The Evolution of Agncultural Irti'plements. 245 
cloths and silk gauzes, into a number of different qualities, and 
these, after being purified of bran by air currents of suitable 
strength, are separately crushed into flour between smooth 
rollers. 
Sifting or “ dressing,” and winnowing or “ purifying,” are 
both extremely important elements in the gradual reduction of 
wheat to flour by roller mills. A word or two will suflfice to 
indicate what has been done of late years in dressing-machines. 
Wire cylinders and brushes gave way more than thirty years ago 
before silk screens, which, though very effective in work, are 
cumbrously long. Shorter cylinders have lately been introduced, 
containing revolving beaters, which throw the meal repeatedly 
but lightly against the silks, and thus greatly expedite the 
operation of dressing. 
The middlings purifier is one of the most important pieces 
of machinery connected with roller milling. By its means the 
semolinas, which it is the object of gradual reduction to produce, 
are freed from the small particles of bran mixed up with them, 
and are themselves separated, according to their densities, into 
lighter and heavier samples. No sifting can accomplish this, 
because the middlings and bran are often of the same sizes 
although differing in specific gravity. 
In the purifiers now universally employed in Austria and 
Hungary, a stream of semolina falls through a vertical pipe 
fitted with bafile boards, so arranged as to cause the stream to 
cross and recross the pipe repeatedly, while the current of air 
is sucked upwards through the latter by means of an exhaust 
fan. The strength of the current can be regulated so as to 
carry away the whole stream of semolina, or only the lightest 
particles of bran. 
In a second type of machine, finding much favour both in 
England and America, the purifier consists of a combination 
of silk riddles and exhaust. The former are given a downward 
beat by the action of a pair of cams, which, compressing four 
india-rubber balls acting as springs at the corners of the riddle 
frame, cause the riddles to rise and strike against a stationary 
buffer ; and the sudden stopping of the sieves, throwing the 
middlings into suspension, enables the exhaust to act with the 
greatest effect upon them. The air current, sweeping upward 
through the silks, carries the lightest particles away altogether, 
depositing them in a chamber provided for their reception, 
while the heavy granules of semolina fall through the riddles. 
The branny particles floating on the silks, through which the 
air current prevents them from passing, are ultimately dis- 
charged as overtails. 
