19 



parts ; hence a rotary advance of one division on the screw head 

 produces a linear advance of one ten-thousandth (o-oooi) of an 

 inch. 



"A vertical scale, divided into fiftieths of an inch, is fastened to 

 the frame of the instrument, and set very close to each screw head 

 and parallel to the axis of the screw ; these serve to mark the 

 starting point of the former, and also to indicate the number of 

 revolutions made. By means of this double instrument, the exten- 

 sions can be measured with great certainty and precision, and 

 irregularities in the structure of the material, causing one side of 

 the specimen to stretch more rapidly than the other, do not 

 diminish the accuracy of the measurements, since half the sum of 

 the extensions indicated by the two screws is always the true 

 extension caused by the respective loads." 



The size of test pieces advised for compression varies with 

 different authorities, the limit of the ratio of the length to the 

 diameter being five times the diameter. The compression test 

 should crush the piece down or shear it at 45, but even with the 

 ratio used the length equal to twice the diameter, as given and 

 used by Prof. Thurston the piece showed a slight buckling in 

 some cases. 



We have obtained the results, from each test by plotting a curve, 

 taking the loads as ordinates and the deflections as abscissas. In 

 this way we see the behavior of the piece at a glance, by the 

 curve, and find the elastic limit very accurately. This latter was 

 obtained by drawing a straight initial line when necessary, and 

 taking it as the point where the curve leaves the line. 



We have given not only the results derived from the most 

 common formula used in designing, or used indirectly, but also 

 the original data of size of test piece, the actual loads, unit strains 

 and stresses as they may be found convenient in some cases and 

 for further information if desired. The formulae from which the 

 results were derived are set at the head of the tables. 



