426 MR ANGUS R. FULTON ON EXPERIMENTS TO SHOW 
the beam, and which is caused by the loading, we have the lateral widening and 
contracting of the section in compression and tension respectively due to the induced 
stresses. In deep, narrow beams this process is not greatly interfered with, but in broad, 
shallow ones there is a constraint due to the method of loading. The more elastic 
a material is, the less will this constraint be felt. Possibly the presence of very pro- 
nounced medullary rays allows the Oak to accommodate itself better than the Ash, 
and the latter in turn more readily than the Box. 
If we assume that all lateral strain is prevented, the apparent increase in value of 
the modulus of elasticity H, compared with the case where there is no such restraint, is 
2 
represented by — 7 where | =Porsson’s ratio. If 1=-25 for Ash and -4 for Box, 
oc — ion oe 
then the ratio fe) = 1°06 for Ash and 1°2 for Boxwood. As the author has had no 
opportunity of measuring Porsson’s ratio for these materials, it is impossible to say how 
far such an assumption is justified. 
CoMPRESSION TESTS. 
In these tests half the number of specimens were cut with the broad side of the 
section tangential, and half with that side radial, and each series consisted of varying 
lengths. The lengths (/) were simple multiples of the narrow dimension (x), and ranged 
from l/=k to l= 13k. 
Fig. 33 shows a set of Oak blocks cut with broad side tangential and exhibiting 
the radial side, after having been subjected to longitudinal compression. The five 
shortest blocks, 7=k to /=8k, show no sign of bending, but have the usual local 
buckling taking place tangentially, as has been previously noticed. 6 (J=10k) and 
7 (J=12k) have both a compound failure, consisting of the usual local crumpling and of 
bending in the plane of the paper, which is of course the plane of the least dimension, 4. 
A similar set of blocks, but cut with the broad side radial and showing the 
tangential side, is illustrated in fig. 34. Here the compound fracture is slightly shown 
in the fourth block (/= 6), which is much earlier than in the other series. This is to be 
expected, for the local buckling and the buckling of the test piece as a whole take place 
in the one plane, that of the paper. Although this appears on shorter lengths than in 
the other series, there is evidently no great difference in the crushing stress required. 
The extra inducement to bend only comes into play after the local buckling has taken 
place and failure has begun. Only in pieces of very short lengths (J=- or /= 2k) is 
there any advantage, and then it rests with those cut with the broad side tangential. 
The Ash and Box tests show very similar results, so it is not necessary to illustrate 
them. : 
Plotting the results obtained with these timbers, and using the crushing pressure per 
square inch as ordinates and the ratio ‘ as abscissee, it is found that, within the limits 
k 
