76 Royal Society:— 



number of segments is inversely proportional to the square root of the 

 tension. This result may be deduced by reasoning from the laws enun- 

 ciatedin38and45,anditsrealizationinfactconfirmstheircorrectness*. 



47. Finally, I have here three wires of the same length and thick- 

 ness, but of very different densities ; one of them is of the light metal 

 aluminium, another of silver, and another of the heavy metal plati- 

 num. I attach the wires in succession to this tuning-fork, and deter- 

 mine the weights necessary to cause them to vibrate as a whole, or 

 to form the same number of ventral segments. The stretching 

 weights here necessary I find [to be directly proportional to the spe- 

 cific gravities of the wires. From this result, combined with that 

 enunciated in 45, it follows that — 



48. The rapidity of vibration of different chords of the same length 

 and thickness is inversely proportional to the square root of their 

 densities. 



49. It is perhaps worth remarking, that, by means of a tuning- 

 fork, the specific gravities of all metals capable of being drawn into 

 wires of sufficient fineness and tenacity may be determined. 



The foregoing laws have been combined in various ways ; and the 

 deductions drawn from them, when subjected to the test of direct ex- 

 periment, have in every instance been verified. 



ROYAL SOCIETY. 



[Continued from vol. xxxi. p. 545.] 



April 12, 1866. — Lieut, -General Sabine, President, in the Chair. 



The following communication was read : — 



" On Uniform Rotation.'' By C. W. Siemens, F.R.S. 



The paper sets out with an inquiry into the conditions of the 

 conical pendulum as a means of obtaining uniform rotation. This 

 instrument, as applied by Watt to regulate the velocity of his steam- 

 engines, is shown to be defective, — first, because the regulated posi- 

 tion of the valve depends upon the angular position of the pendulums, 

 and therefore upon the velocity of rotation, which must be perma- 

 nently changed in order to effect an adjustment of the valve; and 

 secondly, because when the balance between force and resistance of 

 the engine at a given velocity is disturbed, the angular position of 

 the pendulums will not change until a power has been created in 

 them, through acceleration of the engine, sufficient to overcome the 

 mechanical resistance of the valve — giving rise to a series of fluctua- 

 tions before a balance between the power and resistance of the engine 

 is reestablished. 



These defects in Watt's centrifugal governor are shown to be ob- 

 viated in the chronometric governor, an instrument which was pro- 

 posed by the author of the paper twenty-three years ago, and which 

 consists of a conical pendulum proceeding at a uniform angle of 

 rotation, and therefore at uniform speed, which is made to act 

 upon the regulating-valve by means of a differential motion between 

 itself and the engine to be regulated, which latter has to accommo- 



* This experiment, in whole or in part, has been already performed by 

 Professor Foster, of University College. 



