II, LEXGTtf. 63 



variations taking place in the wire can be perceived through the telescope 

 directly magnified and projected on the scale. 



In most cases it will only be necessary in making such experiments to know 

 exactly the absolute length of the body to be measured in equal per cents, as 

 well as the elongation, for measuring which a good scale, or a very simple 

 cathetometer, is all that will be required. 



24 la. Original Cathetometer by D along. 



Polytechnic School, Paris. 



241 b. Cathetometer, with two Levelling Micrometer 

 Telescopes. 



Physical Science Cabinet of the Imperial Academy of 

 Sciences, St. Petersburg. 



241 c. Drawing of a small Cathetometer, used by Prof. 

 Mendeleeff in his investigations on the tension of gases. 



Prof. Mendeleeff. 



In order to eliminate a source of many errors the eyepiece is fixed in the 

 telescope, and the whole cathetometer has to be put at the required distance 

 from the object to be observed. 



The telescope is provided with a micrometer screw. 



253. Great Cathetometer, for reading differential levels 

 more than a metre apart. 



Geneva Association for the Construction of Scientific In- 

 struments. 



This instrument is composed of a tripod supporting a central rod, which 

 bears on its upper part the brass column, or prismatic piece, along which 

 the telescope moves in a right line. The dimensions of the column are great, 

 so as to avoid all flexure. The division on the silver plate is in millimeters, 

 and the vernier of the slide gives readings to the 50th of a millimeter. This 

 instrument has two levels ; the one placed between the rings that support 

 the telescope, the other placed perpendicularly to the first upon the table 

 situated at the base of the column. 



The universities of Berlin, Rome, Dorpat, Neuchatel, &c. have instruments 

 of this pattern. 



273. Cathetometer, by Casella. The telescope moves on a 

 girder-shaped brass bar, to which the scale is attached, and is 

 furnished with a micrometer eyepiece, by means of which readings 

 can be taken without moving the telescope. The instrument is 

 supported by a massive iron frame- work. 



Prof. A. W. Rucker, Leeds. 



292. Cathetometer. C. Bamberg, Berlin. 



The principal division of the instrument is executed on silver to centi- 

 meters. The division into single millimeters has been made on a scale con- 

 nected with the principal slide, whose divided surface is on a level with that 

 of the centimeter graduation. The millimeter-scale moves with the principal 

 slide, which carries the means for reading and adjustment (microscope and 

 ^elescope). The reading of the meter-division is effected (as far as O'OOl 



