X. ANGLES. 89 



435a. Method of ascertaining Angles of Torsion by means 

 of instruments constructed by Professor Wischuegradski. 



Laboratory of Mechanics, Technological Institute, St. 

 Petersburg. 



This is composed of a support fixed with two horizontal screws in the 

 given section of the beam subjected to the process of torsion. This support 

 upholds an horizontal axle, upon which is fixed an arc, bearing the teeth, 

 whose pitch measures an angle of 2,440 seconds. This arc catches an endless 

 screw, the head of which bears a circle divided into 244 equal parts, and 

 furnished with a fixed decimal vernier ; the arc also carries a very sensitive 

 level, placed at the beginning of the experiment in a horizontal position. 



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 wanted. 



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

 Petersburg the well-known apparatus of Mr. 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. 



435b. Dynamometer Waggon, for marking and registering 

 the tractive power, and the distances travelled. 



Eastern Railway of France Company, Paris. 



436. Theoretical Tension-Diagram for calculating the 

 mechanical work in a steam cylinder. 



H. Hadicke, Demmin, Pommerania. 



X. MEASUREMENT OF ANGLES. 



437. A 10-inch Protractor, by Kamsdeu. Royal Society. 



438. Clinometer of Precision, employed in 1865 by Pro- 

 fessor Piazzi Smyth inside the Great Pyramid. 



Prof. Piazzi Smyth. 



This instrument was made to order by J. Cook and Sons, of York, in 1864, 

 at the cost of Andrew Coventry, Esq., of Edinburgh, for measuring the 

 interior slopes of the Great Pyramid. When thus used it was further 

 mounted on a deep wooden beam, 120 inches long, armed with feet of gun 

 metal. 



The angle measuring portion of the instrument is a complete circle, pro- 

 vided with three pairs of opposite verniers, each reading to 10" in order to 

 eliminate errors of division as well as eccentricity, and the whole circle can 

 be moved and clamped on its centre so as to repeat any required angle all 

 round the circumference. On the voyage to Egypt a thermometer broke 

 inside the box, and the mercury tarnished the divided rim in parts. The 

 Pyramid angles were printed in Vol. II. of " Life and Work at the Great 

 Pyramid," by Professor Piazzi Smyth, in 1867. 



