525 
of Edinburgh, Session 1885-86. 
the field ; while the sixth shows the difference of each value of g 
from that preceding it, except in the case of Exp. VII., where the 
difference is between the value of g for its position, and that at 
the centre. 
There is also a table giving two series of pairs of points, the value 
of g being constant throughout each series. Thus there are two 
points in every quadrant, for each value of x , which give the same 
value of g. 
Explanation of the Diagrams ( Plate XVIII.). 
Fig. 1 represents a portion of the Helmholtz field, where 0 is the 
centre, and OY the axis. The two pairs of curved lines are of 
equal values of g, being constructed from the data given in the last 
table. The scale in this figure is 5 divisions per cm. 
Figs. 2, 3, 4, and 5 represent the values of g along OM, 
ON, MA, and N A respectively; the scale for the distance along 
these lines being in every case 5 small divisions per cm. along the 
one co-ordinate, while the values of g are measured along the 
other. 
In fig. 6 the values of g along OA are represented ; but, as 
it is inconvenient to represent 
the actual distances of the 
points along the diagonal on 
same scale, the distances from 
the centre of their projections 
on the axis are substituted. 
The accompanying sketch 
represents the whole of the 
horizontal section along the 
axis of the Helmholtz, indi- 
cating the position of the part 
of the field given in fig. 1, 
CC and C'C' being the two 
coils, and OY the axis of the 
galvonometer. 
