28 THE WORLD'S LUMBER ROOM. 



according as they are measured in the winter or in the 

 summer. It is for this reason that a space is always left between 

 each length of rail, for, if it had not room to stretch, it 

 would bend upwards or outwards, with disastrous results. 



It is, perhaps, not easy to realise the amount of force 

 exerted by a piece of metal in the act of expansion ; but it 

 certainly cannot be ignored with impunity, as the builders of 

 an iron foot-bridge in London have had to learn to their 

 cost. They had covered the iron with concrete, and this 

 again with pavement, forgetting that, although these, too, 

 would expand in their degree, the metal would want to ex- 

 pand still more ; and the consequence was that when the 

 summer came, both were split by the force with which the 

 iron swelled upwards. 



From this it is evident that rocks composed of two or 

 three different minerals are at a special disadvantage with 

 regard to sudden changes of temperature, as one mineral 

 will expand more than its neighbours, and push them slightly 

 out of place a mere trifle it may seem, and yet, when often 

 repeated, it will be quite enough to loosen them. In a 

 damp climate the mischief is greatly increased by the absorp- 

 tion of moisture, and its conversion into particles of ice ; 

 but in the dry air of the Sahara the rocks are splintered into 

 fragments, and reduced to powder merely by the alternations 

 of heat and cold ; and in the Peninsula of Sinai where the 

 sun has scarcely risen before he begins to peel the skin from 

 one's face, though everything may have been covered with 

 hoar-frost during the night the very flints become so rotten 

 as to fall to pieces at a touch, from the repeated expansion 

 and contraction they have undergone. 



