MATERIALS. 107 



The erosive power of sand is well illustrated by the " sand- 

 blast." This consists of a small jet of air or steam carrying sand 

 with it, and usually having a pressure of from 70 Ibs. to 90 Ibs. 

 per square inch. It is used for various purposes, amongst which 

 is the sharpening of hard steel files. 



Glass bottles placed in a sand-drift will very soon have all 

 the glaze taken off them, and the sharp edges of broken glass 

 quickly become rounded under such conditions. 



In the year 1891, an arrangement of low-pressure sand-blast 

 was used for cleaning the marble exterior of the United States 

 Assay Office in Wall Street, New York. In this instance, sand 

 was driven against the work by an air-blast having a pressure 

 of only 2 Ibs. per square inch. This abraded 1 square foot of 

 marble, to a depth of from -<fa inch to % inch in one minute, and 

 left a fresh clean surface. Great storm-waves strike with a 

 force of more than 40 Ibs. per square inch (p. 41). 



Tidal or other currents, carrying with them sand and shingle, 

 will often very quickly wear away the blocks forming a pier- 

 head ; special care is therefore necessary in selecting suitable 

 material for use in such positions. 



Sufficient has now been said to show the necessity of using 

 none but the most wear-resisting materials in positions where 

 they will be subjected to such erosive action as has been 

 described, but I would like to say a few words upon the direct 

 dynamical eroding power of the waves themselves. 



On carefully examining sea-cliffs, or rocky shores exposed to 

 heavy seas, evidences of erosion are easy to be seen. These 

 have already been noticed in the chapter on exposure and wave- 

 power ; but it is not always by such intermittent violent assaults 

 as are there described that the greatest effect is produced. 

 Moderate gales, of frequent occurrence, carry on the work of 

 destruction with none the less certainty because they do it more 

 slowly and with less ostentation. 



Wherever joints occur, either in rock or in artificial structures, 

 both mechanical and chemical action proceed the fastest. 



Apart from the inherent weakness of joints, the air or water 

 confined within them, when struck by a wave, is converted into 

 a very destructive agent. The air, under such circumstances, is 

 compressed and forced along joints or seams, and, immediately 

 upon the wave receding, it expands and tends to loosen and push 

 out pieces of the rock. 



