INTRODUCTOBY. 



Ix all engineering works, whether they be of masonry, 

 timber, concrete, or metal, the question of strength is 

 one of the most important which has to be considered ; and 

 where any material is to be so placed in a structure as to 

 have to withstand considerable stress, some previous 

 knowledge of the strength properties of this material must 

 be within the reach of the engineer who contemplates 

 its use. 



In every engineering structure there are two distinct 

 points with regard to its strength which must be aimed at. 

 These are that, in the first place, the structure must be of 

 ample strength to withstand any and all loads to which it 

 may be subjected without permanent injury to its parts ; 

 and, secondly, that there should be no more material 

 employed in any one part than is sufficient to ensure the 

 fulfilment of the first condition. The chief reason for this 

 latter condition is sufficiently obvious it is simply a 

 question of cost. But, apart from pounds, shillings, and 

 pence, it is obvious that to introduce any such beyond 

 that which is necessary to ensure safety and efficiency 

 would be an unwarrantable waste of valuable material ; 

 and, further, it is in many instances advisable to reduce 

 the weight of a structure as far as possible, so as to thereby 

 lessen the stresses due to the weight of the structure itself. 



In the case of many engineering structures and works 

 it is usual to apply some kind of test load after the com- 

 pletion of the structure, so as to provide a final check on 

 the work before submitting it to its ordinary load. Thus, 

 bridges are often tested in this manner, by putting upon 

 them loads greatly in excess of those to which they may 

 be expected to be subjected in daily use. Again, boilers 

 are treated in a somewhat similar manner, by applying 

 internal hydraulic pressure double, or more than double, 

 the intended working pressure of the steam. There 

 are, however, many structures which cannot be put to 



B 



