TREATMENT OF MANUFACTURED IRON AND STEEL. 283 
particularly require holes in boilers to be drilled, the hand rivetting | 
has, in nearly all cases, been superseded by hydraulic or steam 
rivetters. Many machine rivetters have been invented, but the 
majority of those in use are worked by hydraulic pressure and 
were invented by Mr. Tweddell. 
Before any rivetting is done, the drilled plates are separated and » 
the burrs removed. The plates are then drawn metal to metal by 
the free use of service bolts. If the work is not bolted together 
at every third or fourth hole, faulty rivets will result, and spaces . 
between the plates will exist for oxidation to take place. For 
boiler work the best rivetting machines are arranged to force the 
plates together before the rivets are closed, but the machines used 
for bridge work are not so designed. 
Mild steel rivets having a tensile strength of from twenty-four 
to twenty-seven tons per square inch are usually used where steel 
plates are adopted, but the author thinks that for boiler work, 
superior wrought iron rivets are preferable. The tensile strength, 
shearing resistance and ductility of such iron varies very slightly 
from the rivet steel referred to, and it is known that iron is not 
injured to the same extent as steel when it is worked at a low 
temperature. There is certainly some advantage with regard to 
first cost in favour of mild steel, and there is no reason why steel 
rivets for bridge and such like work should not be adopted. 
During rivetting, care is taken to heat the rivets in a clean fire 
and to knock them while held in the tongs, with the object of 
removing as much of the scale from them as possible. The rivet 
boy is instructed to do this. If this scale is not removed, the 
rivets, when put in place, will appear sound to the tap of the 
hammer, but if the head be cut off by a cold set, the resulting 
jarring will pulverise the cinder and oxidized surfaces, when the 
body can be easily pushed out. The rivets are well heated all 
over the shanks ; if this is not done and the ends only are made 
hot, the portions near the original heads will not fill the holes, even 
when put in by hydraulic machinery, and the rivets will appear 
to be loose, if those forged heads are tried by the hammer. 
