70 TRANSACTIONS OF ROYAL SCOTTISH ARBORICULTURAL SOCIETY. 



"The following remarks are taken from the U.S.A. Forest 

 Service Bulletin, dated May 191 7, Notes bearing on the use of 

 spruce in airplane constructioti. 



" ' For use in airplanes the Sitka spruce combines the qualities 

 of lightness, great strength and stiffness per unit weight, with 

 a considerable degree of toughness. 



" ' Very rapid growth is usually productive of inferior material ; 

 the optimum quality has been found in timber showing 15 

 annual rings per inch radius. There is no material difference 

 in strength, either towards increase or decrease, in timber 

 showing more annual rings per inch radius than this, while 

 timber which shows less than 8-10 is usually weaker and 

 lighter. 



"'Spruces follow the general law that strength increases with 

 the increase in specific gravity. Excellent material is obtained 

 from clear timber when the specific gravity (based on oven-dry 

 weight and volume) is •36. When green, clear wood with this 

 specific gravity would seldom run below 4500 lbs. per square 

 inch in modulus of rupture, or below 2000 lbs. per square inch 

 in maximum crushing strength along the grain. Drying to 

 the condition of aeroplane stock would increase these figures 

 to about 8000 and 4000 lbs. per square inch respectively^.'" 



II. Treatment of Forest Seed-beds with Disinfectants 

 to prevent Damping-off. 



By James Kay. 



Ordinary commercial sulphuric acid appears the cheapest and 

 most effective preventive of damping-off. Both heat and for- 

 maldehyde, the means usually recommended for disinfecting 

 greenhouses and truck soils, proved less reliable as well as 

 expensive. 



In all disinfection of seed-beds by chemicals, the quantity of 

 the disinfectant used per unit area of soil surface seems to be 

 the important variable. The disinfectant must be dissolved in 

 sufficient water to permit its distribution through the soil to a 

 depth of several inches, but within certain limits the concentration 

 of the solution as applied does not appear to be an important 

 factor, for watering the beds frequently during the germination 



