FILTER THE WATER. 93 



water can be always running over them ; 

 regulate your stream by means of stop- 

 cocks, and find a place for the waste water 

 to run. The water would be all the better 

 for being filtered. This is not absolutely 

 necessary, if it is naturally pretty pure, but 

 it is certainly advisable. Filter it through 

 charcoal, gravel, sand, or any other simple 

 and commodious material. The same water, 

 if clean, may be used over and over 

 again ; but fresh water is, of course, pre- 

 ferable. 



If the reader does not understand this, 

 let him call at the < c Field" Office, 346, 

 Strand, and see my apparatus at work, or let 

 him pay Mr. Ponder a visit ; this gentleman 

 (or in his absence his keeper) will be pleased 

 to allow visitors to examine the boxes by 



