170 



ELEMENTS OF GENERAL SCIENCE 



water from the public waterworks, and any fault in the sys- 

 tem affects the whole city, thereby attracting attention. In 

 rural districts, on the contrary, each home has its own inde- 

 pendent source of water, commonly a well, and contami- 

 nation of any one well does not usually affect so many 

 people. It is quite possible, however, that many wells may 

 be in a polluted condition at the same time, and the average 

 character of the water in a rural community may be very 

 bad. Indeed, typhoid and other water-borne diseases are 

 often more prevalent in the country than in the city. City 

 health officers have learned to expect an increase in the 



FIG. 86. Purity of farm wells 



Results of the examination of farm wells in Illinois. Deep wells are more 

 frequently pure. After data by Bartow 



number of cases of typhoid in the city at the time of year 

 when people return to the city from vacations in the country. 

 The water in an ordinary well comes from the rain water 

 which falls upon the ground within a few hundred feet of 

 the well and percolates through the soil until it reaches the 

 well. Such water has usually been contaminated more or 

 less at the surface of the earth with the various bacteria 

 which may be present upon the surface or in the upper 

 layers of soil (fig. 86). As the water percolates through the 

 soil many of these bacteria ' are removed by the filtering 

 action of the soil. If the bacteria are not too numerous, 

 and if the soil is sufficiently compact and the distance the 

 water travels through it is great enough, practically all the 

 bacteria may be removed. On the other hand, if the well 

 is located near a compost heap, cesspool, pigpen, or other 



