Eeference to the Supply of Large Towns. 317 



would be to remove these matters when thus formed before 

 the waters which are contaminated by them are drawn for 

 use. 



To apply these two methods to the purification of the 

 waters of Albany will illustrate their application to the 

 water of any other place, and the investigation of the 

 questions which are involved brings us to many interest- 

 ing facts. 



By reference to the diagram (on page 314) it will be 

 seen that the water destined for use in the city is collected 

 in Rensselaer lake which is a large reservoir, and carried 

 from here to the Bleecker whence it is supplied to the city. 

 When it reaches the first of these lakes it is as pure as 

 ordinary spring waters. It has never yet been analysed, but 

 comparative experiments made at various times by different 

 experimenters show that it contains onlv a small amount of 

 dissolved impurities. 



Rensselaer lake contains much shoal water. The part 

 of its bed which is exposed in a drought is much larger 

 than the whole of the deeper portion that remains covered. 

 The shoal places operate in several ways to cause the 

 evil : the heat of the sun falling upon the water reaches 

 the shallow bottom and warms it, and the water is warmed 

 by contact with it ; there are no currents, and without 

 motion there is no interchange of the cooler with warmer 

 parts; when dry in summer grasses spring up, which, when 

 submerged by the warm flood, cause a growth of infusoria; 1 

 to be followed in turn by microscopic vegetal formations. 



1 The flat in this lake which is usually covered with water and was so 

 covered at the time this paper was written, was overgrown by a crop of 

 grass during the dry weather of the ensuing summer (1866) and was 

 mowed. A spear of this, or any similar grass, put in a glass of water and 

 set in the sun in summer would give birth to a flock of infusoria, so named 

 by reason of this. This is the common and well known method of their 

 preparation for microscopic experiments. 



[Trans. v.~] 41 



