600th ordinary GENERAL MEETING, 



HELD IX COIVIIVIITTEE ROOM B, THE CENTRAL HALL, 

 WESTMINSTER, ON MONDAY, MAY 13th, 1918, 



AT 4.30 P.M. 



Sir Erank W. Dyson, F.R.S., The Astronomer Royal, 

 IN THE Chair. 



The Minutes of the previous Meeting were read, confirmed and signed. 



The Secretary announced the death of the Rev. H. A. Crosbie, an 

 Associate of the Institute. 



The Chairman then called on Dr. Sydney Chapman to read his paper 

 on " Terrestrial Magnetism," which was illustrated by lantern slides. 



TERRESTRIAL MAGNETISM. Bv Sydney Chapman, 

 Esq., M.A., D.Sc. 



r flHE history of our subject for this afternoon began when men, 

 J_ long ages ago, discovered the power of attraction exercised 

 towards iron by the mineral called lodestone. This property 

 is mentioned in the writings of Thales (640-546 B.C.), so that 

 it has been' known for two thousand five hundred years or more. 

 According to Lucretius (99-55 B.C.), the stone received the name 

 of magnet from the place whence it was obtained, among the hills 

 of Magnesia. From this the terms magnetism, magnetic 

 force, and so on, are naturally derived. 



As the attractive property of the lodestone was gradually 

 investigated, it appeared that small pieces of iron are specially 

 attracted by two particular parts of the lodestone, which, in a 

 stone of regular shape, are opposite to one another. These are 

 called the poles of the magnet, and the stone is said to be mag- 

 netized in the direction of the line joining them. 



The next great discovery in magnetism related to the directive 

 property of the lodestone. If a small magnet is mounted on a 

 floating card or board, it is observed that the float and magnet 

 will turn on the water till the magnetic axis, or line of poles, lies 

 along a particular direction. Once the stone has assumed this 

 direction, the tendency to motion ceases — there is no force on 



