INTO THE STRENGTH OF WROUGHT-IRON PLATES. 
715 
In experiment 3, W=10759+18=10777 j ^=27, 
S= 
10777 X 27 X 1-91 
=35 tons, 
15568 
and S,= l 2*6 tons. 
Taking the data of Table XVIII., experiments 7 and 8, 
e= 1-425, e, = 2‘5, t='2, #i = -36, 
Hence we find from equation (2.), A=l-762; from equation (1.), X=l-86, and 
.'. Xi=2-5 — l-86=-64 ; from equation (3.), Ii=6-943; and from equations (4.) and 
(5.), S=W/x -00021 tons. 
In experiment 7, W=3008+l 1=3019, /=48, 
.-. S=3019 X 48 X -00021 =30-4 tons, 
and 
S.= 
•64x30-4 
1-86 
= 10-4 tons. 
In experiment 8, W=3142X 11=3153, /=48, 
.-. S=3153x48X-0002l=31-7tons, 
and 
S> = 
•64x31-7 
1-86 
= 10-9 tons. 
Observations . — The value of S determined from experiment 1, is the resistance of 
the material to extension, whereas the value of S determined from experiment 3, is 
the resistance to compression. Hence it appears, that in beams of this form and 
thickness of plates the resistance to extension is equal to that of compression. The 
same observation applies to the values of S determined from experiments 7 and 8 ; 
and the same law also holds true for experiments 9 and 10. 
These calculations further show, that the material in these beams is not properly 
distributed, for while the thin side of the beam is about to undergo rupture, the 
broad side has not attained one-half of the tension or compression, as the case may 
be, which it is capable of sustaining. 
It will also be observed, that the resistance of the material at the thin side, as in- 
dicated by these calculations, is greater than what it would be under ordinary circum- 
stances, viz. about 25 tons per square inch. This apparent discrepancy may be ex- 
plained as follows : — as a beam of wrought iron approaches the limit of tension it un- 
dergoes an accelerated rate of elongation, even while the cohesion of the material re- 
mains unimpaired*. Now this unusual extension of the particles in the lower laminae 
(in a beam having a single flanch placed upwards) allows a succession of particles 
in the higher laminae to come into full tensile strain, so that the particles at the lower 
edge of the beam apparently attain a tensile strain greater than they would have 
under ordinary circumstances. And it may be presumed, that a similar law obtains 
in reference to the compression of wrought-iron beams. Hence it follows, that all 
calculations which assume the tensile or compressive forces, in beams of this form, 
at the edges of the beam equal to what they are under ordinary circumstances, must 
lead to erroneous results. 
* See remarks on experiment 2, p. 720. 
4 Y 2 
