6 Mr Blythe, On the forms of [Nov. 11, 



thereby produced in the secondary circuit is P/R, and thus the 

 initial angular momentum of the suspended coil is 



Ko' = q ^f ( 10 )> 



where a> is the initial angular velocity. 



But, if 6' is the " throw " produced, we have by (6) w\w = 0/6', 

 so that by (3) and (10) 



--^ w 



Two determinations of C'PN/And' on two different days 

 agreed to within '5 °/ . 



The reversing key mentioned in the paper has been con- 

 structed and measurements of the hysteresis for a specimen of 

 steel wire have given results agreeing, within the limits of accu- 

 racy of the observations, with the values obtained by B-H 

 cycles. For very soft iron in which the maximum value of dB/dH 

 is over 8000, and six times its average value, there is still a 

 difficulty due to self-induction when a single wire about *01 cm. 2 

 is tested. But I hope to overcome this difficulty shortly. 



G. F. c. S. 



(2) On the forms of cubic surfaces containing 27 real straight 

 lines. By W. H. Blythe, M.A., Jesus College, Cambridge. 



The following account of the transition between the various 

 forms of these surfaces is a supplement to the paper in Proceed- 

 ings, vol. VIII., p. 241. 



A simple case of a cubic surface is that of a plane and a right 

 cone shewn by one figure. Take a solid rectangular block, and 

 let its upper surface represent the plane in question. Cut a right 

 cone out of this block, axis vertical, having its vertex in the plane 

 at a point A. Invert the solid cone, vertex at A. The surfaces of 

 the solid and hollow cone, and that of the plane, represent a cubic 

 surface. A straight line can be drawn to cut the surface in three 

 points. Comparing this diagram (1) with (2) and (3), which are 

 vertical sections of a cubic surface represented by the general 

 equation, we see a similarity in form. In the case of (3) the 

 upper hollow portion is connected with the lower. In passing 

 from the form (2) to (3) and then from (3) to (2) it is evident that 

 a " hole" is made, joining the upper to the lower hollow portion. 



In the case of a cubic surface having twenty-seven real straight 

 lines there are three such " holes " or openings. 



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