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-mâché, 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 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 Corrugated Iron, which has come so much into use for temporary buildings, such as schools, places of worship, reading-rooms, &c. It is very light and very strong, and can be used either for roof or walls with equal success.
By means of certain wasps belonging to the genus Polistes, Nature produces corrugated dwellings, which are made of very thin materials, but which are marvellously strong in proportion to their weight.
The insects belonging to this genus are all exotic, but are spread over a very large surface of the earth.
So strong are the nests made by some of these species, that they need no external covering, the corrugated paper supplying at the same time strength and warmth, the latter element being furnished by the air which is entangled between the corrugations.