1893.] Magnetic Qualities of Iron. 75 



solid; and, as in 17 above, we see that there are fifteen independent 

 coefficients in the quadratic function of the strain-components ex- 

 pressing the work required to produce an infinitesimal strain. Thus 

 we realise the result described in 7 above. 



35. Suppose now each of the four tie-struts to be not infinitely 

 resistant against change of length, and to have a given modulus of 

 longitudinal rigidity, which, for brevity, we shall call its stiffness. 

 By assigning proper values to these four stiffnesses, and by sup- 

 posing the tetrahedron to be freed from the two conditions making 

 it our special tetrahedron, we have six quantities Arbitrarily assignable, 

 by which, adding these six to the former fifteen, we may give arbitrary 

 values to each of the twenty-one coefficients in the quadratic function 

 of the six strain-components with which we have to deal when change 

 of bulk is allowed. Thus, in strictest Boscovichian doctrine, we pro- 

 vide for twenty-one independent coefficients in Green's energy-function. 

 The dynamical details of the consideration of the equilibrium of two 

 homogeneous assemblages with mutual attraction between them, and 

 of the extension of 9 17 to the larger problem now before us, are 

 full of purely scientific and engineering interest, but must be reserved 

 for what I hope is a future communication. 



II. Magnetic Qualities of Iron." By J. A. EWING, M.A., F.R.S., 

 Professor of Mechanism and Applied Mechanics in the 

 University of Cambridge, and Miss HELEN G. KLAASSEN, 

 Lecturer in Physics, Newnham College. Received June 7, 

 1893. 



(Abstract.) 



The paper describes a series of observations of magnetic quality in 

 various specimens of sheet iron and iron wire. A principal object 

 was to determine the amount of energy lost in consequence of magnetic 

 hysteresis when the iron under examination was carried through 

 cyclic magnetising processes between assigned limits of the magnetic 

 induction B. For this purpose observations of the relation of the in- 

 duction B to the magnetic force H were made, from which curves were 

 drawn, and the area enclosed by the curves in cyclic magnetising pro- 

 cesses was measured. Many such cycles were gone through in the 

 case of each of the specimens, the limits between which B was re- 

 versed being varied step by step in successive cycles, to allow the 

 relation of the energy expended or of J Hdl to B to be determined. The 

 curves of B and H in these graded cycles are drawn in the paper, as 

 well as curves showing the relation of f RdL to B and to H. Most of 

 these experiments were made by the ballistic method, the specimens 



