EXPANSION OF SOLIDS WITH RISE OF TEMPERATURE. 25 



of a rise in temperature of 25 C. Now Ramsden's value of the co- 

 efficient of expansion of steel is -00001145. Then 400 miles will increase 

 with a rise of 25 by 



400 x -00001 145 x 25 miles = '1145 mile, 

 or over 200 yards. 



Then this distance, at least, must be left in intervals between London 

 and Edinburgh. 



In iron bridges some arrangement must be adopted to allow for 

 expansion without serious change of shape in the structure. The Menai 

 tubular bridge, of which the total length is over 1500 feet, is mounted 

 on rollers, and the joints between the successive tubes are telescopic, 

 and have a play of several inches. In the Forth Bridge rocking columns 

 are interposed between the ends of the central girders and the ex- 

 tremities of the central Inchgarvie cantilever arms, and the shore arms 

 of the side cantilevers are left free to slide on their abutments. 



In the long lengths of iron rod used to work railway points at 

 a distance from the signal box, expansion or contraction through 

 change of temperature might seriously interfere with the working. The 

 rod is therefore divided into successive lengths connected by short 

 cross-pieces. Thus in the simplest case, represented in Fig. 16, it may 



At. 



ID 



C 



FIG. 16. Provision for Expansion in Rods Working Railway Points. 



be divided into two halves AB, CD, with a connecting piece BOO turning 

 about O, which is fixed. If A and D are fixed, expansion is provided for 

 by BC turning round as indicated by the dotted lines. But if A be 

 moved back by the pointsman, D will move the same distance forward. 



An interesting application of the expansion of iron was first made 

 many years ago in order to draw together the two side-walls of a 

 gallery at the Conservatoire des Arts et Metiers at Paris, which were 

 bulging outwards through the weight of the roof. Long iron bars were 

 passed through the two walls, and circular plates were screwed outside 

 on to the two ends of each, till they came against the walls. The bars 

 were heated inside and expanded. The plates, having been thus pushed 

 out, were screwed farther on till they again touched the walls, and, on 

 cooling, the bars contracted and drew the walls together. By several 

 repetitions of the process, the alternate bars being heated in each 

 operation, the walls were brought to their proper position. 



It is well known that a thick glass vessel is very liable to crack, if 

 hot water is poured into it. This is due to the sudden expansion of the 

 inner layers, the outer layers not at once receiving heat, since glass is a 

 poor conductor. The strain in the glass is relieved by the rupture. 



The contraction of iron on cooling is made use of in putting hoops 

 on casks and tires on wheels ; the iron is put in position while hot, 

 and as it cools the contraction keeps it firmly in its place. 



