EELATIONS OF WOOD TO WATER. 57 



or S-shaped iron clamps driven into the wood ; tar, oil, grease, 

 &c., may be smeared on their ends to fill the pores, or thick 

 paper glued on them to keep off the effects of sun and wind. 



When freshly sawn beech planks are stacked, each plank 

 should be separated from its neighbours by small pieces of wood. 

 Valuable timber has been protected against cracks by barking 

 the trees as they stand, binding the bark on the stems, and 

 delaying the felling till the wood has dried. Steaming wood is 

 one of the best preservatives against cracks, but it must then 

 be very gradually dried. Soaking or boiling timber in super- 

 saturated solutions of salt has often given good results in 

 preventing cracks. 



(c) The Swelling of Dried Wood exposed to Moisture. — The 

 swelling of air-dried wood exposed to moisture does not 

 follow a similar course to the shrinking of drying wood. At 

 first it swells rapidly, and after a month or six weeks has 

 resumed its volume when green; no further sensible increase 

 in volume then occurs, but the wood continues to absorb water 

 and increase in weight from one to three years, as the wood- 

 vessels, which may have been filled with air when it was felled, 

 become full of water. It is however evident, that woods which 

 shrink greatly when dried will swell correspondingly when again 

 saturated, and that the swelling is greatest tangentially, and 

 least along the grain of the wood. 



Although the extension of wood in length by the absorption of 

 water is not considerable, R. Hildebrand, by observations made 

 in Wurzburg, has shown that it is greatest (1*52 per cent.) for 

 walnutwood, 0*43 per cent, for oakwood, and is least for the 

 wood of sycamore, beech, Scotch pine and spruce. 



It is evident that in water logs will swell less rapidly than 

 quartered or converted timber, especially if they still retain their 

 bark, also that non-resinous coniferous wood and soft, broad- 

 leaved wood swell more rapidly than highly resinous wood ; 

 these facts explain what kinds of wood are most liable to 

 sink during floating operations. The great force exerted by 

 swelling wood may be estimated from the fact that masons 

 split large blocks of stone by drilling holes into them and 

 inserting wedges of soft wood, which are then soaked with 

 water. 



