Grubb — Some New Forms of Geodetical Instruments. 



389 



instrument as a subtense instrument, and to complete the survey from one single 

 point, it is necessary to project upon the staff a scale, as is seen to be the case in 

 the engraving, PI. xxxiv., fig. 6. The staff used in this case has two marks 

 placed upon it a certain distance apart, say two yards, and at the same time that 

 the bearing of that staff is taken (as in the operation first described for the 

 Plane Table) the interval between those two marks, six feet apart on the staff, is 

 measured by the number of divisions which that space occupies on that scale, the 

 number of the divisions occupied by this space being of course greater as the 

 distance of the staff is less, and vice versa. 



On that side of the base which is parallel to the line of sight, and which points 

 also to the centre on which the base revolves, a scale of unequal parts is cut, and 

 if a mark bo made at the particular division on this base scale, which corresponds 

 to the number of the divisions occupied by the staff on the " ghost " scale as seen 

 in the sight, that jDoint will represent on the paper not only the bearing of that 

 staff as regards the central station, but the actual distance from it, according to 

 whatever scale the instrument is divided for. 



The assistant carrying the staff is directed to walk round the field and plant 

 his staff at every spot where a change in the direction of the boundary occurs, 

 holding the staff upright until the observer signals to him to pass to the next 

 station, and in this way a survey can be completed upon the paper in the time 

 that it takes the assistant to pass round the field from station to station. 



Level. — The Level, with the new 

 sight applied, is shown diagrammatically 

 in plan in fig. 3, and as worked in PI. 

 XXXIV., fig. 8. It is not intended that 

 this Level should be used to supplant 

 the ordinary surveyor's level with the 

 parallel plates, &c., but it is intended 

 to take the place of the class of instru- 

 ment generally known as the " Abney " 

 level, which is held in the hand, and 

 which is very rapid in its working, giving fair results, and sufiicient for ordinary 

 road work, or laying out of grounds. 



In this instrument the sight is utilized for projecting upon the field of view 

 not only a fiducial mark, but an image of the bubble itself, and also that of an 

 arc, which shows the gradient to which the instrument is set when it is necessary 

 to lay off roads or bases that are any particular number of degrees off the 

 horizontal. 



The appearance of the field of view is shown in PI. xxxiv., fig. 7, where the 

 instrument has been set for a level gradient, and it will be seen that the observer 



TEANS. HOY. DUJJ. SOC, N.S., VOL. VH., PABI XV. 



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