716 NEW YORK STATE MUSEUM 



in use, of 30 and 36 inches respectively, the larger mill is worked 

 on tailing from the separators only, the hourly output being. 38-40 

 cwt when ground to about 2^ on 50. The smaller sized mill is 

 worked on tailings from the separators and clinker from the 

 crusher mixed, a rough grating being placed in the clinker hop- 

 per to prevent any pieces larger than a walnut from going for- 

 ward into the mill, as such pieces would choke the worm feed. 

 The smaller mill under these conditions is said to yield 26-30 cwt 

 an hour. 



Mills compared. H. Faija makes the following comparisons. 



The power consumed by the several principles, reduced to the 

 proportions of 1 ton of cement an hour, may be approximately 

 stated as follows: for millstones 30-32 horse power per ton an 

 hour; ball principle 16-18; edge runner principle 12-14. In 

 each case the cement is ground to a fineness of a 5^ residue on 

 a 50' X 50 sieve, and it will thus be seen that the power required 

 is proportionate to the amount of flour produced. 



Butler declares, from microscopic analysis of different cements, 

 that the statement that millstones produce an angular grain, and 

 edge runners a rounded one, is incorrect. 



Testing 



There are as yet no universally accepted standard methods of 

 testing, but the characters which may be, and often are deter- 

 mined are: compressive strength; tensile strength; rate of set- 

 ting; boiling test; abrasion; permanency of volume; degree of 

 fineness; adhesion; specific gravity. 



Mixing the mortar 

 In 1885 the American society of civil engineers suggested test- 

 ing briquets of neat cement, and, in addition, briquets of cement 

 and sand: those of natural cement with one part sand, and those 

 of Portland cement with three parts sand by weight. Some au- 

 thorities advocate the abandonment of the neat cement test, since 

 in use the material is always mixed with sand. The ratio of 



