381 
1907-8.] On the Cohesion of Steel, and Yield Points. 
the cross-sectional area of the specimen at the fractured part presents 
difficulties, especially in the case of flat bars. Having regard to all these 
circumstances, the value of K corresponds fairly closely with the breaking 
stress ; but, as expected, the latter is somewhat lower, the average value of 
K being 59*5 tons per square inch, while that of the breaking stress is 54'3. 
In only four cases, namely for round bars 6170 and 6321, and for flat bars 
6215 and 6437, is the breaking stress greater than K. A column showing 
the reduction of area of each bar in the fractured region is also included in 
the table. 
Summary. 
(a) On the assumptions that resistance to deformation is due to simple 
friction, and that the coefficient of friction is independent of the load, the 
ratio of the yield point in tension to the yield point in compression, for 
what is ordinarily known as mild steel, is calculated as 2'384 to 3*384, or as 
0'705 to 1. Experimental results so far obtained do not agree well with 
these figures, the value for the tensile yield point being relatively high, and 
that for compression relatively low. 
( b ) On the further assumption that a cohesive force acting between the 
metallic particles gives rise to a frictional resistance which may be added 
(algebraically) to that due to the effect of the external load, the value of 
this cohesive force is deduced as equal to 3'384 times the stress which 
corresponds with the tension yield point, or to 2 '384 times that correspond- 
ing with the compression yield point. Experimental results from a large 
number of tests agree very fairly with the calculated figures for the case of 
tension. 
Note ( June 11, 1908). — The value of tjc may be obtained in a more 
simple manner than that given on page 377 ; for since 
a = (45° + <f>/2 ) , and /3= (45° - <£/2) , 
it follows that 
t sin 2 a — c sin 2 /3 = c cos 2 a 
and 
t = cot 2 a = cot 2 50° = (0-839) 2 = 0'704. 
{Issued separately July 20, 1908.) 
