230 MADRAS FISHERIES BULLETIN VOL. XIII, 



is placed a filter cloth, then a few inches of filtering material 

 perhaps of different kinds ; this arrangement is duplicated, so that 

 the total depth of the filtering bed may be perhaps a foot. On the 

 top of the filtering layers is placed a perforated pliite similar to 

 that at the bottom. Above this is placed a screw set centrally in 

 a frame which is affixed to the top of the filter body ; by this screw 

 the filtering media can be conveniently packed and compressed to 

 any desired degree. An outflow tube leads away the filtered oil 

 from the top of the filter which is on the upward flow principle. In 

 working the filter the oil is led by a pipe from a cistern placed a 

 few feet above the filter, into the bottom of the filter below the 

 perforated plate, through which and the superincumbent filtering 

 media, the filtrand passes upward, issuing clear and bright above 

 the top plate. The upward system is generally adopted in these 

 filters, especially if solid matters, heavy coagulations, etc., are 

 present in the filtrand, for these at once fall to the bottom of the 

 cone (whence they are periodically removed through the wash-out 

 cock) and do not affect the filter proper, whereas in a downward 

 flow filter all the impurities, coagulated matters, etc., collect on the 

 top of the filter and obstruct filtration. A rough sketch is append- 

 ed ; a galvanized iron filter precisely of this pattern was made for 

 the Tanur Yard. 



Moss is said to be particularly suitable for oil filtration; this is 

 abundant, especially in the monsoon, on the West Coast ; it should 

 be gathered during the rains, washed from earth, etc., and care- 

 fully dried. Coir, well washed, is also suggested ; it would require 

 considerable compression to act satisfactorily but might have other 

 materials mixed with it. Cotton waste, paper pulp from unsized 

 paper, and other forms of cellulose are much used in filters, but 

 when used with oil the material must be kept from the air, since 

 the rapid oxidation of the oily cellulose frequently causes spon- 

 taneous combustion ; hence the oily mass must never be left lying 

 about in bulk, but should be pressed and washed at once. 



The filtering media require occasional changing, when the used 

 material should be pressed, washed with alkali to recover any 

 residual oil— as a rough soap — and then either prepared for 

 further use or thrown away. 



There are many other forms of filter, including the simple bag 

 filters of gunny, flannel, cotton, etc., or suction filters in which the 

 filtrand is sucked by a pump through a filter drum or chamber 



