1J2 SEC. 3. MEASUREMENT, 



The angle of torsion between the two given sections of the beam is calcu- 

 lated by two instruments exactly similar. The deformation of the twisted 

 beam causes an inclination of the levels of both instruments ; they are re- 

 stored to their original position by means of the endless screws, and then is 

 effected the reading of the angles described by the arcs of the two instru- 

 ments. The difference between these angles is the angle of torsion required. 



In the Laboratory of Mechanics of the Technological Institute of St. 

 Petersburg the well-known apparatus of Wohler is used for the torsion 

 of trees, the photograph of which, taken together with the instruments for 

 measuring the angles of torsion, is exhibited. For demonstrating how to use 

 the instruments, a provisional apparatus is exhibited, wherewith the torsion 

 of the beam is effected by means of a simple lever. 



442. Clinometers, devised by the Rev. Professor 

 Hen slow, one of which was used by Dr. Hooker in his 

 Himalayan journeys. J. I). Hooker, M.D., P.JR.S. 



442a. Three Clinometers. G. W. Strawson. 



443. Protractor, with scale, vernier, and magnifying glass. 

 Reads to 1 min. Prof. Baron von Feilitzsch, Greifswald. 



443b. Instrument for the Measurement of Angles. 



Dr. Fr. Holler, Selbo Drontheim, Norway. 



440-1. Drawings and Photographs of Dividing Ma- 

 chinery. Messrs. Trougliton Simms. 

 Fig. 1. General view of dividing machine. 



A. The circular table with racked circumference containing 4,320 teeth, 



each tooth, therefore, equal to five minutes of arc. 



B. The screw by which movement is communicated to Table A. 



C. A ratchet wheel attached to the screw shaft. 



D. A crank arm which during one half of a revolution gives a forward 



movement to the screw ; during the remaining part of its revolution 

 the screw is at rest. The axis which carries the crank arm has a 

 bevelled wheel upon it, serving to communicate motion to the 

 cutting apparatus. 



E. The cutting frame. 



F. A cam to give movement to the dividing knife or other tool by which 



the division is made. 



The apparatus is so arranged that the division may be cut whilst 

 the circular table is at rest, the tool being lifted by a second earn (not 

 well seen in the drawing) when the table is in motion. 

 Fig. 2. Plan of cutting apparatus showing the relation it bears to the 

 circular table and screw. 



Fig. 3. Section of table and axis. 



Fig. 4. Drawings of cams and cutting frame, the cam " h " for lifting the 

 tool (just seen in Fig. 1) is here shown. 



