178 THE JOHORE SURVEY. 



who would take trouble to think the matter over and to 

 plot a few lines that such measurements might be made 

 with the ordinary theodolite by taking two observations on 

 a staff and two angles ; but on account of the number of 

 observations entailed, the risk of personal error was so 

 greatly increased that the method did not recommend itself 

 except in very exceptional cases. The author proposed, for 

 the sake of convenience, to draw an arbitrary line between 

 tacheometers and telemeters, Tacheometers he should de- 

 scribe as instruments embodying the principles of Porro's 

 lenses, though in a simpler form of the analatic lens. Tele- 

 meters, on the other hand, might be described as instruments 

 in which an arm of known length, carrying a microscope 

 moving on a horizontal scale, was attached to the trunnion of 

 a telescope, the axis of the arm being perpendicular to the 

 axis of the telescope. The scale referred to has, as regards 

 its length and sub-division, a certain ratio to the length of 

 the staff. 



It would be easily seen, on consideration, that the move- 

 ment of the telescope and microscope being similar, the hori- 

 zontal distance of any object could be obtained by direct 

 telescopic observation upon a graduated staff, the distance 

 being read off through the microscope ; such is the prin- 

 ciple of Eckhold's Omnimeter. In the tacheometer a 

 frame is used carrying a diaphragm with five parallel 

 lines, the middle one of which corresponds to the ordinary 

 horizontal wire of the level, the two remaining pairs being 

 placed symmetrically on each side of the middle one. By 

 use of the analatic lens, the reading of the distance on a 

 graduated staff intercepted between either the outer or 

 inner pair of lines, when multiplied by a corresponding 

 constant, gives the horizontal distance of the vertical staff 

 from the centre of the observer's instrument. When taking 



