Gibbs on Dry Rot. 15 
chant ships are at times troubled with it. Our ships of war 
being built of live oak, cedar, and locust, are less exposed 
to this evil. The live oak appears to be almost indistructi- 
ble, except perhaps by its contact with other species of 
wood, the juices of which, as in treenails,* may injure it.— 
But the time is not far distant, when we must bewail this ca- 
lamity, or discover some preventive. : 
e same evil attends the construction of modern built 
ises. The timbers of the roof of Westminster-Hall have 
been in place six hundred years, and I have examined in 
S$ country some which were placed one hundred and fifty 
years ago, and are seemingly uncorrupted and incorruptible. 
But no architect now would calculate on a durability of half 
the latter term. I have been informed that some of the 
floors in the new City-Hall, in New-York, though finished 
} Gee only six years, have been removed on account of the 
TY 80-25 4% pe acre ake : Cres 
Considering these and other facts before the public, I 
_ have been led to believe, that the dry rot is owi the 
nature of the wood, rather than to the deficiency of ordina- 
ry preparation. . 
~The wood of a tree consists of the heart and the albur- 
mum, or sap wood which forms the external concentric 
Layers. This last is the vehicle of the sap. In young 
trees, it extends to the centre, but as the tree grows, the 
heart becomes firm, and ceases to circulate the sap, and 
this process continues during the life of the tree. In aged 
trees the. sap wood forms only a small part of the timber, 
till at length a process similar to ossification in the o) 
of animals takes place, and the tree dies for want of nourish- 
- 
ment. ie 
: The durability of heart, and the pernicious effeets of sap 
wood, are well known ; but as timber bears a high price, 
workmen content themselves with taking off the coloured 
sap wood, without regarding the remaining part in the tim- 
ber. An oak tree, at the age of eighty years, is generally 
ofa size fitted for timber for large vessels. But if we com- 
pare this tree to one of the same size, but two hundred 
years old, we shall find the real proportion of sap wood and 
heart very different in the two specimens. Now if we con- 
*7 5? so aced by the shi t Editi 
Trunnels ? 50 y 1 by the ship carper Editor 
