SNOW, ICE AND FROST 177 



the wind has cut and poUshed the hard pebbles held 

 fast in the clay of which the cliff is made. Then you 

 will know how terrible the force of the wind-driven 

 sand must have been. 



Many things we learn to do in a better way by 

 watching nature and seeing how she works. A man 

 once noticed the force with which the wind drove 

 sand against a cliff, and he thought about it so much 

 that he invented a new way to decorate glass. It is 

 called engraving glass with a sand-blast. There is a 

 pattern cut as you cut your pattern for stencil-work. 

 Then the glass, excepting where the pattern is to 

 be, is covered with wax, and a blast of air full of fine 

 sand is blown against it. Wherever the sand touches 

 the glass the surface is made rough and white, not 

 transparent as it was before. 



The wind helps, also, in blowing about the finer 

 particles of sand and dust which have been worn 

 from the rocks. You have already learned how it 

 piles up heaps of sand along the shore and moves 

 them almost as if they were marching. It does the 

 same thing in the desert. 



The rain has another helper in the cracks or fissures 

 in the rocks themselves. For the rain is caught and 

 held in these cracks ; then comes the frost and freezes 

 the water. You know that water frozen into ice takes 

 up more room than it does as water. That is why 

 tumblers are broken when water freezes solid in them. 

 Ice breaks rock m the same way, making the fissure 

 wider and deeper and splitting off many pieces from 

 its sides. At the end of the winter you will often find 

 these fragments of rock lying in a heap at the base 

 of a ledge. 



