PRESIDENTIAL ADDRESS. 491 
devote a short hour to the consideration of some of the available means which 
an engineer can use as a guide for his applications of science to construction, 
since of whatever kind are the professional activities he pursues, his place in the 
scheme of afiairs mainly depends on his ability to make machines and structures 
for directing and modifying natural sources of power in known ways, or applying 
them to new purposes as scientific discoveries advance the boundaries of 
knowledge. 
The power to do this depends, to no small extent, upon the ability to deter- 
mine the distribution of stress in a structure, and the skilful manner in which 
material can be disposed for the required purpose. 
It is of some help to our appreciation of the achievements of the great con- 
structors of past ages, if we remember that they probably all held the erroneous 
view that materials of construction are perfectly rigid bodies, and, indeed, we 
know that as late as 1638 Galileo Galilei was of that opinion, and that he came 
to an entirely wrong conclusion as regards the stress distribution in a loaded 
cantilever. 
It required the genius and insight of Robert Hooke to make a really great 
step, with his celebrated theory of the linear relation of stress to strain, and we 
can appreciate the glow of pride and satisfaction which he must have felt at his 
great discovery, when he records in 1675 that ‘his Majesty was pleased to see the 
experiment that made out this theory tried at Whitehall, as also my spring 
watch.’ 
Hooke had, in fact, discovered the fundamental principle upon which a 
theory of the elasticity and strength of materials could be based, and it would be 
interesting to trace the great advances which were rapidly made from this new 
vantage-grouud, whereby the main facts of the distribution of stress in simple 
members of structures became known, and a foundation was laid for the great 
advances of the mathematical theory. If I am silent upon the enormous develop- 
ments of the modern theory of the strength of materials it is not from lack of 
appreciation, but because I do not deem myself adequately fitted to discuss the 
great work of the elasticians, which all engineers admire, and so few are 
equipped to follow with the full battery of mathematical tools which have been 
pressed into service in the pursuit of this great science. 
Among the greatest of the services rendered by early pioneers was that of 
Young, who was the first to notice that the elastic resistance of a body to shear 
was different from its resistance to extension or contraction, and this led him to 
define a modulus of elasticity for materials in compression. As Professor Love 
remarks, ‘This introduction of a definite physical concept, which descends, as 
it were, from a clear sky on the readers of mathematical memoirs, marks an 
epoch in the history of the science.’ 
From the standpoint of the engineer, nothing is of more practical importance 
than the great discoveries of Hooke and Young, that bodies like metal, wood, and 
stone are ‘springy’ and have a simple linear relation between stress and strain. 
It is probably within the mark to say that nine-tenths of all the experimental 
investigations on stress distributions in structures have been entirely based on 
the fundamental principles which they enunciated, and new uses are continually 
arising. The recent application of the steam turbine to the propulsion of ships 
produced a profound change in marine-engine practice, and incidentally involved 
an entire reconstruction of methods for obtaining the horse-power developed, 
which had been gradually perfected from the time of Watt, but were absolutely 
useless for the new system of propulsion. Hooke’s discovery of the essential 
springiness of metals enabled engineers quickly to devise new instruments capable 
of accurately measuring the infinitesimal angular distortions of propeller shafts, 
and from these to determine the horse-power transmitted by the aid of an appro- 
priate modulus. 
The construction of tall buildings affords another example where advantage 
has been taken to determine the loads upon columns by measuring the minute 
diminutions of length as the structure proceeds, thereby affording a valuable 
check upon the calculations for these members, and a reliable indication of the 
pressures supported by the foundations. 
The distribution of stress in buildings constructed of composite materials like 
concrete reinforced with steel has also been examined by similar methods, and 
