II. LENGTH. 49 



257. Curious Steel Callipers for very accurate measure- 

 ment, by Paull of Geneva, 1777. Royal Society. 



267. Apparatus for measuring the exterior diameter of the 



Gun Barrel and the interior diameter of the rings to be spanned 

 on the same ; constructed by G. Brauer. 



Arsenal of St. Petersburg. 



The apparatus consists of a rod (bar), which is placed right across the 

 cannon, and suspended, on which two arms can be moved ; the one arm is 

 brought into contact on one side with the surface of the cannon, whereupon 

 the other arm with its contact lever is brought into connexion with the sur- 

 face of the cannon on the other side, in such a manner that this contact lever 

 at its upward and downward motion through the vertical screw at the greatest 

 diameter indicates zero. In order to exactly determine the diameter, the 

 divided movable scale is pushed, after taking off the apparatus from the 

 barrel of the cannon, between the two arms, and glided so until the contact 

 lever again indicates zero. The same scale serves also for measuring the 

 interior diameter of the rings. The apparatus has been constructed and 

 made by G. Brauer at St. Petersburg. 



267a. Photograph of an Apparatus for measuring the 



eccentricity of the chamber and the curve of the bore of cannons. 



G. Brauer, St. Petersburg. 



The apparatus consists of two parts : 



1. A body, which is squeezed into the mouth of the cannon by means of 

 an endless screw. In this screw a telescope is fixed which can be turned 

 about its own axis, and is provided with a Filar micrometer and a position 

 circle. 



2. A piece, which can be slided along the barrel, being turnable about the 

 axis of the bore, and in whose centre is a glass plate with engraved cross. 

 This cross is viewed through the telescope before mentioned, and the deter- 

 mination of the position of the cross on the Filar micrometer indicates the 

 elements for determining the curve of the bore and the eccentricity of the 

 chamber. 



The apparatus was constructed by the exhibitor for the Russian Marine 

 Artillery Department. 



267b. Instrument for Measuring the Bore of Cannons 



(Etoile Mobile). G. Brauer, St. Petersburg. 



This instrument consists of a ring, in which slide two parallel rods length- 

 ways side by side, and of which the one carries the scale, the other the 

 vernier to it. The sliding rods are driven asunder by means of springs, so 

 that their exterior steel ends touch the side of the bore to be measured. As 

 this contact must not take place during adjustment, the sliding rods are 

 pushed together by a bolt. A screw perpendicular under the sliding rods is 

 intended for adjusting them according to the greatest diameter, which has to 

 be done afresh at each measurement. If the chamber of a breech-loading gun 

 is to be measured with the apparatus a set of two rings has to be added, of 

 which the one in front carries a telescope for viewing the scale. The two 

 exterior rings are joined to each other by four bars, and these bars have a 

 movement in the centre ring, which during the operation of measuring is 

 squeezed into the back part of the cannon bore by means of four screws, and 

 the whole apparatus is then moved as required. 



39508. D 



