106 



DRESSING OF ORES 



it is desirable that every part of a machine should bo as light as possible, in order 

 that it may be carried by mules or drawn by oxen ; in the above machine this object is 

 easily obtained, and several have been; constructed for the gold-mines in Africa, 

 Australia, and America, as well as for the mines of Cornwall and other parts of 

 Europe. 



JIGGING MACHINERY. 



In the jigging sieve only the initial velocity of the substances to be separated 

 is obtained at each stroke. Were, however, the sieve plunged to a depth of, say 

 20 or 30 feet, the various grains would settle themselves according to their sovor/Tl 

 velocities of fall, one over the other, assuming them to be of a uniform sixr. 



The following Table shows the fall of various spheres of water in one second, the 

 depth being in millimeters : 



Let it be supposed that it is necessary to know the relative velocities of fall in 

 water, of grains of gold, galena, blende and quartz, each 1-08 millimeter diameter. 

 An inspection of the table shows the fall of gold to be 653 millimeters per second, 

 galena 393, blende 266, quartz 194. Then, let it bo assumed that the diameter of the 

 grains vary ; the foregoing table will show that gold of 6 millimeters would settle 

 at bottom at the same instant as grains of galena 17'4 millimeters diameter, and that 

 grains of galena 3 millimeters diameter would fall at about the same velocity as grains 

 of quartz 11^ millimeters diameter. 



If, further, it be supposed that the grains varied between 8*71 and 17'4 millimeters 

 diameters, some time would elapse after gold of 8 '71 millimeters had settled before 

 the galena would begin to deposit itself. With blende, however, of 8*71 millimeters, 

 and quartz of 17'43 millimeters diameter, the grains of 'both would appear at the 

 bottom almost at the same instant. 



Two rules have been promulgated for determining the relative volumes of grains of 

 different densities having the same velocity of fall in water, each rule giving a slightly 

 different result. One in which the proportion between the maximum and minimum 

 size of the grains is as the specific gravity of one to the other ; the other in which 

 the proportion is found by deducting 1, the equivalent of water, from the specific 

 weight of the substance, and dividing the less into the greater. 



Rule 1. Suppose a pile of stuff to consist of galena, blende, and quartz, and it is 

 necessary to determine the maximum and minimum diameter of grains of each of 

 these substances which shall have an equal velocity of fall in water. Then, 



Galena and Blende 4'0 *. 7'5 : 

 Blende and Quartz 2'6 : 4'0 : 

 Galena and Quartz 2-6 : 7'5: 



Diameter of Grains having equal Velocity 

 of Fall 



1 = 1'87 or Galena 1 diam. Blende T87 diain. 

 1 = 1-53 Blende 1 Quartz 1-53 

 1=2-88 Galena 1 Quartz 2-88 



Rule 2. If a pile of stuff consists of galena, blende, copper pyrites, and quartz, 

 what will bo the maximum and minimum diameter of the grain having an equal 

 velocity of fall in water ? 



Diameter of Grains having equal Velocity 

 of Fall 



Galena 

 Blende 



Tben Fd =2-1 



Galcna * diam> 



2-1 



