388 Gruspp—Some New Forms of Geodetical Instruments. 
paper by a quantity less than the breadth of a pencil line. It is evident, there- 
fore, that the accuracy obtainable by this method is as great in most cases as can 
be recorded upon the paper. ‘The system is very much more direct and rapid, the 
plotting upon the paper being effected directly from the sighting observations, 
whereas, in the case of the Theodolite, the angles have to be measured and noted 
down and then laid off again on the paper with another instrument of the form of 
a protractor. So far as the ultimate survey is concerned there is no use in noting 
these angles except as a means to an end, the final result being the plotted survey, 
which, by the new method, is done directly and without the measurement of any 
angles. 
Instruments of this class are capable of doing good work in the hands of com- 
paratively inexperienced observers, for it is not necessary that the manipulators of 
these instruments should understand anything about angles at all, or be able to 
read verniers, and the process being a direct one, instead of one that is the result 
of a series of observations and steps, the risk of error is very much lessened. 
Plane Table.—The Plane Table, as 
adapted for the new form of sight, is 
shown diagrammatically in fig. 2, and 
im use in Pl. xxxiv., fig. 5. It consists, 
as usual, of a levelled drawing-board, 
covered with a sheet of paper, on which 
it is desired to plot a survey, a base 
(which may be of any desired form, but 
which in this case, for reasons of con- 
venience, partakes of the form of a set 
square), and a sharp steel pin which can 
be inserted in the drawing-board to form 
a centre for the whole of this base to 
turn upon, and on this base one of the 
new sights. 
If the instrument is to be used as a 
Plane Table and for taking bearings 
simply, it is only necessary to have pro- 
jected on the object a single vertical line. 
By superposing this upon the staff at the different salient points of the survey the 
bearing of each object can be recorded upon the paper, and when this is completed 
the whole Plane Table can be moved to another position with a measured base 
line between the two, and angles taken over again from this second station as at 
the first, the intersection of the two sets of bearings giving a record of the points 
required of the survey. 
If, however, the area to be surveyed is small, and it be required to use the 
