38 



D. P. PENIIALLOW ON 



"We may, therefore, seek an answer to this in a comparison of the ratio now obtained, with 

 the weight per cnbic foot, and the coefficient of ehisticity : 



No. VI. 



RELATION OF RELATIVE VOLUMES OP SUMMER AND SPRING WOODS, AND WEIGHT PER CUBIC FOOT 



AND COEFFICIENT OF ELASTICITY. 



The' relations thus exhibited go far to confirm the view already expressed, since with one 

 exception, a diminishing ratio between summer and spring woods is directly related to 

 an increase in weight, and also an increase in the coefficient of elasticity. The exception 

 as found in 789 is so marked as to point to some exceptional conditions, the nature of which 

 does not appear. 



"While these relations are extremely suggestive and show the direction in which future 

 investigations should be pursued, the data are altogether too limited for the deduction of a 

 general law. It is clear, however, that if these relations do express a general law, then we 

 have at once a means of determining the relative strength of timbers either by ascertaining 

 the weight of a cubic foot, or by determining the ratio of summer and spring woods. 



It should be pointed out that these relations were discovered too late in the course of 

 our investigations to admit of more extended examination. 



Among other variations to be noted in this wood, and one which has a more or less 

 direct bearing upon the question of strength, is that which relates to the size of the tracheids 

 as exhibited in transverse section. In each case the dimension of a tracheid is laken in two 

 directions from centre to centre of the wall. In the case of the spring wood those tracheids 

 which were first formed, and which lie immediately external to the summer wood of the 

 previous year, are uniformly selected. The average value is then obtained by measuring in 

 each direction — radial and tangential — as many tracheids as lie within the limits of a micro- 

 meter scale of 5 mm. in length. In the case of the summer wood, the tracheids forming the 

 outermost portion of the growth-ring are uniformly chosen, and for an average value, as 

 many tracheids are measured in the two directions, as will lie within a micrometer scale of 

 5 mm. in length. "When the total thickness of the summer wood does not exceed the micro- 

 meter scale of 5 mm., all the tracheids lying on a given radial line are measured for an 

 average. The average areas may then be brought into comparison. In this way the values 

 exhibited in the following table have been obtained : 



