PHYSICS. 315 



obtain the elastic constants of small bodies. The appHcation of the 

 displacement method proved at once to be astonishingly easy in a 

 case where a degree of rough handling is inevitable. 



Chapter IV contains appUcations of the rectangular interferometer, 

 using achromatic fringes, to geophysical problems. A method for the 

 determination of the Newtonian constant is worked out. Again, the 

 same interferometer is associated with the horizontal pendulum for 

 the detection of small changes in the inclination of the earth's surface. 

 A series of observations extending between January and August are 

 recorded. 



The last chapter is devoted to corresponding methods for the inter- 

 ferometry of vibrating systems. The luminosity of the achromatic 

 fringes lends itself easily to this purpose, and it was merely necessary 

 to design an appropriate vibrating telescope. To test the method a 

 study is made of the vibration of telephonic apparatus. Clear-cut 

 interference vibration curves are shown for two identical telephonic 

 systems joined directly in series, while these forms subside completely 

 when the telephones are joined differentially. Such a system is also 

 an electric dynamometer capable of appreciating an average alternat- 

 ing current well within a microampere. It could, moreover, be syn- 

 chronized relative to an external electric impulse by aid of the Lissajous 

 curves, with the same accuracy as two tuning-forks. 



Howe, Henry M., National Research Council, Washington, D. C. Research 

 Associate in Metallurgy. (For previous reports see Year Books Nos. 6-16.) 



Most of the year 1917-18 has been devoted, under the joint direction 

 of the National Research Council and the Ordnance Department of 

 the War Department, to seeking material with great baUistic resist- 

 ance when in the form of the very thin sheets used for helmets and 

 body armor. The general pohcy of these bodies forbids my giving any 

 indications of the results reached. An interesting phenomenon is the 

 spontaneous development of large volute aging cracks, sometimes with 

 a loud report, weeks and even months after ballistic testing. 



I have proved that agitation during solidification, for instance by 

 rotation at a contmuously var^-ing rate, prevents in metals the for- 

 mation of the coarse columnar crystals of the class which in steel 

 ingots do so much harm by concentrating at their boundaries certain 

 of the impurities contained in the molten metal, and thus creating 

 regions of great enrichment in those impurities and consequent brittle- 

 ness of the mass taken as a whole. 



I have done much work in systematic attempts to correlate the 

 microstructure, thermal treatment, and mechanical properties of steel, 

 and to throw hght on the mechanism of plastic deformation. 



