20 BULLETIN 556, U. S. DEPARTMENT OF AGRICULTURE. 



tree taken at random will be below 91 per cent of the species average, 

 one chance in four that it will be above 109 per cent, and two chances 

 in four, or one chance in two, that it will be between 91 and 109 per 

 cent of this average. There is also about one chance in 100 that 

 the random tree will vary from the average an amount equal to four 

 times the probable variation, or in this case 36 per cent. 



GLOSSARY. 



AIR DRY. 



(See p. 7.) 



Air-dry condition is the normal condition, with respect to moisture, 

 of wood exposed to the air, although this condition may have been 

 obtained by artificial means. The term "air dried" means dried by 

 exposure to the air, while "kiln dried" indicates artificial drying. 



Air dry is a very general term and may mean any degree of dryness 

 from about 6 per cent moisture, as in furniture stock, to over 30 per 

 cent moisture, as in timber dried to reduce its shipping weight. The 

 degree of dryness in timber depends upon species, size, and the con- 

 ditions under which the material is dried, especially such as humidity, 

 method of piling, shelter, time of drying, etc. For instance, the 

 wood of the conifers dries much more rapidly, on the average, than 

 that of the hardwoods. Douglas fir bridge timbers will fall to about 

 30 per cent moisture in 2 years. Inch lumber of the same species, 

 under the same conditions, will dry to 15 per cent moisture in con- 

 siderably less time, and small-sized timber dried in a heated room 

 will in some cases reach 6 per cent moisture. The same species, in 

 the same sizes, piled in the same manner under shelter out of doors, 

 will scarcely ever fall below 12 per cent moisture. 



DENSE. 



Dense, as applied to wood, means compact, heavy (when dry), 

 containing much wood substance in small space (see footnote, j>- 5). 

 For example, hickory is a very dense wood. 



The oven-dry specific gravity is a measure of the density of wood. 

 This figure is based on the weight, exclusive of moisture, but including 

 rosin and other substances not volatile at 100° C. 



ELASTIC LIMIT. 



(See pp. 13, 14, and 15.) 



The elastic limit (sometimes called proportional limit) is that 

 point where the distortion ceases to be in proportion to the load. 

 For example, if a beam deflects one-sixteenth of an inch with a 50- 

 pound load it will deflect one-eighth of an inch with 100 pounds, and 

 so on, each additional load of 50 pounds causing an additional de- 

 flection of one-sixteenth of an inch until the "elastic limit" is reached, 



