some of the borders might furnish useful hints to artists.

|| Britton's 'Archit. Antiq.,' vol. ii., p. 83. ¶ Ibid., p. 85.

Every permanent support should be formed of a good and sound piece of timber; inferior kinds should be used only for temporary purposes, or where no strain occurs, and where they can be easily renewed without injury to the strength of the building.

Mr. Barrow, in writing on this subject, very judiciously remarks, "that the felling of timber while young and full of vigour, making use of the sap-wood, and applying it to ships and buildings in an unseasoned state, have no doubt contributed to make the disease of dry rot infinitely more frequent and extensive than it was in former times, when our ships were hearts of oak, and when, in our large mansions, the wind was suffered to blow freely through them, and a current of air to circulate through the wide space left between the panelled wainscot and the wall. In those old mansions, which yet remain, and in the ancient cathedrals and churches, we find nothing like dry rot, though perhaps

" perforated sore And drilled in holes, the solid oak is found By worms voracious eaten through and through." *

462. In regard to the durability of different woods, the most odoriferous kinds are generally considered to be the most durable; also woods of a close and compact texture are generally more durable than those that are open and porous; but there are exceptions, as the wood of the evergreen oak is more compact than that of the common oak, but not near so durable.

Sir H. Davy has observed that, "in general, the quantity of charcoal afforded by woods offers a tolerably accurate indication of their durability; those most abundant in charcoal and earthy matter are most permanent; and those that contain the largest proportion of gaseous elements are the most destructible. Amongst our own trees," he adds, "the chestnut and the oak are pre-eminent as to durability, and the chestnut affords rather more carbonaceous matter than the oak." * But we know from experience, that red or yellow fir is as durable as oak in most situations, though it produces less charcoal by the ordinary process. The following Table of the quantity of charcoal afforded by 100 parts of different woods is added, for the information of the reader: -

* ' Encyclopaedia Britannica,' art. Dry Rot.

In Count Rumford's experiments a longer period was allowed for the process; and, in consequence, his results represent more nearly the real quantities of carbon in each wood than the others. But even according to the common process it docs not appear that the proportion of charcoal is a satisfactory criterion of the durability.

* 'Agricultural Chemistry,' p. 254, 8vo edit.; p. 221, 4to edit.

† ' Chemical Essays,' vol. iii., p. 27.

‡ ' Philosophical Magazine,' vol. ii., p. 183.

§ Dr. Thomson's ' System of Chemistry,' vol. iv., p. 185.