EFFECTS OF EXPANSION. 771 



come 1-0000294 x 1-0000294 x 1-0000294 = l cc -0000882. 

 The volume has thus increased by 0*0000882, and this 

 number is therefore the coefficient of cubical expansion 

 of zinc. But 0-0000882 is equal to 3 x 0-0000294 ; and 

 if a like calculation is made for other substances, it 

 appears that the coefficient of cubical expansion of a 

 substance is just three times as great as that of its linear 

 expansion. Similarly the coefficient of superficial 

 expansion of a substance is twice as great as its co- 

 efficient of linear expansion. 



Solid bodies which are heated do not increase their 

 volume to any considerable extent, at least not within 

 the ordinary ranges of temperature ; but the force ex- 

 erted in expanding is very great for it is equal to 

 the external force required to compress the expanded 

 body to its original volume without altering its tem- 

 peratureand the expansion is therefore in many cases 

 of great practical importance. 



An iron rail may be exposed in the winter to a tem- 

 perature of 30, and in the summer to one of 50. If 

 the length of the rail be 6 m at 0, it will become shorter 

 in the winter by 30 x 0*0000123 x G ra ; this is equal to 

 O m -002214 = 2 mm -214 ; in the summer it will lengthen 

 by 50 x 0-0000123 x 6 m = O m -00369=3 mm -69. The total 

 variation in its length may thus possibly amount to 

 5 mm *904; and in laying down a line of railway this 

 change of length must be taken into account, and space 

 must be left between the rails to allow of their expansion ; 

 if they were laid down in the winter, end to end, they 

 would expand in the summer, and then either become 

 bent or forced altogether out of position. 



Iron telegraph-wires hang in the summer more 



3 T) 2 



