18 



and knotty on either side of middle line on the thickness side. 

 Broke just under 750 pounds, with 0-8 1 1 inches deflection. 



(33.) Zopilote. 1 1 -inch supports. With a knot near one sup- 

 port gave away by shearing at the place when under 2, 5 50 pounds. 

 Stopped the test at this point and took this as the rupturing load. 



The compression tests were made on the Olsen machine of 

 14,000 pounds capacity. The machine consists of four columns 

 bolted to a plain cast-iron bed, and supporting on top a plate, in 

 the middle of which one end of the test piece, if for tension, or the 

 rod which holds the measuring apparatus are held by means of steel 

 wedges. 



Through the four angles of this bed and through those of that 

 below pass four screws. To these latter is secured a plate, which pulls 

 or compresses the piece, as the case may be. The plate rests upon 

 four knife edges, on three beams or levers, and their ends rest together 

 upon a link hung from knife-edges on each side of another beam, 

 which latter is linked to the end of the scale in the same way. The 

 machine is very sensitive. The scale is divided into divisions of 

 five pounds. The load is applied by means of a crank or a lever, 

 ^turning a central wheel geared to four others, one on each screw. 

 A powerful leverage and a steady vertical motion of the plate is 

 obtained. The deflections were measured by means of micrometer 

 screws, using the electrical contact. I quote Prof. Thurston's full 

 description of the instrument : 



" The instrument consists essentially of two very accurately- 

 made micrometer screws, working snugly in nuts secured in a 

 frame which is fastened to the head of the specimen by a screw 

 clamp. It is so shaped that the micrometer screws run parallel 

 to and equidistant from the neck of the specimen on opposite sides. 

 A similar frame is clamped to the lower head of the specimen, and 

 from it project two insulated metallic points, each opposite one of 

 the micrometer screws. Electric connection is made between the 

 two insulated points and one pole of a voltaic cell, and also between 

 the micrometer screws and the other pole. As soon as the micro- 

 meter screw is brought in contact with the opposite insulated point, a 

 current is established, which fact is immediately revealed by the 

 stroke of an electric bell placed in the circuit. The pitch of the 

 screws is 0-02 of an inch, and their heads are divided into 200 equal 



