under endlong compression. 193 



about 50 per cent, greater since the moment of the " rectangular 

 forces " is greater than that of the " triangular " in the ratio §. 



As an example of this effect the case of a square sectioned strut 

 may be cited ; for any given elastic deformation the required 

 bending moment will be the same whether bending occurs about 

 an axis parallel to one side of the square or about a diagonal 

 or about any intermediate axis. When the elastic limit is 

 exceeded, this state of affairs no longer holds — thus if bending 

 occurs about the axis AB in Fig. 2 the limiting bending 

 moment, as previously explained, is about 50 per cent, greater 

 than the maximum elastic one. In Fig. 2 a however, where 

 bending occurs about an axis such as CD, the limiting bending 

 moment is only about 40 per cent, greater than the maximum 

 elastic one. 



In other words, the strut is weaker when bent as in Fig. 2 a 

 than as in Fig. 2, therefore when such a strut collapses it might 

 be expected to do so by bending about a diagonal axis. The 

 author tested this theory by taking a f-inch square-sectioned 

 strut and loading it till instability set in and collapse occurred — 

 it was found that the bending was about a diagonal axis, and 

 seven repetitions of this experiment gave the same result in every 

 case. 



2. This helps to explain the experimental results obtained by 

 the author in the following experiments. The material used was 

 mild steel and was in the form of circular rods of diameter 

 = 0256 inch, the ends of the rods being coned to receive steel balls, 

 and these balls being used to transmit the endlong forces and at 

 the same time to allow bending to occur with as little friction at 

 the ends as possible. Preliminary experiments showed that the 

 elastic limit of this material was about 33 tons per square inch 

 and that the breaking load in direct tension was about 41 tons per 

 square inch. The rods, each 24 inches long, were placed upright 

 in the apparatus shown in Fig. 3 and loaded with a force P at 

 the top whilst being pulled out sideways by a weight W. The 

 lower block LB and the screw above were both recessed conically to 

 receive steel balls (031 2 inch in diameter) and between these balls 

 was placed the rod to be tested. The block LB rested upon the 

 cast-iron platform of a steelyard so that any downward force 

 exerted upon LB was measured by reading the steelyard lever — 

 this steelyard read up to about 1 ton. The downward force P was 

 exerted by screwing down the wheel SW until the strut-beam 

 being tested was subjected to the required degree of endlong com- 

 pression, this compression being easily measured as stated above. 

 In addition to this endlong loading a lateral one was introduced 

 by suspending a weight W from the cord shown on the right of the 

 diagram. Both P and W were next increased — one at a time — 



