INTRODUCTION. 



The average weights of equal volumes of "green" 

 wood, expressed in terms of a chosen standard, also 

 afford him information of service ; and when these are 

 compared with the average weights of similar volumes 

 of the same wood in a dry state, he obtains yet other 

 data, which lead him moreover to approximate notions 

 as to the quantities of water contained in the former. 

 Comparisons of the changes in volume undergone by 

 his specimens as they lose or absorb water also help him 

 to estimate the capacities for shrinking and swelling 

 which his specimens exhibit. 



But it must be confessed that his methods and con- 

 clusions in this connection are very rough indeed ; for, 

 while on the ^ one hand the same wood holds very 

 different quantities of water according to the time of 

 year at which it is felled, the soil on which it grows, 

 and the climate in which the tree flourished, on the 

 other hand the observations on dry wood have been 

 usually recorded for " air-dry " specimens — i.e. speci- 

 mens allowed to dry until they are judged to be dry in 

 the popular sense. 



Now it can be readily shown that such wood is never 

 really dry, and that it gives off and takes up considerable 

 quantities of hygroscopic moisture from the air ; and 

 there are many other fallacies at the bottom of these 

 conclusions. 



The same criticism applies to the weights of different 

 pieces of wood, as given in the tables of the engineer 

 and builder, etc. The numbers there found refer to 

 pieces which vary in volume — because their volumes 

 alter differently as the state of the atmosphere changes 

 — and which contain different quantities of water — 

 because different pieces of wood give up or retain dif- 

 ferent quantities of water as the state of the air varies. 



