409 



showing that the strength of the wood was the same whether 

 the moisture was obtained from soakage in water or whether 

 it consisted of the natural sap. It will be seen from the curve 

 that the strength in compression falls off very rapidly as the 

 moisture increases above the 10 per cent, or thereabouts con- 

 tained in well -seasoned wood in this climate, the diminution 

 in strength being practically proportional to the increase in 

 the percentage of moisture until the strength becomes less 

 than half that of well-seasoned wood when the moisture con- 

 tents amount to 25 per cent, of the dry weight. From this, 

 point on the diminution in strength will further increase 

 in moisture is much less marked. With 10 per cent, of 

 moisture the average crushing strength is 4,600 lbs. per sq. 

 inch, at 25 per cent, it has fallen to 2,250, and at 50 per cent, 

 of moisture it. is 1,940 lbs. per sq. inch. 



Similar sets of tests were made upon blocks of Pinus- 

 maritima, Pinus canariensis , and Oregon, with results that are 

 shown upon the curves of fig. 1. The curve for Pinus maritirna 

 is very similar to that of Pinus insignis. At 10 per cent, of 

 moisture it indicates a strength of 5,600 lbs. per sq. inch, and 

 at 25 per cent, a strength of 2,750, a little less than half, 

 while with a further increase of moisture up to 70 per cent, 

 the strength is reduced only to 2,450 lbs. 



Wood when placed in water not only increases in weight 

 by absorption but expands in volume. This is a feature that 

 causes much practical difficulty to engineers when using wood 

 blocks for street paving, but the author is not aware of any 

 attempts having been made to measure the force which the 

 wood can exert in this way when prevented from expanding. 

 With this object in view a rectangular block of Pinus insignus, 

 3 1 iin. x 3 in. and 4 in. high was placed in a, flat dish on the 

 compression table of the Riehle testing machine. The grain 

 was horizontal and the rings as shown in fig. 2. A tightening 

 load of 600 pounds was put upon it, and the block was thus 

 held between two cast iron plates, top and bottom, which 

 could not move, but the upward force on the top plate could 

 be measured at any time by balancing the lever of the 

 machine. Water was then placed in the dish, nearly, but not 

 quite up to the top of the block. This was done at 10 a.m. 

 and gradually throughout the day, as the block absorbed more 

 water, it exerted a greater and greater upward force on the 

 top block. By noon this force was 1,100 lbs., and at 5 p.m. 

 it was 1,520 lbs. It was left all night and next morning it had 

 dropped to 1,360 lbs., and continued to drop slightly through- 

 out the day. Next morning it was down to 1,280 lbs. On 

 removal from the machine it was found that the block ex- 

 hibited a typical compression failure, as though it had actually 



