664 Tranmcfionp of the Royal Society of So2ith Africa, 
On the lines through b and c perpendicular to the rod are set off lengths 
— 14-8 and — 1"3, giving the points e and f. 
Pins are inserted at a and at e, and the cardboard parabola adjusted till 
it is in contact with these points, its axis being perpendicular to the beam ; 
- 
IT 
in - 
+ - 
in in- 
+ T- 
to >~ 
-Q ^ 
— O 
o o 
o o 
II II II 
M ^ ^ ^ 
3 Q Q O 
0 
C Q) C 
Q) ^ O 
-J S 
\ 
n 
- 
this orientation is ensured by keeping lines of the cardboard parallel to the 
corresponding lines of the paper. 
The abscissa readings of a and e on the cardboard are noted down 
(-16-2 and +23-8). 
The same process for e and f gives abscissa readings — 16'8 and 3-2; 
and for f and d —10-7 and 9*3. 
