G INTRODUCTORY. 



boilers, railway appliances, and steamships. In most ot 

 these strength is of the first importance, although con- 

 siderations of form do in most cases affect the design. In 

 addition to iron and steel there are other metals in use for 

 engineering purposes, although, in these, other qualities 

 rather than strength render their use desirable. Such are 

 copper, brass, gun-metal, tin, zinc, lead, aluminium, and 

 various anti-friction alloys. Of these there are several 

 where considerations of strength do enter largely into their 

 use. For instance, copper is used for boiler fire boxes, 

 stays, steam pipes, and for overhead electrical conductors 

 where it has to withstand great tensile loads ; gun metal 

 also is often used in machine and engine parts where it 

 has to undergo considerable tension or compression. The 

 increasing use of aluminium and its alloys makes it 

 necessary for its strength properties to be known ; and 

 lastly a knowledge of the compressive strength of anti- 

 friction alloys in bearings is often needed. 



Timber is used to a great extent in constructive work, 

 especially for temporary work. It is, however, too uncer- 

 tain a material to allow structural parts to be designed 

 with the same certainty that exists in the case of the 

 metals, without the use of a large margin of safety. Still, 

 tests of timber, although approximate in character, are 

 often necessary and useful, and give reliable information 

 when properly carried out and judiciously applied. 



Last among the materials of construction which are 

 tested are those substances which are employed in works 

 of masonry and those of a like nature, chief among these 

 being the different kinds of natural stone, bricks and 

 terra-cotta, cements and limes, and these combined with 

 other substances to form the various kinds of concrete 

 which are used. So much depends on the strength and 

 reliability of these, which are very largely used in the case 

 of buildings, retaining walls, bridges, foundations, and 

 harbour works, and so great is the variation in quality of 

 apparently similar substances, that they present a large 

 field for testing operations. 



It will be clear from what has been said that the variety 

 of substances which are subjected, or may be subjected, 

 to tests is very great, and not only do these many sub- 

 stances used in constructive work present many differences 

 in their qualities and behaviour under test, but there are 

 many ways in which any given substance may be tested. 

 Take for example mild steel. It may be used for boiler 



