258 AGRICULTURE ON THE PACIFIC SLOPE 



or make a cup by folding a piece of clean paper. Drink- 

 ing cups of paper or rubber may be bought at drug 

 stores. 



Each one of the bacteria in the gelatin in a short 

 time produces millions more like itself. We can thus 

 see how a few cholera or typhoid bacteria may quickly 

 infect a water or milk supply. 



Colon bacillus. If bubbles of gas appear in the gela- 

 tin, it means that your water or milk supply contains 

 bacteria which live in the intestinal tract, and sooner 

 or later cholera or typhoid bacilli may be found in such 

 milk or water. If you find such bubbles, you should 

 report it to your doctor or health officer. 



One of the most interesting maps ever made is shown 

 in figure 154. It shows how the cases of cholera, in the 

 great epidemic of 1892, stayed on the Hamburg side of 

 the line, although the city of Altona just over the line 

 used the same water supply. You will notice that in 

 some places all the houses on one side of the street 

 have cholera, while those on the other side do not. 

 The reason is that Altona filtered its water supply, 

 but Hamburg did not. 



Filtering the water supply. All that is needed to 

 filter the water is to pass it through several feet of sand. 

 A film or scum forms on the surface of the sand, and this 

 catches the bacteria and holds them back. Twice a 

 day the sand is cleaned by sending a strong current of 

 water through it in the opposite direction. This water 



