DUST MADE BY FRICTION. 5 



vehicles are evident enough ; for the streets are constantly 

 needing to be repaired. 



On the large flagstones in the main road between the 

 West India Docks and Whitechapel, for instance, where the 

 traffic is, of course, exceptionally heavy and unceasing, three 

 inches of hard granite were worn away in the course of 

 forty years; and the roads of Funchal, in the island of 

 Madeira, which run up steep slopes and are paved with 

 basalt, have become polished and slippery from constant 

 use.* 



Every one has noticed, too, how very dusty railway- 

 carriages are apt to be; and though some of the 

 dust in them consists of particles rubbed from the cushions 

 and curtains, and some of sand from the road, yet a large 

 proportion will adhere to a magnet and prove to be 

 minute fragments of iron and steel worn from the wheels 

 and rails. 



But the world's dust is not made solely by man. Nature, 

 too, is for ever making it in a variety of ways ; and as dust 

 of all kinds finds its way into our houses, it is not too much 

 to say that, before we can account satisfactorily for all the 

 contents of the dustman's cart, we must know something of 

 Nature's labourers and their methods of working. The 

 oyster-shells are, ot course, indebted to her dust-makers 

 and dust-carriers for the materials of which they are made, 

 and the same frost and heat which have broken countless 



* Mr. Bates, in his " Naturalist on the Amazons, 1 ' mentions that 

 those mother turtles which had laid eggs the previous year were easily 

 known by the fact that the horny skin of their breast-plates was worn by 

 their having crawled over the sand. 



