441 



can only answer the purpose for a certain time ; for it 

 must become clogged with the impurities and sediment 

 from the foul water, and will require renewal more or 

 less often, in proportion to the foulness of the water. The 

 sand should be well washed before it is used, and it should 

 contain no earthy matter, as this would defeat the object 

 of filtration. Clean sharp sand is best, and it should be 

 separated by sieves into various degrees of coarseness, 

 to place in different layers, the finest being put at the 

 bottom. 



It may be remarked, that when water is filtered by nature 

 through beds of sand, it ascends to the surface, by which 

 the purification is more completely effected than by de- 

 scent. In the latter case, some impurities might be forced 

 through by the weight of the water, or by their own gra- 

 vity ; but, in the former case, gravity must oppose the 

 ascent of the impurities, which are therefore more likely 

 to remain behind. Filters have been executed on this 

 principle, by making the water pass upward through the 

 sand and charcoal, or other filtering materials. 



A very simple apparatus of this kind was made long ago 

 by M. Parrot, of Paris, which 

 has been the origin of much 

 of the recent apparatus for 

 this purpose. As it may be 

 very easily executed, it 

 might be sometimes useful 

 to travellers, who may find 

 it difficult, in some situa- 

 tions, to procure pure water, a b c, represents a 

 curved tube, either round or square, into which sand, or 

 sand and charcoal, are put, up to the level of the dotted 

 line at c. A little flannel bag is put into the end a, and 

 water poured into this has its coarsest impurities retained 

 by the flannel ; and in passing through the sand, in the 

 19* 



