388 



Grubb — Some Nciv Forms of Geodetical Instnmients. 



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 

 in use in PI. 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 us 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 i)oints 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 



Fig 2 



