MATHEMATICAL AND SURVEYING INSTRUMENTS. 6] 
vanes must lie in a vertical plane with the edge of the ruler. When this 
edge is so adjusted, it becomes a fiducial edge. For the sake of sighting both 
forwards and backwards with the same position of the ruler, each vane con- 
tains both slit and wire, one above the other; the slit being inferior in the 
one, and superior in the other. A magnetic needle, I’, is sometimes attached 
to the ruler, in which case the erecting compass can be spared, as it is only 
necessary to draw a sight line at the commencement of operations, and 
marking the direction of the needle with reference to this line, to cause the 
same direction to be maintained at each successive erection of the table. 
The form of ruler represented in pl. 5, fig. 25, naturally serves only for 
short distances, as far as the unarmed eye can see clearly. To sight at 
greater distances, the diopter ruler, with telescope (fig. 26), has been con- 
structed. <A plate, C, is fastened to the ruler AB, carrying a telescope of 
feeble magnifying power. The ocular of the telescope is at E, and in its 
focus is a vertical cross-hair, which supplies the place of the object diopter. 
In the better instruments, the tube can be taken out of its bed and reversed : 
by this means, both forward and backward sighting can be attained. For 
reduction to the horizon, and sighting objects above or below the horizontal 
plane, the axis of the telescope can be turned, in a vertical plane, about its 
axis of rotation. A graduated arc, F’, is often attached, which is fastened 
to the tube, and turns with it, while an index is fixed at G, to the frame. In 
this manner angles of elevation or depression may be measured. If the 
index point to the zero of the scale, the tube will be horizontal, and may 
then be used in levelling. 
To illustrate the mode of using the plane table, reference must be again 
made to fig. 57. ‘Taking for instance the figure on jig. 57, the first thing to 
be done is to determine the base ab, which it is of great importance to lay 
out properly. Its chief condition is, that as many points as possible may be 
determined from it, by intersections which are not too acute. This line 
must be measured as accurately as possible by the chain and staves. The 
plane table is then erected horizontally, at A, with the point a over the 
point a of the ground, and the north and south line marked. An assistant, 
with a signal, is then dispatched to b. A needle is now stuck in a, the point 
6 sighted with the diopter ruler, and the line ab drawn. With the needle 
still sticking in a, sight lines are drawn to all the principal points visible, as 
C, D, BE, F, G, H, J, K, L, M, N, O, P,Q, RB, S; TT, which are rum out to 
the edge of the paper, and indicated by letters or numbers, for subsequent 
identification. In this manner are produced the sight lines ab, ac, ad, .... as, at. 
When the objects have no sharp outline, or are difficult to recognise, as at 
O and 8, signals must be set up, and allowed to stand until the operation is 
completed. After this has been done, and all the sight lines again gone 
over, the station A is left, with a signal fixed in it, and the table carried to 
B, where it is again set up, after having marked on the paper the length of 
the base ab, on a reduced scale, thereby determining the point 6. This 
point 6, on the paper, is brought over the extremity b of the base line, first 
by the eye, and then by the fork,—the table erected horizontally, the ruler 
laid along the line ab, and the table turned until the sight line of the ruler 
61 
