1802.] testing the magnetic properties of iron and steel. 335 



Suppose, for instance, that it is desired that a movement of the 

 spot of light through 10 centimetres horizontally should corre- 

 spond to a magnetic force iuside the solenoid equal to 100 C.G.S. 

 units. A known current is sent round the coil L, and this coil is then 

 so adjusted that the spot of light is deflected through the proper 

 distance cor-responding to the calculated value of H. To standardise 

 the vertical motion of the spot, a magnetised steel wire may be 

 placed inside the solenoid M, through which no current is passing, 

 and the solenoid is then adjusted until the spot of light shows a 

 deflection in the vertical direction of 10 centimetres for each 1000 

 C.G.S. units of intensity of magnetisation of the steel wire. The 

 intensity of magnetisation of the steel wire can be determined by 

 the use of an ordinary magnetometer. If the cross section of the 

 wire to be tested is different from that of the steel wire an 

 appropriate factor must be introduced. 



Although for very accurate observations in the subject of 

 hysteresis the use of two separate instruments, galvanometer and 

 magnetometer, will probably still be necessary, yet I think that 

 the instrument I have described may be found useful as a means 

 of rapidly gaining an approximate knowledge of the form of the 

 hysteresis curves for various samples of iron and steel without 

 any calculation. For this purpose it may be a useful instrument 

 in the Lecture Room. 



May 30, 1892. 

 Professor G. H. Darwin, President, in the Chair. 



The following communications were made : 



(1) The hypothesis of a liquid condition of the Earth's interior 

 considered in connexion with Professor Dariuins tlieory of the 

 genesis of the Moon. By Eev. O. Fisher, M.A., F.G.S., Hon. 

 Fellow of Jesus College. 



In a series of papers in the Philosophical Transactions, Parts I. 

 and II. 1879, Professor Darwin has developed the theory of tidal 

 action in the solar system. 



At p. 23 of his paper on Bodily Tides of Spheroids he 

 gives " a dynamical investigation of the effects of a tidal yielding 

 of the earth on a tide of short period according to the canal 

 theory." A numerical estimate will afford a clearer idea of the 

 effects produced. 



