14 TRANSACTIONS OF ROYAL SCOTTISH ARBORICULTURAL SOCIETY. 
if the span is a big one, and secondly, because it is a kind of test 
which can be carried out quite readily on the works, with a fair 
degree of accuracy, without any elaborate apparatus. Two knife- 
edges, or supports, for the ends of the beam, a cradle to sling 
from the centre, in which pig-iron or any other convenient kind of 
heavy weight can be placed, a stretched cord, and a 2-foot rule, 
are practically all that is needed to give fairly good tests when 
this description of stress and strain is adopted. 
I give again ina Table some of Bauschinger’s results, made on 
beams 74 inches by 74 inches, with a span of 98 inches. 
Kind of Wood. | eiatiety. | Blastienty.| Surease | Density: | Percent 
i | 
Scots Pine— Ibs. sq. in. |Ibs. sq. in. | Ibs. sq. in. | 
Summer, . : - | 1,535,000 |) 2,867" |) 6.720 | 0°50 23 
Winter, . : . | 1,465,000 | 3,136 6,405 0°55 | 33 
Norway Spruce— | 
Summer, . : . | 1,563,000 | 3,247 | 5,957 0°45 | 29 
Winter, . , . | 1,650,000 | 3,719 6,406 0°45 27 
It is desirable that I should point out at once that the 
calculated stresses produced in the outer fibres of a beam when 
it is subjected to cross bending, as determined by the ordinary 
formula employed for that purpose, are only reliable, and only 
represent the real condition of affairs when the stress does not 
exceed the elastic limit, because beyond that elastic limit the 
assumptions upon which the ordinary beam formule are based 
do not in the least apply; hence the figures given in the above 
Table, under the heading “ Breaking stress in lbs. per square 
inch,” are not really actual stresses at that instant, but they are 
figures useful enough for comparison purposes. 
In the following Table, results are given which were obtained 
by Professor Lanza in a number of experiments on American 
timbers, and in this case I have given the maximum, minimum, 
and mean results obtained for each quality of timber, in order 
to show the very great variation there is in the strength of 
timber of apparently the same quality. 
