NATURE ON THE ROOF. 91 



thin and drooping for lack of soil, are sometimes seen 

 there, besides grasses. Ivy is familiar as a roof- 

 creeper. Some ferns and the pennywort will grow 

 on the wall close to the roof. A correspondent 

 tells me that in Wales he found a cottage perfectly 

 roofed with fern it grew so thickly as to conceal the 

 roof. Had a painter put this in a picture, many 

 would have exclaimed: "How fanciful! He must 

 have made it up ; it could never have grown like 

 that!" Not long after receiving my correspondent's 

 kind letter, I chanced to find a roof near London upon 

 which the same fern was growing in lines along the 

 tiles. It grew plentifully, but was not in so nourish- 

 ing a condition as that found in Wales. Painters are 

 sometimes accused of calling upon their imagination 

 when they are really depicting fact, for the ways of 

 nature vary very much in different localities, and that 

 which may seem impossible in one place is common 

 enough in another. 



Where will not ferns grow ? We saw one attached 

 to the under-side of a glass coal-hole cover ; its green 

 could be seen through the thick glass on which people 

 stepped daily. 



Eecently, much attention has been paid to the dust 

 which is found on roofs and ledges at great heights. 

 This meteoric dust, as it is called, consists of minute 

 particles of iron, which are thought to fall from the 

 highest part of the atmosphere, or possibly to be 

 attracted to the earth from space. Lightning usually 

 strikes the roof. The whole subject of lightning- 

 conductors has been re-opened of late years, there 

 being reason to think that mistakes have been made 



