1 
102 Modern Improvements in Ccrra-Millvag Machinery. 
already explained, these " grits " are mixed with bits of brau 
of the same sizes, and many of them have bran attached to them. 
They requii'e to be purified, and the class of machines to which 
the name " purifier " has been applied is certainly the most 
important in the histoiy of modern milling. 
There are two types of these machines in general use : one | 
is known as the wind machine or gra\4ty purifier, while the I 
other is the sieve purifier. la the former machine the semolina 
to be purified falls through a succession of graduated and con- 
trollable currents of air, by means of which the heavy pure 
semolinas are freed from all the lighter particles, whether of brau | 
or of branny semolina, and these latter are again divided and j 
subdivided into various qualities. These wind machines can be ! 
used only for the larger sizes of semolina. For the smaller j 
sizes and for middlings sieve purifiers must be used. The I 
essential part of these machines common to the veiy many con- j 
structions in existence is a chamber, air-tight on all sides except ' 
the bottom, which is formed by a sieve covered with silk cloth 
and having a rapid reciprocating or joggling motion ; the air in 
the chamber above the sieve is drawn away by an exhaust-fan, , 
so that a current of air is created passing upwards through the , 
meshes of the silk cloth of the sieve. The material to be puri- 
fied is delivered in an even stream to the head end of the sieve, i 
which is of a long and naiTOw rectangular shape, and passes I 
along it in a thin layer towards the tail ; by the mere shaking ! 
of the sieve the middlings arrange themselves in horizontal 
strata, the lighter and more branny particles rising to the sur- 
face, and the heavier and purer next the silk cloth. The slight 
cuiTent of air passing upwards through the sieve assists this 
operation, and the meshes of the silk cloth being proportioned 
to the sizes of the middlings, the heavier and pure particles j 
fall through and are collected in a receptacle below the sieve, | 
while the lighter and inferior particles are kept in suspension ' 
and pass onwards to and over the tail of the sieve, and any 
very light particles are lifted up and earned away by the cur- 
rent of air to a dust-room provided to catch them. " Dust- 
catchers," which are in their various forms air-filters, are here 
employed to catch all these very light particles. Recently, puri- * 
fiying machines have been constructed containing a filter in I 
themselves, as well as a device by wliich particles that have been ' 
lifted from the mass, but are too heavy to be earned away by l 
the air-current, and would therefore fall back again, are inter- 
cepted in their fall and divided fi-om the middlings. 
All these processes of purification are very delicate opem- 
tions, and all the gritty particles require to be accurately " sized," 
80 that each size may be treated by a machine having a current : 
