66 SEC. 3. MEASUREMENT. 



motion independent of the lower plate. The lower plate carries the fulcrum 

 of a bent lever, whose arras are at right angles and as 50 to 1. This lever 

 moves in a vertical plane, so that the longer arm lies by gravity alone on the 

 undulating edge of the compensation bar. The upper plate is pressed end- 

 ways against the shorter arm of the bent lever by means of a spring keeping 

 them always in contact. By a kind of parallel motion the two plates are 

 attached so as to allow of the very small motion required in the upper plate 

 independently of the lower. The compensating bar, which is of the length 

 of the screw, has 50 narrow slips of metal placed upon it, each having an 

 adjusting screw by which the ends of the pieces may be placed in a continuous 

 line, or above or below the line, as required by the mode of adjustment. This 

 bar is carried by a pivot at one end, and the other end is raised or depressed 

 by a screw, which adjusts the compensating bar to the total length moved 

 through by the guide screw. 



In the case of dividing a scale, the swing frame carrying the cutter or 

 diamond point is attached to the framing of the machine, the scale to be 

 divided being placed upon the upper plate. 



In the case of cutting a screw, the tool holder is fixed upon the upper plate 

 and the screw to be cut is placed between centres parallel to the motion of the 

 table and to the guide screw, having motion imparted to it by a train of 

 wheels connecting it with the screw of the machine. The compensating bar 

 being adjusted for total length, and the small pieces of metal upon the same 

 being adjusted to the intermediate errors of the guide screw, it will be seen, 

 that by the passage of the longer arm of the lever over the edge of the com- 

 pensating bar, a slight motion will be imparted to the upper plate independent 

 of the lower, so that, in other words, if by the error of the screw the lower 

 table is moved through too great a space, the upper table is made to move 

 (by the action of the lever) through a space equal thereto in the contrary 

 direction, and ewe versa. 



Note. A description somewhat more in detail and of the manner of 

 adjustment will be found in " Holtzapffel's Turners' and Mechanics' Manufac- 

 tures," 2nd vol., p. 651 et seq. 



Many scales were divided and many screws cut by this machine, of which 

 some were given to various scientific friends, and Sir ,T. Whitworth, amongst 

 others, had a scale and a screw about the year 1843 which have served him a& 

 standards. 



265. Machine for dividing right lines, by Nicholas Fortin. 



MM. Fortin Hermann Bros., Paris. 



This machine is the one constructed by the celebrated inventor in 1787, 

 and used in the works connected with the adoption of the metrical system. 



The pitch of the screw is exactly one millimetre. (Fortin's machine for 

 dividing circles, as well as this machine, was presented to the Conservatoire 

 des Arts et Metiers by MM. Fortin Hermann Bros., in 1876.) 



297. Micrometer Dividing Machine. 



Voigt and Hochgesang (Gust. Voigi), Gottingcn. 



The pitch of the screw is mm., its head is divided into 200 parts ; each 

 part, therefore, corresponds to -gfa mm., reading by the vernier to ^ of this 

 value. By a spring fixed in the nut " loss of time " is completely removed. 



The tracing appliance is constructed in the simplest manner possible. The 

 tracing point a diamond is lifted by a mechanical contrivance, and let 

 down again. 



The slide allows of drawing a line of 30 millimeters in length. 



