Micro- Metallography of Iron. 59 



As a third example, suppose the section perpendicular to the long 

 axis uniform in shape and area, we then get 



(79). 



A special instance of this last case is presented by an elongated 

 cylinder rotating about a perpendicular to its length through its 

 centre. Putting B, = o> 2 , we have 



(80). 



Comparing the several cases of rotation, we have an interesting 

 illustration of how the effects of the " centrifugal " force increase as 

 the mean distance of the substance of the solid from the axis of rota- 

 tion becomes larger. If the limiting angular velocity permissible in 

 the elongated ellipsoid rotating about a short axis be taken as 100, 

 then the limiting angular velocities in the thin elliptic disc and the 

 elongated cylinder rotating about their short axes the material, and 

 the length of the long dimension being the same for all are approxi- 

 mately 87 and 71 respectively, both on the stress-difference and 

 greatest strain theories. A caution must, however, be added that m 

 bodies of such elongated form rotating about a short axis, a sudden 

 change in the angular velocity may prove disastrous. 



" Micro-Metallography of Iron. Part I." By THOMAS ANDREWS, 

 F.R.S., M.Inst.C.E. Received December 15, 1894, Read 

 January 24, 1895. 



Secondary Micro-crystalline Structure in Metallic Iron. 



The term metallography appositely describes, in one word, that 

 department of metallurgical science which deals with the accurate 

 study and delineation of the ultimate formation or structure of 

 met ;ils, a knowledge of which is of the utmost importance. 



This development of the science of metallurgy is destined to prove 

 of incalculable value in the study of the ultimate micro-structure 

 of iron and steel. 



Experiments on the microscopic structure of iron and steel appear 



* This is more exact for a long beam of rectangular cross section than the result 

 I obtained in the ' Quarterly Journal* for 1888, p. 29. 



