2 INTRODUCTORY. 



these final tests, owing to the nature of their load and 

 the position of the structures themselves. Examples of 

 such are to be found in roofs and bridges subject to 

 wind pressures. But even if it were possible to apply 

 these after tests in all cases, it would be preposterous 

 to construct a great and costly work entirely by "rule 

 of thumb," and trust to luck that it would be strong 

 enough to withstand the loads which might be applied 

 to it. 



Some previous knowledge, both of the loads to which 

 each part of a structure may be expected to have to 

 withstand, and of the strength of the materials forming 

 these parts, is absolutely necessary. In former times, 

 before such a knowledge existed of the strength of the 

 various materials of construction as is now the case, an 

 engineer had to rely to a great extent on his own or 

 other people's experience, and on his own judgment and 

 instinct for the dimensions of the parts of his structures. 

 Such a plan is of necessity still followed out to a great 

 extent, especially in the case of small and unimportant 

 parts, and also very often in timber structures. It 

 is not an uncommon thing in drawing offices to see a 

 draughtsman vary a dimension several times until he 

 thinks " it looks right." This is perfectly legitimate and 

 allowable in many cases, and is, indeed, necessary in such 

 cases, for instance, as the frames of machines and other 

 structures subjected to a great variety of indeterminate 

 stresses, or to very slight stresses, where stiffness alone is 

 necessary. But in large and important structures and 

 pieces of machinery a certain definite plan must be, and 

 nearly always is, followed. 



In the first place, the engineer must know what loads 

 his projected structure will have to sustain as a whole, 

 what part of these are live loads, what part dead loads, 

 and if any shocks are to be expected, and of what magni- 

 tude. 



Secondly, knowing what the loads upon his structure 

 are to be, he must next be able to calculate what will be 

 the effect of these loads in producing stresses in the 

 several parts of the structure, what are the magnitudes of 

 these stresses, and what their nature ; that is, whether the 

 stresses are those of tension, compression, bending, or 

 torsion. 



Thirdly and lastly, he must have an intimate know- 

 ledge of the properties of the materials he intends to make 



