INTRODUCTORY. 3 



use of, so as to be properly guided in determining the 

 shape and dimensions of all the parts. 



Of these three divisions the first entirely depends upon 

 the knowledge which the engineer may have as to the con- 

 ditions pertaining to the particular case in question ; the 

 second requires calculations depending upon the principles 

 of mechanics, and may be performed by simple arith- 

 metical or algebraic methods, or the problem may be 

 attacked graphically. When these two first conditions 

 have been determined, the last step in the design of the 

 structure may be taken; that is, knowing the stress 

 which may be expected to act on each part, both as to its 

 nature and magnitude, and also knowing what materials 

 are to be used, the engineer can at once proceed to ascer- 

 tain the form and dimensions of this part, provided he 

 is fully acquainted, or has some means of becoming 

 acquainted, with the strength properties of the materials. 

 It is towards the fulfilment of this last mentioned condition 

 that the " testing of materials" is undertaken. 



For such a case as we have just considered, it is usual 

 to test several small samples of the material to be employed, 

 as in the case of a strip of steel cut from a boiler plate, 

 on the assumption that the whole of the material is 

 uniform and similar in its properties to the specimens 

 tested, or, as in the case of a chain or rope, to subject an 

 actual piece of the work itself to the test. Such tests have 

 been called "commercial tests," and may be either 

 specially made in connection with the structure in 

 question, or have been previously made ,on some samp'es 

 of similar material. Besides tests of materials carried out 

 for purely commercial and constructive purposes, there is 

 a great deal of work being done at the present time, and 

 has been done in the past, of a more refined and scientific 

 character, with the intention in most instances, of eluci- 

 dating the more hidden and complex phenomena displayed 

 by materials under various stresses and under different 

 conditions of stress. Such research has been going on for 

 many years, and is going on most actively at the present 

 time. The knowledge obtained in this way is to a great 

 extent permanent, and though as time goes on more facts 

 are constantly being unearthed with regard to the strength 

 properties of materials, still these new facts are not, as a 

 rule, such as to render unimportant the knowledge already 

 gained, although it is certainly so in some cases. The 

 value of this scientific or research testing may not be at 



