Under the heading of "Volume of Feed" R. T. Misler makes a comparison of the work of a tube mill with varying amounts of feed in which he uses constants based on the Rittinger theory. A perusal of his work will give an idea of the method of computation.

The method employed is first to make a screen analysis of the ore entering and leaving the mill. The percentage of each grade is multiplied by its energy number and the difference of the sum of all the energy units in the feed and discharge represents the number of energy units used in crushing. This figure is divided by the number of tons crushed per unit power. By this method the work done by two mills operating under like conditions or one mill operating under unlike conditions can be compared with a fair degree of accuracy.

In all crushing problems the mill product must be screened before and after going through the machine, and when we state that a certain per cent, of the output will pass a certain screen it becomes of prime importance that the size of these grains be fixed beyond dispute. In the following pages this subject is briefly treated. It will be good policy in future to either use the Institute .Mining and Metallurgy screen scale or that known as the Tyler standard screen scale, the latter being used in the United States while the former is used mainly in South Africa. Gage. The (U. S.) Steel Wire gage 1 or Washburn and Moen gage ("W & M") is the standard for iron, steel, and tinned wire cloth, the old English gage for brass, copper and bronze wire cloth and the size of needle punched and slotted steel or iron plates.

The diameter of wire used in screens should be measured with a micrometer as shown in the illustration, Fig. 65, and recorded in decimals of an inch. This method is far better than with a discgage because it shows decimal not vulgar fractional sizes and because fine wires are liable to stretch when forcing the wire in the notch of the disc.

Mesh. The work "mesh" in wire cloth means the number of openings per lineal inch, measured from center of wire, Fig. 66, and does not indicate the size of the opening or "spaces" unless the size of the wire is also known. For example, a 30-mesh steel wire cloth may be made of wires between No. 27 and 36 giving size of openings between 0.0163 and 0.0243 in., or a 24-mesh screen No. 24 wire will give about the same size opening as a 35-mesh cloth with No. 34 wire.

In 1907 the Institute of Mining and Metallurgy after exhaustive study adopted screen sizes shown in the following table where the size of the openings is the same as the size of the wire, giving a constant ratio of unity between the area of open space

Makers of screens have such different ideas of mesh values that it is impossible to order screens without stating the size of aperture required. The table below will show how makers gage their screens.

For laboratory work we must use screens that have fixed mesh values and ones that can always be duplicated in any locality. As all screening work on samples is done in the laboratory and not in the mill, when we say that a certain proportion of the mill pulp will go through an 80-mesh screen or as expressed by metallurgists 80-mesh, we mean that in the assay laboratory using a standard set of screens a certain per cent, will go through that screen; therefore, every screen should have a fixed standard value. and of metal.