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Page:Popular Science Monthly Volume 15.djvu/548

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THE POPULAR SCIENCE MONTHLY.

as dry-rot can only thrive upon decaying timber, it is apparent that the best protection against both these evils is careful seasoning. When wood dries gradually in the air by the process of natural seasoning, it should be placed in a dry yard and sheltered from sun and wind. This method may be recommended for specimens of moderate thickness; and the time needed is two years for timber used in carpentry, which in this period loses one fifth of its weight. Four years are needed for timber that is to be used in joinery, in which time it will lose one third of its weight. It is important that timber be reduced to its proper size for use before seasoning; for, however dry it may become, when it is cut smaller it will shrink and lose weight. At first the seasoning should proceed slowly, and the pores upon the surface should remain open to permit the free evaporation of internal moisture. It should be set on bearers to admit a circulation of air all around it. The sleepers at the bottom of the pile should be perfectly level and solid; for timber bent in seasoning will retain the same form when dried. The time required for drying under cover is shorter than in the open air in the proportion of five to seven. Three years are required to season ship-timber; the timbers are shaped a year before they are formed, and then left a year in a skeleton shape to complete the seasoning.

Sappy timber that must be seasoned quickly, in cases where strength is not chiefly required, should be immersed in running water as soon as felled. It should be chained down beneath the surface, as partial immersion is very destructive. Boards placed end on at the head of a mill-race for two or three weeks, and then set upright in the air, and turned daily, are said to floor better than timber that has been years in dry seasoning. The longer wood has been under water the faster it dries. The process of water-seasoning is easily explained. Sap is denser than pure water, and it is inclosed in membrane. By osmotic action pure water takes the place of the sap and so renders the wood less liable to ferment. Again, pure water evaporates more readily than water which contains certain principles in solution, and hence water-soaked timber dries more rapidly. Timber steeped in water has some of its substance dissolved thereby. Boiling and steaming are said to prevent dry-rot by getting rid of spores and coagulating albumen.

 

GENERIC IMAGES.
By FRANCIS GALTON, F. R. S.

IN the prescientific stage of every branch of knowledge, the prevalent notions of phenomena are mainly founded on general impressions. But when that stage is passed, and the phenomena are submitted to measurement and numbering, very many of the notions