Reference to the Supply of Large Towns. 321 



reservoir has to be kept to draw on in a dry season, for 

 without it the necessary city supply might fail. If a 

 stream were introduced large enough to obviate the danger 

 of this, thus doing away with the necessity for keeping a 

 large amount stored up, it might assist as long as such an 

 abundant supply was used by setting aside the necessity of 

 having Rensselaer lake at all, in which the difficulty first 

 occurs, thus producing the conditions of the original stream. 



But few cities, however, have a sufficient source of supDly 

 to rely on throughout the year without a reservoir in 

 reserve, as is the case in Albany, and all such will be liable 

 to this constantly recurring evil. With the increase in 

 this city that is indicated by its previous growth, reservoirs 

 will soon have to be reestablished here even if they are tem- 

 porarily set aside by additional supplies, that can only be 

 drawn from the limited sources of water high enough to 

 be led into the upper parts of the town. 



It is not a difficult matter to prevent the stagnation of a 

 body. of water as large as Rensselaer lake. It should be 

 deepened into a reservoir (as indicated by the line E, page 

 314), that has uniform depth and is capable of holding as 

 much as it does now. If the limited current it will then 

 possess is insufficient for the purpose, motion can be given 

 it by mechanical contrivances. For this purpose, if it is 

 made elliptical in form, with its foci connected by a wall 

 higher than the surface, a motion around the lake as rapid 

 as the current which purifies the river, can be given the 

 water at but moderate expense, during the short time in 

 the heat of summer that a lake with proper depth would 

 require it. 



We are all familiar with the rapid current made by the 

 wheels of a steamboat when aground, or fastened to and try- 

 ing to move some immovable object, as another grounded 

 boat. Such a small steamer of the commonest model, 

 anchored on one side of the lake would produce the neces- 



