In the tests described in this bulletin, the Department of Agriculture employed a rotary digester of its own design,[2] comprising a shell 5 feet 5 inches in length by 4 feet in diameter, capable of holding about 300 pounds of air-dry hurds. It is believed that a test of this size is large enough to give satisfactory results and that the results are susceptible of commercial interpretation, while at the same time they are sufficiently small for complete control and to afford fiber-yield figures which are both accurate and reliable. Two such rotary charges gave enough fiber for one complete paper-making test. [Footnote 2: For a description of this rotary digester, see Brand, C. J., and Merrill, J. L., Zacaton as a paper-making material, U. S. Dept. Agr. Bul. 309, p. 28, 1915.] =OPERATIONS INVOLVED IN A TEST.= A complete test on hurds comprises seven distinct operations, and the method will be described, operation by operation, in the order in which they were conducted. _Sieving._--The hurds for the first test were not sieved to remove sand and dirt, but the resulting paper was so dirty that sieving was practiced in all subsequent tests. The hurds were raked along a horizontal galvanized-iron screen, 15 feet long and 3 feet wide, with 11-1/2 meshes per linear inch, the screen being agitated by hand from below. Various amounts of dirt and chaff could be removed, depending on the degree of action, but it was found that if much more than 3 per cent of the material was removed it consisted chiefly of fine pieces of wood with practically no additional sand or dirt; in most of the tests, therefore, the material was screened so as to remove approximately 3 per cent. It became apparent that a finer screen would probably serve as well and effect a saving of small but good hurds.