humidity of 65 per cent. When so desired strength is reported as 

 pounds per square inch, as determined by the standard American 

 testing machines, no correction being made in this case for variation 

 due to the moisture content of the air, as these machines give only 

 approximate results. 



Unless otherwise requested, weight, thickness, and strength will be 

 expressed in the metric system. 



(1) Kind of fibers or stock is determined by microscopical exami- 

 nation. 



( 2) Quantity of each hind of fiber or stock is determined microscopic- 

 ally by a count of the fibers of the different materials present, and will 

 be expressed as "percentage by count" on the basis of the stock. 

 The Bureau is not yet ready to say how close this "percentage by 

 count" will agree with "percentage by weight/' but there is every 

 evidence that the error, if any, is not greater than obtains in the man- 

 ufacture of paper. The actual percentage of stock in a paper is 

 found by substracting the sum of the moisture, ash, sizing, and color- 

 ing from 100. As the determinations of sizing and color are not usually 

 made, the percentage of stock in all such cases can only be approxi- 

 mated. 



(3) Average length of fibers is determined by microscopical meas- 

 urements and is expressed in millimeters. 



(4) Weight is expressed in grains per square meter, or, when so 

 requested, in pounds per standard ream (24 by 36 inches). 



(5) Thickness is expressed in millimeters, or, when so requested, in 

 thousandths of an inch. 



(6) Mean breaking weight is the average weight in kilograms (1 

 kilogram = 2. 2046 pounds) required to break a strip of air-dry paper 

 15 mm wide and 180 mm long (0.5905 inch wide and 7.087 inches 

 long). 



( 7) Mean breaking length is the mean length of moisture-free paper 

 expressed in meters (1 meter = 3. 2808 feet) which if suspended by 

 one end would break of its own weight. 



(8) Loss of strength on folding is the decrease in strength caused .by 

 folding the paper a stated number of times under a uniform load of 

 7 kilograms, and is expressed as percentage on the basis of the mean 

 strength. 



(9) Mean stretch is the elongation before breaking of the paper 

 when subjected to a pull sufficient to break it, and is expressed in 

 percentage. 



(10) Loss of stretch on folding is the decrease in stretch due to fold- 

 ing the paper a stated number of times under a uniform load of 7 

 kilograms, and is expressed on the basis of the mean stretch. 



[Cir. 84] 



