616 proceedijSigs of section h. 



12.— ON A NEW FORM OF TELEMETER. 



By G. H. KNIBBS, Lecturer in Surveying, University of Sydney. 



The coustructive features of a surveying instrument lately in- 

 vented by Mr. J. Short, an instrument maker, of London, and called 

 by him a " gradient telemeter level," are, so far as I am aware, 

 novel in respect of their application to telemetry. I propose 

 therefore to discuss the limitations, in regard to accuracy, of tele- 

 meters constructed on this principle, without adhering rigorously 

 to the details of Mr. Short's instrument in its present form. As 

 it now stands the instrument may be thus briefly described : — 

 A large telescope, similar in all respects to that of an ordinary 

 level, is attached, at an angle of less than 90°, to an axis, the 

 revolution of which is, by an attached pointer, measured on a 

 graduated circle set at right angles thereto. This axis rotates 

 within a second one, carrying the graduated circle ; but the two 

 axes, instead of bein^ concentric or parallel, as in a theodolite, are 

 inclined at an angle equal to the before mentioned defect from 90°. 

 This second and outer axis is made vertical in the ordinary way by 

 means of a level tube, and the instrument is adjusted so that the 

 sight-line is horizontal, and is in the jilane containing the two axes 

 when the pointer (or vernier) is at the zero of the circle. A needle 

 compass is also fitted to the instrument for the measurement of 

 directions. 



With an instrument adjusted in the manner above defined, if the 

 inner axis remain clamped, the rotation of the outer simply causes 

 the sight-line to trace out a horizontal plane, as in the case of an 

 ordinary level, and this is the condition maintained while the instru- 

 ment is used merely as such. If, however, the inner or inclined 

 axis is rotated the sight-line departs from the horizontal, continu- 

 ally increasing its slope until an arc of 180° has been turned, when it 

 diminishes, becoming horizontal again at 360°. The circle there- 

 fore serves as a measure of the degree of slope of the sight-line. 



The telemetric theory of the instrument may be thus stated : — 

 If g denote the natural cotangent of a sloping line, so that g ex- 

 presses its grade, as so many horizontal to 1 perpendicular, then 

 the intersections (s) of such a line with a vertical one {i.e., a 

 graduated staff), horizontally distant b from the other terminal of 

 the slope line {i.e., the intersection of the inclined axis with the 

 sight-line of the telescope) is bjg below a point on the vertical 

 line level with the terminal ; and, similarly, it is bjg' for a second 

 intersection (s') of a line of the grade g' . Therefore the distance 

 is given by the formula 



b= l_^l^{s^s') (1) 



ff ^ y' 



that is to say, the difterenoe of the intercepts on the staff, when 

 the pointer is set at specific grades, permits of the finding of the 



