1903.] Stress and Strain in the Cross-section of a Beam. 15 



mirror, N, is attached to F on a vertical spindle, about which it can be 

 turned by the fine adjustment screw G. 



When the instrument is set up, a scale is placed in an upright 

 position in front of the mirrors, and some distance away, and the two 

 images of the scale are brought together in the field of a telescope by 

 means of the screw G. A convenient mark on the N image is then 

 taken as an index, and the reading coinciding with it on the 

 other image is noted each time the load on the beam is altered. 

 From the difference between the readings before and after applying a 

 load, the transverse strain is calculated across the breadth of the beam. 



The instrument may be placed either below the specimen, when the 

 screws AA are used, or above, when BB perform exactly the same 

 functions. 



The pressure between the screws and the specimen is maintained by 

 a stiff spring, Q and the weight of the instrument taken up by 

 attaching a thin cord to G, which, passing over a light pulley, carries 

 a balance weight. It will be seen that the instrument is free from 

 strains, which vary with the load on the specimen. 



The dimension from the axis to the plane of the tilting mirror was 

 20 cm., while that between the centre-line of the set-screws and the axis 

 was 5 cm. A magnification of four was thus obtained in the instrument 

 itself, For a long time consistent reading could not be obtained, and 

 it was only after very many trials of the instrument and alterations 

 in its design that the causes of the inconsistency were successively 

 eliminated. 



The apparatus was very sensivive to vibrations, and it was soon 

 found that accurate readings could only be obtained when the 

 machinery in the adjacent laboratories was at rest. Vibrations caused 

 by traffic some distance away were quite noticeable. 



The most troublesome factor was probably the small unavoidable 

 amount of jerk accompanying the application of the load. Occasionally 

 this would cause a very slight displacement of the tilting mirror, 

 which could not be detected on looking in the telescope, but neverthe- 

 less was sufficient to vitiate the accuracy of the readings. Under 

 these circumstances it was impossible ever to depend on a single 

 observation. The load was invariably applied and removed several 

 times, and if the readings were consistent, and returned each time to 

 their original value when the load was taken off, they were accepted as 

 correct. 



Any imperfection in the adjustment of the instrument or in the 

 manner of applying the load was always made evident by the erratic 

 nature of the readings. 



