748 SEC. 15. GEOGRAPHY. 



and its height over or under the level line of the instrument ; the former with 

 the accuracy of a good chaining, the latter -with the precision of a perfect 

 levelling, and accomplishes the work of theodolite, level, and chain with a 

 great economy of time. 



3109. Tacheometer of Gentilli, a telescopic instru- 

 ment for measuring heights and distances, in surveying difficult 

 country. Without calculations it measures accurately distances 

 up to 400 meters, or over 1,300 feet, with an error of, less than 

 ^--oVo"' Dr. Karl von Scherzer. 



This instrument was invented by M. Amade% Gentilli, an eminent Austrian 

 engineer at Vienna. Its use is for the measurement of heights and distances in 

 the survey of difficult ground, and it has proved especially useful in surveying 

 the contour-lines of mountainous districts. The means by which it measures 

 the distance is an apparatus which obliges the telescope to traverse a precise and 

 unvarying angle. The test of value of this instrument is the fact that, with 

 a magnifying power of 40, it measures distances up to 400 meters with such 

 exactness that the maximum error is less than s , 6 1 00 of the distance. 



The horizontal axis of the instrument revolves in the head of a vertical 

 arm, which can be clamped to it Avhen the measurement of a distance is to 

 be made. The motion of the lower end of this arm is effected by a slow- 

 motion screw, and limited in each direction by an adjustable stop. 



3109a. Iiaslett's Metroscope. For measuring inaccessible 

 heights and distances, and for levelling. Thomas N. Laslett. 



For the use of architects, engineers, and the military. Heights are read 

 off as decimal parts of the horizontal distance. Distances are measured by 

 readings from the two ends of a base line, which base line may be vertical, or 

 lie in the direction of the object, or at right angles to that direction. 



Lineal dimensions only are used ; no degrees angles, or tables. With the 

 index at zero the instrument becomes simply a spirit level. 



3109b. Drawing of Metroscope* For the determination 

 of dimensions of distant objects. 



Dr. Sncllen, Physiological Laboratory and Ophthalmo- 

 logical School, Utrecht. 



Before the objective of a telescope are placed two mirrors, one above the 

 other, each occupying one half of the field of view, and inclined to the plane 

 of the objective at an angle of 45, the angle betAveen them being 90. On 

 a cross bar, at each side, a mirror parallel to the first is movable. Looking 

 through the telescope, by means of these two sets of mirrors, two objects, seen 

 straight above each other, must be, at whatever distance they are from the 

 observer, at a mutual distance equal to the distance of the two outer mirrors. 

 (Handbuch der Ophthalmologie, von Graefe und Saemisch, III. p. 203.) 



3124. Universal Instrument, by Breithaupt and Son, 

 Cassel. Royal High School of Industry, Casscl ( W. Narteii). 



Universal Instrument, with distance-measurer for levelling and measuring 

 horizontal and vertical angles. This instrument is intended for large surveys 

 and levellings, which require to be performed with great accuracy and as 

 little expenditure of time as possible. It corresponds with the tacheometer, 

 but exceeds this in the amount of work that can be got through with it. The 

 telescope rests with prisms and screw-heads upon the steel plates of a bearer, 



