190 NATURAL HISTORY 
About half an inch seems to be a sufficient layer 
foraday. Thus careful workmen, when they build 
mud-walls (informed at first, perhaps, by this little 
bird), raise but a moderate layer at a time, and then 
desist, lest the work should become top-heavy, and 
so be ruined by its own weight. By this method, 
in about ten or twelve days, is formed an hemispher- 
ic nest with a small aperture towards the top, strong, 
compact, and warm, and perfectly fitted for all the 
purposes for which it was intended. But then no. 
thing is more common than for the house-sparrow, 
as soon as the shell is finished, to seize on it as its 
own, to eject the owner, and to line it after its own 
manner. 
After so much labour is bestowed in erecting a 
mansion, as nature seldom works in vain, martins 
will live on for several years together in the same 
nest, where it happens to be well sheltered and 
secure from the injuries of weather. The shell or 
crust of the nest is a sort of rustic-work, full of 
knobs and protuberances on the outside; nor is 
the inside of those that I have examined smoothed 
with any exactness at all, but is rendered soft and 
warm, and fit for incubation, by a lining of small 
straws, grasses, and feathers, and sometimes by a 
bed of moss interwoven with wool. In this nest 
the hen lays from three to five white eggs. 
At first, when the young are hatched, and are in 
a naked and helpless condition, the parent birds, 
with tender assiduity, carry out what comes away 
from their young. Was it not for this affectionate 
cleanliness, the nestlings would soon be burned up, 
and destroyed, in so deep and hollow a nest, by 
their own caustic excrement. In the quadruped 
