EXCRETION 
461 
cattle tick, was so successful that in 1918 hides valued at 
many millions of dollars were added to the general supply 
in the United States. Another biological factor in the pro¬ 
duction of good leather has reference to the care that the 
cattle receive. If they are well fed and properly housed 
during the winter and keep free from disease, then their 
hides have a better texture and are finer grained. The 
coarser grades of leather are made from older cattle and those 
that have little or no shelter in winter. Man has never been 
able to originate or create leather. It must be first grown as 
the skin of some animal. Man’s skill consists in his manipu¬ 
lations of the hide after a living animal has grown it. 
Sewage. — In every town and city where a general water 
supply is established, it is necessary to provide means for the 
removal of the waste water. This water comes from homes, 
places of business, and various manufactories. Not only 
the wastes from the human body but also from the street 
washing, the waste products of various factories, and the 
annual rainfall and snow are all added to the waste waters of 
a city or town. Such water is known as sewage. A great 
deal of study is being given to this problem that becomes more 
and more difficult as the number of people living in a given 
place increases. 
It is now known that there is an average of one hundred 
gallons of sewage daily for every inhabitant of a city. The 
daily sewage from homes averages about thirty gallons for 
each member of a family; but when we add the street flush¬ 
ing and wastes of various manufactories the total amount 
per capita is not far from the larger amount named. Thus 
in a city of 100,000 inhabitants, there will be about ten million 
gallons of sewage a day. What must be the daily average 
of sewage in your city? 
Disposal of Sewage. — The question of what shall be done 
with all this vast amount of sewage is one of the most 
difficult that cities are trying to solve. The cities that are 
