130 
HINTS TO TRAVELLERS. 
the general directions of the views and the kind of intersections which 
may be expected. 
Suppose two suitable enlarged pictures have been selected to commence 
plotting from such as we know, from cursory inspection, are likely to 
give good intersections over a fair area. Tire next practical step is to 
select and to mark, on the picture , with tiny dots and numbers in red ink, 
the points which it is desired specially to plot by intersection. The same 
numbers should be given to the same points in both pictures (or, indeed, 
on any pictures where they are visible). When the pictures have been 
carefully overhauled and marked in this way, the next thing to do is to 
mark off along one edge of a narrow band of paper the exact horizontal 
distance from the median vertical line of each point, and note the 
appropriate numbers on the band near the points. One or more separate 
bands are used for each picture. Next we must fix the position of the 
horizontal trace of the picture plane on the plan for each picture. This 
is done by first setting off from the stations the correct directions of the 
distance lines of the views by aid of a good protractor, and prolonging 
these distance lines until their total length equals exactly the equivalent 
focus for each picture. Lines drawn through the distal extremities of 
the distance lines so set off and accurately perpendicular to them are the 
horizontal traces of the picture planes. 
The marked paper strips or bands are then laid on the plan so that the 
marked edges coincide with the picture traces and the zero of each band 
coincides with the point where the distance line meets the trace of the 
picture plane. The strips are then held in position by pins or paper¬ 
weights. 
Next, pins are driven into the station points, and hairs or threads of 
silk or cotton, looped at one end, are slipped over those pins. At the 
other end they are tied to elastic threads, which are fixed at their distal 
ends to paper-weights, so that when the weights are laid on the plan and 
the elastics stretched a little the threads must be straight. 
Now, if the weights be shifted on the board or table until the threads 
(always moderately tight) pass through a dot of the same number on the 
two slips, the intersection of the threads marks the position of the point 
on the plan. In this way, all points which are common to the two 
pictures, and which have been marked on the paper strips, can be very 
rapidly plotted. The same process can be repeated with any number of 
