219 
Professor MituerR asked whether a zenith sector referred to 
by the author was the one used at the equator, some faults of 
which he pointed out; he also enquired whether observations 
had been made to determine the eccentricity of the quadrants 
used in the Lapland measurements. 
Mr TopHUNTER said the sector was not the same; and that 
there was no clear information given on the other points. 
Mr Goprray asked if the toises used in the observations 
had been compared. 
Mr Topuunter replied in the affirmative. Some further 
conversation occurred in which the President (Prof. CAYLEY) 
and Mr ELuts took part. 
May 15, 1871. 
The PRESIDENT (PROFESSOR CAYLEY) in the Chair. 
The Treasurer (Dr CAMPION) gave a statement of the finan- 
cial condition of the Society, which he pronounced to be satis- 
factory. A vote of thanks, proposed by Prof. BABINGTON and 
seconded by Prof. SELWYN, was heartily accorded. 
Communications made to the Society : 
(1) On Dr Wiener’s Model of a Cubic Surface with 
27 lines; and on the Construction of a double-sixer. 
By Prof. Cavey. 
I call to mind that a cubic surface has upon it in general 
97 lines which may be all of them real. We may out of the 
27 lines (and that in 36 different ways) select 12 lines forming 
a “double-sixer,” viz. denoting such a system of lines by 
Ay Ar) Ay Ay Usy Upp 
Dp Op xy We Wy Os 
then no two lines a meet each other, nor any two lines , but 
each line a meets each line }, except that the two lines of a 
pair (a, b,), (dy b,)s-++(Ges Bs) do not meet each other. And such 
