III. SURVEYING. 747 



estimated in lOths of divisions by three apparent wires, the means 

 of these three estimations only being used. 



3rd. The division is decimal ; each hundredth of the quadrant is num- 

 bered, and divided into five parts. 



The error of collimation is ascertained by reversing the ring that supports 

 the wires. This instrument, constructed for topography, is of saaall dimen- 

 sions. In the larger model the division is carried to the thousandth of the 

 quadrant. 



3089b. Aba, or new Altazimuth. Designed by the exhibitor. 

 Antoine cTAbbadie, Membre de VInstitute. 



The sextant, so useful at seaf is not a traveller's instrument, fo it requires 

 on land an artificial horizon, and cannot give, without much trouble, the small 

 angular heights and true bearings of distant terrestrial objects, so useful in 

 mapping. With common altazimuths time is lost ia adjusting the levels and 

 collimation error. These two adjustments are avoided in the " aba ;" by con- 

 struction the errors are brought down to narrow limits, are determined by re- 

 versals, and remain practically constant. The telescope is very powerful for the 

 size of the instrument in order to ensure easy vision. The observer can read his 

 verniers without shifting his position. There is no inferior or fiducial telescope, 

 as the observer, after going round his horizon, takes care to repeat his first 

 bearing, so as to provide against any motion in azimuth. A pinion and tooth- 

 rack supply the place of clamp and screw, thus saving time and trouble. 



The verniers read to the ten thousandth part of the quadrant or right angle 

 (equal to 32 //> 4), this decimal division -being preferred, on trial, by persons 

 already accustomed to sexagesimal divisions. It is likewise more easily 

 learnt by beginners. The figures indicate hundredths of the quadrant, and are 

 marked so as to be very easily read. From the jutting position of the object 

 glass it is possible to take the bearing of a signal at the very foot of a tower 

 or wall. As screws get loose in travelling, the maker has employed fewer of 

 them than it is usual in altazimuths. A powerful screw is attached to the 

 stand and fastens the box on its top, before sliding off its superior part, which 

 can thus be quickly put on again in case of a sudden shower of rain. The 

 small box is destined to contain a thermometer for calculating atmospheric 

 refraction, sun glasses, &c. 



By taking corresponding azimuths of the sun on the same day or on two 

 ensuing ones, the latitude is obtained together with true bearings of all visible 

 objects, while the compass gives the needle's variation. 



A person familiar with the use of the sextant only, on observing circum- 

 meridian altitudes for his first attempt with the " aba," obtained his latitude to 

 within 4". 



3101. Geodetical Tachygraph, a representation of an in- 

 strument invented and constructed by the exhibitor, for conducting 

 measuring operations in their minutest details. 



Prof. Joseph Schlesinger, Vienna* 



This instrument consists of a circle with divisions, mounted in a movable 

 frame, that can be adjusted so as to place the circle in any given direction. 

 It is also provided with a rule to plot angles. 



3102. Eckhold's Omnimeter, for measuring linear distances 

 by one and the same operation. Elliott Brothers* 



This instrument is a transit theodolite, with an apparatus for measuring 

 by one operation the distance of an object or staff of a determined length, 



