CEREAL IMPROVEMENT SELECTION 181 



methods of cleaning and grading grain : (1) by air move- 

 ment or wind, (2) by simple gravity, and (3) by specific 

 gravity. The same general principle, differences in 

 gravity, is effective in the first and third methods, while 

 in the second the size and shape of seed are also effective. 



179. Air movement. The simplest and the most 

 primitive operation of cleaning by force of air is the 

 winnowing process, in which the grain is held aloft in a 

 bucket or basket and allowed to spill in a thin stream in 

 the face of a strong breeze or wind. Even in this process 

 the grain is also roughly but incompletely graded. In 

 oriental countries this winnowing process is commonly 

 the only method of cleaning ever employed, and often 

 follows similarly primitive thrashing operations. 



In recent times the fanning mill has come into use. 

 Though a fan is attached to all thrashing machines, it is 

 sometimes the practice to use a fanning mill following 

 the thrashing, as by means of it and its numerous sieves 

 of different sized meshes, the grain can also be graded. 



180. The gravity method is the only one which essen- 

 tially is not a gravity process at all, but is so called 

 because the grain passes through the machine by its own 

 weight, and there is no machine motion. The machine 

 is principally a vertical series of sieves set each at an 

 angle with the one next below, the sieves becoming finer 

 toward the bottom. It is chiefly a method of grading, 

 though some foreign seeds may be eliminated. The 

 grading is effected by differences in the size and shape 

 of meshes in the different sieves. The seed passing 

 through each may be caught separately or the products 

 of two or more sieves may be run together. Of course 

 the separation in this process is chiefly on the basis of 

 size and shape of kernel. 



