TIMBER FOR AEROPLANE AND PIT-PROP PURPOSES. 99 



In view of the fact that all the transverse tests for both the 

 home and the foreign-grown woods were very concordant, it has 

 not been thought necessary to give the transverse tests separately 

 for the two planks of each sort of wood ; the table gives, therefore, 

 in each case the mean result of eight separate tests. In the 

 case of the compression tests, though the eight test pieces from 

 each plank gave results in close agreement with one another, the 

 mean results for each of the two planks differed rather widely, and 

 it is desirable to give their mean results separately in the table. 



It will be observed that while in the home-grown stuff, narrow 

 annual rings corresponded to high compressive strength, the 

 reverse is the case with the foreign-grown stuff, a rather in- 

 explicable result. 



It was intended to make in similar fashion a set of com- 

 parative tests of Scots pine home and foreign-grown pit props ; 

 unfortunately it was not possible to obtain in the short time 

 available for these tests well-seasoned Atholl-grown Scots pine 

 pit-props, but a set of four barked foreign-grown Scots pine 

 props was obtained, and these gave an average compressive 

 strength of 4059 lbs. per sq. inch, a result which agrees fairly 

 closely with the results obtained in the earlier tests of the 

 Finnish red wood. 



Comparing the results of the transverse and compressive tests 

 it will be seen that good home-grown Scots pine is quite as 

 strong as an average imported sample of the same timber, and 

 there seems no adequate reason why, with proper development 

 of our forests, we should not be able to supply our home require- 

 ments for timber of this class. 



It should be explained that, in the case of all the pit-props, the 

 figures given in the tables for the maximum compressive or 

 crushing strength were obtained by dividing the number corre- 

 sponding to the total recorded load carried by the prop, by the 

 number corresponding to the number of square inches in the 

 central cross-section of the prop. If each prop had been a 

 perfect cylinder with the resultant of the applied external load 

 coinciding with the geometrical vertical axis of the cylinder, 

 then the above method would give the real maximum com- 

 pressive stress set up in the material. Most of the props were 

 however, slightly bent, some almost inappreciably, before the 

 test ; when, therefore, they were subjected to the compressive 

 load, bending stresses were set up in addition to the pure 



