II. LENGTH. 65 



A very severe proof consists in sighting a distant object with the telescope, 

 which is then reversed in its sockets, and the apparatus turned round 180, 

 at which the object should be intersected again by the eyepiece cross. 



The immovable cross in the ocular is cut on glass, in order to prevent 

 hygroscopical and other interruptions. For the purpose of obtaining the 

 rectangular position of the telescope, the supporter may also be placed with 

 one end between points, while an elevation screw is fixed to the other. The 

 essential point for effecting the before-mentioned correction by employing the 

 attaching or adjusting water-level consists simply in adjusting the water-level 

 axis exactly to the leaning face by means of the correction arrangement 

 marked a in the drawing. The proof is effected by reversing the angle 

 vertically, the water-level thus turning between its. points. If after the 

 proper attachment the bubble deviates from the centre, half of this deviation 

 must be corrected by the regulating screws of the tripod, and the other by the 

 correction arrangement a. It is, however, to be mentioned that, previous to 

 the above proof, the parallel position of the water-level axis towards its 

 points of attachment is to be examined, which can effected by reversing 

 between its two points, and thereby a deviation of the bubble, if there be any, 

 Avill be removed half by the adjusting screw b, and the other half by the 

 arrangement a. Finally, there remains the examination and correction of the 

 water-level sideways to be made, which is done in the usual manner by the 

 screw c. This attaching or adjusting water-level may also be recommended 

 for other purposes, for instance, in mounting of machines, &c. 



Regarding the peculiar construction of the aforesaid adjusting water-level, 

 the suspension between two points, in general, it may be remarked that the 

 same has been derived from the compensation-level constructed by F. W. 

 Breithaupt and Son some years ago (vide Dingler's Polytechn. Journal, 

 vol. CLIV. p. 401). In what manner this principle has been adopted in other 

 mechanical workshops, and represented partly as an invention of their own, 

 has been proved by an article in Carl's Repertorium, vol. IX., p. 127, by the 

 addition of an arrangement or simplification totally at variance with the 

 construction. 



E. DIVIDING ENGINES. 



248. Instrument for dividing Mathematical Scales or 

 Rules. 



H. M. Commissioners of Patents 



This instrument is to be used for dividing scales according to the French, 

 Swiss, or English measures of length, and is provided with a vernier for 

 obtaining the smaller divisions of the scale. It can also be adapted to 

 the production of diagonal scales. 



248 a. Dividing Engine, made by the late Mr. Bryan 

 Donkin, F.R.A.S., in the year 1828. Bryan Donkin. 



The principle involved in the construction of the machine is the employ- 

 ment of a compensating arrangement, by which great accuracy is obtained, 

 notwithstanding the inequalities of the screw used in the machine for advancing 

 the cvitting tool. The machine consists, first, of a table moving upon wheels 

 on a railway. To the under side of the table is attached a clasp nut in two 

 parts, moved by the main screw, which is below the table, and exactly 

 parallel with the line of motion. To effect the compensation the table con- 

 sists of an upper and lower plate, the upper one being capable of a small 

 40075. B 



