784 SUMMARY OF CURRENT RESEARCHES RELATING TO 



the advantage of the Microscope in saving, in the case of specimens 

 readily determined to be bad, the expense of further testing, and the 

 risk of using it in construction. The samples shown this evening 

 are of bridge timbers, and the lesson they are intended to convey is 

 that had this method of examination been followed, the material which 

 was proved to be faulty after being built into the bridge, would have 

 been promptly thrown out. The samples shown were photographed 

 by Mr. W. E. Partridge, of New York, a professional engineer who 

 is an enthusiastic amateur photographer, and to whom I am indebted 

 for the particulars concerning them. 



The timber from which the poor specimens were taken came in 

 the form of a chip broken off when a highway bridge was wrecked in 

 1879-80. The timber formed a portion of the sill of a draw-bridge, 

 which consisted of two 12-inch sticks, lying one on the other. The 

 turntable casting having been somewhat too small, this 24-inch 

 timber had to support one of the A frames of the bridge at a 

 distance of about twelve inches outside of the bed-plate. After a 

 few days of service, while an empty truck was passing over, the 

 strain became so great that the A frame sheared the 24-inch sill, 

 wrecking the whole bridge. The timber was so exceedingly poor 

 that upon mounting it on the Microscope the porous and weak nature 

 of its structure was at once discovered. Its annular rings are some- 

 thing like three times the distance apart which would be found in a 

 piece of thoroughly good wood of a similar character. The medullary 

 rays are few in number and short in length, while in good wood they 

 are of considerable length, and so numerous that the tangential 

 sections appear like a series of tubes seen endwise, or a number of 

 parallel chains. After once seeing and comparing two samples of 

 wood it is very easy to recognize their characteristic features by the 

 use of a pocket magnifying glass. 



The trunks and limbs of exogenous trees are built up of concentric 

 rings or layers of woody fibre, which are held together by radial 

 plates, acting like the trenails of a wooden vessel, or the " bonds " in a 

 brick or stone wall. The rings or layers representing successive 

 years' growths, are composed of tubes, the interstices between which 

 are also filled with cellulose. The slower the growth of a tree the 

 thinner these yearly layers, and the denser and harder the wood, 

 other things being equal. This is true as between one kind of tree 

 and another, and also between different individuals of the same kind. 



Not only is the closeness of the growth an indication of the 

 hardness and strength of the timber, but the size, frequency, and 

 regularity of distribution of the radial plates which bind the layers 

 together may be taken as a very close illustration or sign of the 

 character of the wood and its ability to resist strain, especially that 

 from crushing stress. 



The micro-photographs of the sections of good and bad timber 

 show that in the strong specimens the concentric rings are close in 

 texture and of light width ; and the radial plates frequent, wide, long, 

 and thick, while in the poor material,, the reverse characteristics are 

 shown. 



