48 SEC. 3. MEASUREMENT. 



263. Pocket Telemeter. By M. Gautier. 



M. Tavernier Gravet, Paris. 



264. Telemeters. Fortin, Hermann Bros., Paris. 



285. Collection of War Telemeters. These instruments, 

 which are based on the speed of transmission of sound, are 

 intended for measuring distances in the field. 



Le Boulenge, Liege. 



307. Instrument for Measuring Distances, according to 

 the systems of Kleinschmidt and Breithaupt. 



Royal Museum at Cassel (Dr. Pinder, Director), 



The instrument for measuring distances was executed completely in brass 

 by J. C. Breithaupt, during the second half of the 18th century. It consists 

 of a rail of 0' 978 m. in length, serving as measuring-base, on both ends of 

 which is attached a movable telescope for sighting the object the distance 

 of which is to be determined. By the known length of the base, and the 

 indicated angles which the adjusted telescopes form with the fundamental 

 line, the distance looked for is ascertained by trigonometrical calculation. 



C. GAUGES AND CALLIPEES. 



236. Sliding Calliper Gauge, with tangent screw and 

 vernier for reading 10 1 00 th of an inch inside and outside measure- 

 ment. Elliott Brothers. 



237. Decimal Gauge, German silver, with screw and ratchet 

 motion for measuring to -j-^Voth of an inch. Elliott Brothers. 



246. Aerial Spider Line Micrometer of great delicacy, 

 measuring an object to the 100,000th of an inch. 



Dr. Royston-Pigott, F.R.S. 



The image of sets of spider lines of a recording micrometer placed beneath 

 the stage, is formed by a half inch objective, five inches from the spider 

 lines. This image is in fact a miniature diminished exactly seven times. 

 The micrometer reads to the l-20,000th of an inch, 20 o p" 5 consequently 

 the image is measured seven times more minutely. This would be the 

 1-1 40,000th, or y^^^th of an inch (English). On the whole, therefore, 

 the instrument may be said to measure to the 1-1 00,000th, i.e., l66 1 6oo th of an 

 inch. 



These aerial spider lines are made to move about the object to be measured 

 at the will of the observer, and come into the focus of the microscope by 

 regulating the plane of the aerial spider lines. 



256. Callipers, for clock and watch making. 



Geneva Association for Constructing Scientific Instru- 

 ments. 



Much used in clock and watch making for measuring thicknesses. This 

 instrument gauges to the fraction of ^th of a line, or -j-^^th of a millimetre. 



The divisions traced on the steel arc are not equal, but are calculated to 

 measure equal increments of the interval between the two nibs. They 

 increase therefore with the chords. 





