4 TRANSACTIONS OF ROYAL SCOTTISH ARBORICULTURAL SOCIETY. 
carried out in America on specimens of short-leaf, or yellow, pine 
(Pinus echinata, Mill.) sap-wood. The vertical ordinates give the 
crushing strength in lbs. per square inch, and the horizontal 
abscisse give the percentage of moisture in the different specimens. 
It will be observed that the strength falls off exceedingly rapidly in 
the same quality of timber as the percentage of moisture increases ; 
and further, it is interesting to note an important point in 
connection with this law of variation of strength with variation 
of moisture. When the moisture reaches about 4o per cent., 
any further increase appears to have but small influence upon 
the strength of the wood; this is due to the fact that any 
increase in the moisture present beyond about 4o per cent. 
merely fills up the empty cell-spaces with moisture, and this 
does not affect in any way the strength; it is the wetting of 
the cell-walls by the first portion of the moisture absorbed 
which is injurious and reduces the strength of the timber. 
The equation to the curve shown in Fig. 1 at once enables 
the strength at 15 per cent. moisture to be calculated, when 
the strength at some other percentage of moisture has been 
experimentally determined. 
Another good series of thoroughly scientific tests upon timber 
was that carried out on behalf of the Forest Department of 
the Board of Agriculture of the United States, between the 
years 1891 and 1895. In this great series of tests the 
mechanical experiments were carried out mainly by Professor 
Johnson of Washington University, St Louis, U.S.A.; while 
the microscopic and other examination of the cellular structure 
of the woods was carried out at Washington itself. The 
experiments were carried out on wood specimens prepared 
from about three hundred trees, embracing about ten different 
kinds of pine or needle-leaf trees, that is, soft woods, and about 
five different kinds of broad-leaf trees, or hard woods, of the 
United States; and when each tree was felled, an accurate record 
was kept as to the condition of the soil in which the tree grew, 
the climatic conditions of the district in which it grew, the 
size of the tree, the conditions of its growth, the age of the 
tree, and the date of felling. You will see, therefore, that 
most careful records were kept not only of what happened 
during the actual tests, but of the previous life-history, so to 
speak, of the trees from which the samples were prepared, and, 
unless such a record is kept, the value of the research, from the 
