34 



thousamltlis of an inch upon a micrometer screw held in the top iron crossbeam. By this means 

 a rigid connection is obtained, through parts not under stress, from the end supports to the center 

 bearings. The movement of the center witli reference to the ends is therefore obtained, regardless 

 of the absolute movements of the parts. The load is put on by the hand wheel and power screw, 

 and the weighing beam kept in balance by putting on overweights and moving the poise. Three 

 men are required to make this test. One moves the power screw, wliich has one-fourth inch pitch, 

 so as to make one revolution every two minutes, and he continues this uniform motion till rup- 

 ture occurs. Another keeps the scales balanced and calls off the even hundreds of pounds. 

 Another keeps the micrometer screw in contact with the head of the power screw, reads it for cer- 

 tain even hundred-pound loads called off, and records the time of each such reading to the nearest 

 minute, the load, and the corresponding reading of the micrometer screw. Here also the end and 

 center bearings are protected by iron plates large enough to prevent any appreciable distortion 

 from lateral compression. 



After rupture occurs the stick is bored for samples from which to obtain the moisture tests, 

 and the uninjured ends sawed off and used for the remaining tests, as described below. 



The moisture text. 



The borings are taken from two holes, L'O inches from each end, and at about one third the 

 width of the stick, from cither side. Tliese borings are first weighed on a delicate balance then 

 placed in a drying oven, at a temi^eraturc of 212° F. until tliey have reached a nearly constant 

 weight, when they are reweighed. The dry weight is taken as the basis on which to compute 

 the ^percentage of moisture. Thus, if the original weight is twice the final weight, then there 

 was as much water as woody fiber in the stick, or one lialf or 50 per cent of the original weight 

 was water. But when computed on the basis of the dry weight there would be 100 per cent of 

 water. The advantage of computing the percentage on the dry weight is that it furnishes a con- 

 stant basis of comparison, whereas if computed ou the actual or wet weight the basis on which the 

 percentage would be computed would vary with every change in the amount of moisture. 



The upccific grarity. 



The specific gravity is found by taking one of the end pieces, usually i by 4 by 8 inches, 

 measuring carefully its lateral dimensions by calipering them at tlie middle i)oiuts of the sides 

 at the central section, measuring the length in a similar manner, and taking the product of these 

 three dimensions as the volume. From the total volume and the actual total weight, the weight 

 per unit volume or per cubic foot is found, and from this the specific gravity, which is the weight 

 per cubic foot divided by the weight of a cubic foot of distilled water. It must be understood 

 that all the small {i by 4 inch) beams are planed up true and rectangular b.-fore testing and 

 that all the crosscuts are made by a power saw so adjusted as to cut truly at right angles to the 

 sides. The volume can therefore be very accurately computed from the dimensions as above 

 described. 



The tension test. 



The tension test piece is cut from one end of the broken beam. It is 16 inches long, 2i inches 

 wide, and IJ inches thick. Its thickness at the center is reduced by cutting out with a band saw 

 circular segments, leaving a breaking section of some 2^ inches by three-eighths inch. This 

 specimen is then placed between the plane wedge-shaped steel grips, and pulled, the same as a 

 bar of iron, in the Universal Machine, shown on Plate iv. This simple method has been found 

 very satisfactory in practice, and is fuUy illustrated on Plate V. For this test care is taken to cut 

 the specimen as nearly parallel to the grain of the wood as possible, so that its failure will occur in a 

 condition of jjure tension. 



The endwise compression test. 



Most of these tests are made on sticks 4 inches square by 8 inches long, the ends having been 

 cut perfectly true and at right angles to the sides. They are tested in the Universal Machine, 

 the compression continuing until the stick has been visibly crushed and has passed its maxinuim 

 load. The crushing usually manifests itself over a plane section, by crushing down, or bending 



