II. LENGTH. 57 



Long callipers used for fmuing the internal length of a cask from head to 

 head. 



Cross calliper. Used for finding the external diameter of a cask. 

 Stave gauge. For finding the thickness of the stave in a cask. 



236. Sliding Calliper Gauge, with tangent screw and 

 vernier for reading TW<j tn ^ an ^ ncn ms ide an d outside measure- 

 ment. Elliott Brothers. 



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

 motion for measurin to ^h 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, a ^ oa /r ; consequently 

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

 l-140,000th, or -j^i^^th of an inch (English). On the whole, therefore, 

 the instrument may be said to measure to the l-100,000th, i.e., l66 * 06o 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. 



246a. Wollaston's Single Lens Micrometer. 



Wollaston Collection, Cavendish Laboratory, Cambridge. 



(Phil. Trans., 1813, p. 119.) 



256. Callipers, for clock and watch making. 



Geneva Association for the Construction of Scientific In- 



struments. 



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

 instrument gauges to ^V tn f a nne > or TlM n f 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. 



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 shrunk 

 on the same ; constructed by G. Brauer. 



Arsenal of St. Petersburg. 



The apparatus consists of a bar, with two adjustable arms, which is sus- 

 pended across the cannon ; one arm being brought into contact on one side 

 with the surface of the cannon, the other arm with its contact lever is brought 

 into contact with the surface of the cannon on the other side, in such a 

 manner that this contact lever at its upward and downward motion, by means 

 of the vertical screw at the greatest diameter, indicates zero. In order to 

 exactly determine the diameter, the divided movable scale is adjusted, after 



