THE CUBIC PARABOLA. 51 
Li Ti. iit. IV. 
FeS, SulphideofIron 1°42 “45 “42 
PbSO, Sulphate of Lead 19°36 19°80 77°60 84°61 
PbS Sulphide of Lead ... 710) 2-20 ‘96 
Insoluble matter  °30 1:60 4°50 1:00 
99°77 99-73 99°93 99-93 
see a 
It will be seen from these analyses that the composition of the 
substance is too variable to allow of its being classed as a mineral 
species. A polished surface of the substance was treated for some 
hours with a solution of acetate of ammonia with the result that 
some of the sulphate of lead has been removed, Jeaving the 
sulphide of silver in relief. 
_ Tue CUBIC PARABOLA As APPLIED TO THE EASING or 
CIRCULAR CURVES on RAILWAY LINES. 
: By C. J. MERFIELD. 
(Communicated by Mr. G. H. KNIBBs. ) 
[With Plate X.] 
[Read before the Royal Society of N.S. Wales, June 5, 1895.] 
Tue cubic parabola having been adopted by engineers and 
Surveyors of this Colony, as an easing curve to the circular arcs 
railway and tram lines, exact and extended tables will be 
: Fequired. The following investigation and tables are intended to 
supply this want. Many of the methods that have been published 
are defective, and unsuited to the requirements of the engineer. 
The use of the cubic parabola, as a transition or easing curve, 
is by no means new. The American and Continental Railways 
are mostly so located, that a curve of varying radius may be 
