368 REPORTS ON THE STATE OF SCIENCE, ETC. 
Many other conclusions might be added for particular metals, butitis believed that 
the foregoing summary gives a general survey, and affords a suitable basis for criticism 
of the theory that has been set forth. 
The diagram fig. 7 represents two fatigue tests on samples of the same steel with 
different heat treatments, and serves to illustrate generally two typical forms taken 
by the hysteresis-time diagram. The total hysteresis measured may be regarded 
as comprising three parts: transient, or primary; continuous, or secondary ; final, 
or tertiary. 
Transient hysteresis has been ascribed above to gliding action, and is associated 
with all the phenomena of cold-work. It preponderates in static measurements of 
hysteresis, and in the earlier stages of fatigue tests on an annealed metal. Being 
usually completed long before the appearance of the rapid rise that signals the approach 
of fracture, and absent in many metals that are none the less liable to fatigue, 1t is not 
regarded as the direct cause of fatigue, but rather as an imperfect protection against 
fatigue. 
Continuous hysteresis maintains a variable activity throughout the whole duration 
of a fatigue test, and, in a test-piece that is going to break, increases in magnitude for 
a long period before the appearance of the final rise. This continuous hysteresis is 
conceived to be due to the dual process, of decrystallisation and recrystallisation, that 
has been described as comparable with the process of solution and recrystallisation that 
occurs in a mixture of strained crystals immersed in their saturated solution. Con- 
tinuous hysteresis is regarded as evidence of an action that is thermodynamically 
irreversible and mechanically irreversible, or reversible according as the metal will or 
will not eventually fail by fatigue. 
Final hysteresis occurs immediately before fracture, and is regarded as evidence 
of the gradual extension of small cavities to form cracks. The action may be only a 
development of the preceding continuous hysteresis, but may differ in respect that it 
is often associated with gliding movements and ductile strain. 
As regards the bearing of this theory, and of the foregoing experimental work, on 
the practical testing of fatigue in metals, it appears that the observation of hysteresis 
should be a useful weapon of research, adding greatly to the information derived from 
any fatigue test in which a considerable body of metal is subject to uniform range of 
stress, 
IV. 
Stresses in Bridges. 
By J. 8. Wison, A.C.G.I., Assoc. M.Inst.C.H., and Professor B. P. Hatcu, 
D.Sc., M.Inst.C.E. 
In recent years the stresses in iron and steel bridge members have been investigated 
in detail, and have been demonstrated by experiment and mathematical analysis 
to be of a complex nature. The investigations have an important bearing on the 
choice of maximum or nominal stresses adopted in bridge design. The question 
of allowable stress and its complement, loading, in relation to railway bridges, has 
during recent years received a great deal of attention in America, India, and this 
country. Here and in India elaborate experimental researches are still in progress. 
These investigations were rendered necessary by the gradual increase in locomotive 
weights and the necessity of deciding on the renewal of some of the older bridges. 
in this country we have the oldest railway bridges in existence, many of which 
have successfully carried all locomotives and trains without restriction for upwards 
of sixty years, and, as the stresses in these bridges have an important bearing on the 
subject, the system followed in their design is of interest. 
Method of Design by Coefficient. 
It is important to note that the strength of these early bridges and girders was 
not estimated in relation to the stresses in them till 1859. Before that time the 
working strength was always referred to the breaking strength of the girder, which 
was calculated from the dimensions with the help of a coefficient determined experi- 
mentally for the different types of girder section. Thus at about the date mentioned, 
Sir William Fairbairn wrote : 
‘ After the completion of the Conway and Britannia tubular bridges, and during 
the early stages of wrought-iron bridge constructions, the engineers of this country, 
