Stresses  on  the  Elastic  Properties  of  Steel.  211 
this  kind  were  made.  Mr.  J.  J.  Guest,  in  England  (Phil. 
Mag.  [5]  vol.  1.  p.  69;  Proc.  Physical  Soc.  of  London,  Sept. 
1900),  carried  out  a  series  of  tests  to  obtain  the  effect  of 
combined  stresses  on  ductile  materials,  the  materials  used  in 
the  tests  being  wrought  iron,  mild  steel,  copper,  and  brass. 
The  results  of  these  tests  also  lead  to  the  same  conclusion, 
that  the  elastic  properties  are  lowered  when  the  materials 
are  subjected  to  combined  stresses. 
The  slight  knowledge  on  this  subject  and  the  entire 
absence  of  any  information  available  for  engineers,  have  in- 
spired the  writer  to  carry  out  a  series  of  investigations  that 
should  give  some  information  immediately  available  for 
practical  work.  The  series  contemplated  includes  the 
following  : — 
(a)  Tests  of  steel  and  iron  solid  rounds  and  hollow  tubes  in 
tension  while  under  torsion. 
(/>)  Tests  of  steel  andiron  solid  rounds  and  hollow  tubes  in 
torsion  while  under  tension. 
(c)  Tests  of  steel  and  iron  solid  rounds  and  hollow  tubes 
with  increasing  tension  and  torsion. 
(d)  Tests  of  steel  and  iron  solid  rounds  and  hollow  tubes  in 
compression  while  under  torsion. 
Thus  far  only  a  part  of  series  (a),  on  solid  steel  rounds,  has 
'  been  carried  out,  and  the  results  given  in  this  report  are  taken 
from  these  tests. 
Apparatus  for  Testing. — The  difficulty  of  making  tests  of 
materials  under  combined  stresses  lies  principally  in  the  fact 
that  no  machines  are  available  for  such  tests.  The  apparatus 
used  in  the  tests  under  consideration  is  shown  in  fig.  1  (Pl.VL). 
It  consists  of  two  specially  constructed  heads  fitted  to  an 
ordinary  100,000  lb.  tension  testing-machine.  Each  of  these 
heads  consists  of  a  flat  cast-iron  base  fitting  into  the  slot  in  the 
head  of  the  testing-machine  provided  for  the  insertion  of  the 
wedges,  as  ordinarily  used.  The  outer  side  of  this  casting  is 
finished  to  provide  for  three  concentric  rows  of  hardened  steel 
balls,  which  rest  upon  a  steel  plate  and  are  covered  by  another 
steel  plate.  The  outer  side  of  this  latter  plate  is  a  spherical 
cup.  This  receives  the  large  casting,  or  chuck,  carrying  the 
wedges  and  arms.  The  spherical  bearing  allows  the  specimen 
to  "  line  up  "  properly.  The  construction  of  the  heads  is 
seen  in  fig.  2.  Each  head  is  provided  with  two  arms 
5  feet  long,  the  lower  arms  having  steel  rollers  that  bear 
against  stationary  knife-edges,  while  the  upper  arms  are 
provided  with  knife-edges  and  stirrups  to  which  the  cords 
carrying  the  loads  are  attached. 
In  order  that  the  loads  in  torsion  might  be  applied,  two 
