The Clifton clarion., 1889-05-29
CONCENTRATION OF ORES
\ Series of Descriptive Articles Showing
 the Various Methods Employed.
BY ANDREW WALLACE.

the jig: and, as the jig plunger in its l upward stroke necessarily creates con- siderable vacuum below it, the water
 supply must be placed at sufficient r height to enable it to flow rapidly into
 the jig. If the supply of water is not
 placed, at least, ten feet above the jig
 overflow, the effect on the jig will be
 that some of the lighter and worthless
 rock will be sucked down to and through the screen, thus causing the
 lower discharges to be dirty, and prob ably require them to be redressed.
 As a matter of fact, the jig plunger does not displace at each stroke as
 many cubic inches of water as would
 be represented by its area multiplied
 by the length of its stroke, and it is not necessary to provide water enough
 for this. lii reality, there is more or
 less air always mixed with the water
 directly under the plunger. and when it is remembered that air is very elas- tic, and capable of being compressed, it will readily be seen that only part
 of tie space vacated by the plunger
 need be filled with water, the rest of the space being compensated for by the expansion of that air which was compressed on the down stroke; in
 practice this will be apparent to any
 thinking person, but it is more notice able in those machines which are treat ing the coarser sizes. For instance. take that jig which is working on 12 inch ore; in that jig, the stroke of the
 plunger would be about 1-2 inches. and about 100 strokes per minute, the plunger working under these condi- tions would displace nearly 300 gal | Ions of water every minute; whilst the size of pipe, with ten feet head. need ed to supply the jig, would not require i to be more than two inches. As there must be ono hundred full strokes per minute, it can readily be seen that
 were the full supply necessary it
 would require to be furnished in 80 | seconds. because the other half minute | is taken care of by the down stroke
 when water is being forced out. The presence of air in the water serves as cushion or spring to the plunger and prevents too sudden changes ill ] the flow of the stream through the screen, and it also deadens the shock which at each stroke is given to the machinery. Were there no elastic medium ill the water or between it and the plunger, either the plunger working close ill its seat would draw i the water back through the seive, and
 with it some worthless sands; or, the
 plunger working loose in its seat, allows the water to escape too rcadi- l ly, and as it is manifestly impossible topass water having ten foot head
 through two inch pipe at the rate of ten gallons per second, the effect would
 bc that the plunger would leave the
 water at every stroke, and on its re- turn would strike the water with such force that the machinery would soon
 be wrecked. Reasoning in this way. it is evident that the plunger ought to
 be made as tight as possible, consist
 ent with the least amount of friction.
 In regard to the stroke of the plunger, and the manner in which it should be applied, the theory is short ly explained ill this way; the stroke should approximate as nearly as pos sible to that obtained in hand work, that is, the plunger should make the
 upward stroke slowly and the down.
 ward stroke quick. Various mechan- ical devices have been planned to achieve a perfect motion, notably that
 of Mir John Collom; the writer also has at divers times designed and built jigs having the quick and slow motion
 to the plunger. Experience, however,
 teaches that ell those devices are very
 expensive to maintain; they require
 expensive labor to operate them;
 and, excepting in the coarsest sizes where the stroke is greatest, the ac- cruing benefit is not appreciable.
 The simplest and most practical me- . chanical movement, and that which is 5 almost exclusively used at this time is the eccentric; but, in speaking of | the eccentric, l do not wish it to be ? understood that 1 claim for it less 3 friction than for the other devices l named. The fact is, the eccentric , wears faster and requires more power