Aug. 15, 1884.] 



• KNOWLEDGE • 



139 



to percolate through the filter. This is secured by a 

 washer, which forms a tight joint around the ball. The 

 water may be permitted to pass through imfiltered along 

 the passage B B, Fig. 21 ; but when the packed handle C is 

 revolved so as to bring the filter into operation, the water 



Fig. 21.^Seotional view ..f " The Grant Revolving Ball Filter." 

 A A, patent cup and joint washer, packed by pressui'e of water ; B B, 

 direct way for water through ball icithout filtering ; c, packed 

 handle to reTolre ball ; D D, bearings upon which ball revolves ; E E, 

 animal bone, charcoal, or filtering medium : F f, bars inside of ball 

 npon which the water strikes when filtering, preventing drilling of 

 charcoal; H H, shell containing ball; i, section of wire screen or 

 sieve which holds charcoal into the ball, and through which water 

 passes when filtering. 



strikes upon the bars F F, thus breaking its initial force, 

 and causing it to spread over the whole body, whence it 

 finds its way through the charcoal with an equalised 

 pressure, thereby securing perfect filtration with a minimal 

 disturbance. The matters which accumulate in the filter 

 may be removed by a simple revolution of the ball, and the 

 entire system thus cleansed in a few seconds. An objec- 

 tion to this method of cleansing is, that it is at most only 

 approximately effective, but that is all that is really neces- 

 sary in a filter of this kind ; and, moreover, should the 

 objector be unusually fastidious, he may, at the worst, 

 remove the charcoal, wash it thoroughly, and return it to 

 its receptacle, or supply its place with a fresh charge, which 

 is easily done at but a nominal expense. The entire appa- 

 ratus is inexpensive, portable, almost indestructible, 

 effective, and even elegant ; it speaks very highly for the 

 ingenuity of its transatlantic inventor. The carbon recom- 

 mended for use is in a sufficiently fine state of division to 

 act as a thorough mechanical strainer ; it also acts chemi- 

 cally, if we are to judge from analyses of the filtrate. 



Type II. — Although we strongly object to the old- 

 fashioned " block " principle in filters, we are constrained 

 to admit that there are some of them which it would be 

 unjust to leave unnoticed, since they achieve results which 

 place them amongst the best of modern inventions. Of 

 these, " Doulton's Manganous Carbon Filters " deserve a 

 special notice ; they may be seen and tested in the western 

 annexe of their Pavilion in the " Central Gallery." 



Before we proceed to describe the action of these filters, 

 we desire to draw attention to one of the most important 

 features in the construction of filter-cases which is strikingly 

 exemplified in Messrs. Doulton's patent. These are the 

 only filters extant where anything of the kind has been 



Fi" 



adopted. Fig. 22 will help us to explain more clearly than 

 any written description alone, 

 how every drop of filteied 

 water becomes available, and 

 is prevented from getting 

 stale by the gentle slope of 

 the floor of the reservoir to 

 the tap, which is situated 

 below the level of that floor. 

 In most filters this item is 

 generally overlooked, with 

 the consequence that the 

 lowest stratum of water may 

 lie unused within the reser- 

 voir of the filter sufliciently 

 long to become either mawk- 

 ish or positively harmful 

 through the accidental in- 

 troduction of aerial germs, 

 which usually gravitate to 

 the bottom of the water. To 

 this class belong certain forms 

 of the minuter Algce and 

 Rotifera, not to sjieak of Bac- 

 teria, which in their totality may, and almost always do, pro- 

 duce, in about ten or twelve days, an undesirable adherent 

 slimy layer at the bottom and lower sides of the reservoir. 

 This state of affairs may be observed in the form of filter- 

 case shown at Fig. 23, where the position of the tap 



necessitates a constant 

 remnant of unusable 

 water, which has there- 

 fore to be cleansed out 

 periodically. As the 

 majority of filters now 

 in use are all modelled 

 upon the type of Fig. 23, 

 we would here offer our 

 practical advice to such 

 of our readers as may^ 

 possess filters of this 

 sort, which is : — To be 

 careful to thoroughly 

 wash out the reservoir 

 at least once a week ; 

 ''"S- ^^- this will be sufiicient to 



insure them against the possible sources of contamination 

 to which we have alluded. 



Another praiseworthy fact about Doulton's filters is^ 

 that their taps are made of ^^^ 



stoneware, and hence are not 

 liable to the inevitable corro- 

 sion, to which metal taps are 

 subject, which, if not positively 

 dangerous, are at least objection 

 able on the score of cleanliness. 

 "We would, of course, expect 

 to find elegantly-shaped and 

 beautifully-decorated filters at 

 Messrs. Doulton's, nor are 

 we doomed to disappointment. 

 Apart from strict utility there 

 is an indescribable charm 

 about an artistically finished 

 production, such as that re- 

 presented at Fig. 24, which 

 raises it above the common- 

 place, to rank amongst the 

 things that are to be desired. 





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