Gruspp—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, Pl. xxxtv., 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 be 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 point 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 mtended 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- Fig. 3. 
ment generally known as the “Abney” , S— 
level, which is held in the hand, and 7” “°° 
which is very rapid in its working, giving fair results, and sufficient 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 
are, 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 Pl. xxxiv., fig. 7, where the 
instrument has been set for a level gradient, and it will be seen that the observer 
TRANS. ROY. DUB. SOC., N.S., VOL. VI., PART XV. 2L 
End Elevation. 
