362 FORESTRY INVESTIGATIONS U. S. DEPARTMENT OF AGRICULTURE. 



OBSERVATIONS AND DEDUCTIONS. 



(a) The difference between the values for the large beam and the average for the small beams is not at all 

 constant, either in character or quantity ; the large beam may be stronger (20 per cent of the cases) or practically 

 as strong— i. e. ; within 10 per cent (57 per cent of the cases)— or it may be weaker, and vary often considerably from 

 the average (23 per cent of the cases). 



Of 6% tests on small beams 235 famished results smaller than that of the large beam. Again, out of 396 small 

 beams fully 40 per cent were weaker that the large beam, while of another series of 300 only 24 per cent gave lower 

 values. 



(&) There are in every case some small beams which far excel in strength the large beam; even in such cases, 

 where the average strength of the small beams is practically the same as that of the large beam, some small beams 

 show values 25 to 30 per cent greater than the large beam. 



(c) In only 6 per cent of the cases each of the small pieces gave a higher result than was obtained from the 

 large beam, but in these cases the latter was evidently defective. 



(d) In all beams the differences observed between the several small beams themselves are far greater than that 

 between the average value of the small beams and the value of the large beam from which they are cut. 



From these observations, which are fully in accord with the observations on the numerous tests of the large 

 general series, it would appear that — 



(1) Size alone can not account for the differences observed; and, therefore, also that a small beam is not propor- 

 tionately stronger because it is smaller, for it may be either stronger or weaker; but that if it is stronger, the cause 

 of this lies in the fact that the larger beam contains weak as well as strong wood, besides other defects, which may 

 or may not appear in the small stick. 



(2) Generally, but not always, a large timber gives values nearer the average, since it contains, naturally, a 

 larger quantity as well as a greater variety of the wood of the tree; and, therefore, also — 



(3) Small beams, for the very reason of their smallness, containing, as they do, both a smaller quantity and 

 variety of the material, give results which vary more from the average than results from large beams, and, there- 

 fore, can be utilized only if a sufficient number be tested; but it also appears that — 



(4) To obtain an average value, even a very moderate number of smaller pieces, if they fairly represent the 

 wood of the entire stem, give fully as reliable data as values derived from a large beam. 



(5) Average values derived from a large series of tests on small but representative material may housed in practice with 

 perfect safety, and these averages are not likely to he modified "by tests on large material. 



It might be added that both the practicability and need of establishing a coefficient or ratio between results 

 from tests on large and small beams or columns falls away. To deserve any confidence at all, only a large series of 

 tests on either large or small beams would satisfy the requirement of establishing standard values, while a series of 

 small pieces has the preference, not only on account of greater cheapness and convenience in establishing the values, 

 but still more for the reason that only by the use of small, properly chosen material is it possible to obtain a 

 sufficiently complete representation of the entire log. 



Before these results, part of which were published, by installments, bad all been computed 

 and arranged, the results of the work made it possible to publish, for the first time in the English 

 language, a brief exposition of the technical properties of wood in general, which appeared as 

 Bulletin 10 of the Division. This little booklet was copied verbatim several times by different tech- 

 nical journals of this country, was embodied in toto in one of the best works on the materials of 

 engineering, and was even translated into French by one of the foremost publishers of France, 

 besides being used itself as a text-book by several of our largest colleges. In addition to the 

 discussions of the several technical properties of wood, this booklet contains the first attempt in 

 the English language at a key by which our common woods may be safely recognized from their 

 structure alone. The key and some of the tables in this bulletin have been reproduced in an 

 earlier part of this report. By this time, when the work was interrupted by superior orders, there 

 were brought together the strength values for the wood of 32 species, of which 2G were represented 

 by more than 200 tests each (the longleaf pine by over 6,000), 17 of them by over 400 tests per 

 species, and seven by over 1,000 tests. These results were published in full in Circular No. 15 of 

 the Division, from which the following extract is here repeated: 



Summary of Mechanical Tests on Thirty-two Species op American Woods. 



general remarks. 



The chief points of superiority of the data ohtained in these investigations lie in, (1) Correct identification of 

 the material, it being collected hy a competent botanist in the woods; (2) selection of representative trees with 

 record of age, development, place and soil where grown, etc. ; (3) determination of moisture conditions and specific 

 gravity and record of position in the tree of the test pieces; (4) large number of trees and of test pieces from each 

 tree; (5) employment of large and small-sized test material from the same trees; (6) uniformity of method for an 

 unusally large number of tests. 



The entire work of the mechanical test series, carried on through nearly six years intermittently as funds 



