480 



NATURE'S TEACHINGS. 



All who have tried their hand at practical geology must be 

 aware of the multitudinous casts of perished beings which are 

 found in various strata. Sometimes the casts are those of 

 vegetables, the original material having been decomposed, and 

 stony matter taken its place. Sometimes there are casts of 

 fishes or echini, while shells, and even insects, are found to 

 have been cast almost as perfectly as could be done with plaster 

 of Paris at the present day. 



As might be anticipated, the chalk deposits are peculiarly 

 rich in these casts, the fine particles of the chalk taking the 

 place of the plaster of Paris. 



In the illustrations are shown examples of casting in Art and 

 Nature. On the right hand is a cast of fruit and leaves, which 

 may afterwards be reproduced in plaster, wax, papier-mache, 

 or electrotype. On the left is shown one of the shells so 

 common in the chalk, the upper figure representing the shell 

 itself, and the lower the mould that has been formed around it. 



CORRUGATED IRON. 



"WE have already seen that the Wasps are paper-makers. 

 We may now see how some of the Wasps have anticipated a 



NESTS OP POLISTES. 



COEEUGATBD IRON. 



valuable invention of man, namely, the principle of corrugation, 

 whereby a thin plate gains strength. 



Even a sheet of paper gains great strength by corrugation, 

 as is seen in those paper covers which are so much in use for 

 the decoration, or rather the concealment, of flower-pots. But 

 the best example that can be given of this principle is the 



