632 report— 1884. 



as to ensure almost absolute accuracy. This method, which is explained, is not 

 suitable for the second kind of application, where the forces transmitted have to be 

 considered. The paper discusses the practical mode of dealing with this question, 

 and by means of diagrams and a model (which was exhibited) shows that the trans- 

 mission of moderate forces can be satisfactorily accomplished where accurate 

 velocity ratios are not required. 



4. On some Irregularities depending on Temperature in Daily's experi- 

 ments on the Mean Density of the Earth. By Professor W. M. 

 Hicks,. M.A. 



5. On Safety Fuses for Electric Circuits. 

 By Professor Sir William Thomson, LL.D., F.B.S. 



6. A Lecture Experiment on Induction. 

 By Professor Lord Rayleigh, LL.D., F.R.S. 



It is well known that an electro-magnet, interposed in the circuit of an alter- 

 nate current machine, diminishes the effect far more than in a degree corresponding 

 to the resistance of the additional wire. This behaviour of an electro-magnet may 

 be exhibited to an audience in an instructive manner, by use of a helix wound with, 

 two contiguous wires (such as are commonly used for large instruments), one of 

 which is included in the circuit of a De Meritens machine, and a few incandescent 

 lamps._ If the circuit of the second wire be open, the introduction of a few stout 

 iron wires into the helix causes a very marked falling off in the incandescence. On 

 closing the second circuit, currents develope themselves in it of such a kind as to. 

 compensate the self-induction, and the lights recover their brilliancy. Even, 

 without iron, the effect of closing the second circuit is perceptible, provided the 

 degree of incandescence be suitable. 



An arrangement suitable for illustrating the same phenomenon with currents- 

 of small intensity was described in ' Nature ' for May 23, 1872. 



7. On Telephoning through a Cable. 

 By Professor Lord Rayleigh, LL.D., F.B.S. 



The principles of this subject were laid down thirty years since by Thomson, 

 but the author had not met with an application to the circumstances of the- 

 telephone. 



A periodic variation of potential, imposed at one end, is propagated along the 

 line in accordance with the law 



' V ' i, ' * cos. (nt— a /— . .r j. 



in which — is the frequency of the electricrd vibration. For Atlantic cable* 



the constant k, depending upon the resistance and the capacity, has in C.G.S. 

 measure such a value as 2 x 10 16 . The distance, in traversing which the ampli- 

 tude is reduced in the ratio e : 1, is given by 



,1k 2xl0 3 ,. . 



x = v — = centimetres. 



n v»i 



If we take a pitch rather more than an octave above that of middle c, we have 

 n =-■ 3,600, Vn = 60, so that 



x «= 3 x 10° centimetres = 20 miles approximately. 



A distance of twenty miles would thus reduce the intensity of sound to almost 

 a tenth, an operation which could not be often repeated without rendering it 



