34 SECTIONAL ADDRESSES. 



was made by the president (Lord Armstrong) of this Association in his 

 Presidential Address in 1863. Lord Armstrong said : ' The sympathy 

 also which appears to exist between forces operating in the sun and 

 magnetic forces belonging to the earth merits a continuance of that close 

 attention which it has already received from the British Association, and 

 of labours such as General Sabine has, with so much ability and effect, 

 devoted to the elucidation of the subject. I may here notice that most 

 remarkable phenomenon which was seen by independent observers at 

 two difierent places on September 1, 1859. A sudden outburst of light, 

 far exceeding the brightness of the sun's surface, was seen to take place 

 and sweep like a drifting cloud over a portion of the solar face. This was 

 attended with magnetic disturbances of unusual intensity and of exhibi- 

 tions of aurora of extraordinary brilliancy. The identical instant at which 

 the effusion of light was observed was recorded by an abrupt and strongly 

 marked deflection in the self-registering instruments at Kew. The 

 phenomena as seen was probably only part of what actually took place, 

 for the magnetic storm in the midst of which it occurred commenced 

 before and continued after the event.' 



Much progress has been made in our knowledge of the sun and its 

 radiations since the pronouncement of Lord Armstrong. One of the first 

 theories j)ut forward attributed magnetic storms to the magnetic fields 

 produced by streams of charged particles from the sun acting like an 

 electric current and producing a direct magnetic effect. Schuster showed 

 that such a stream moving between the sun and the earth would move 

 in a magnetic field of constantly increasing intensity, and would be subject 

 to a retarding force also continually increasing. Schuster considered a 

 particular magnetic storm, and showed that on such an assumption the 

 passage of the stream from the sun to the earth would take about a year, 

 and hence the intimate connection between solar acti^aty and magnetic 

 storms would be very far from apparent. 



Lindemann has overcome this difficulty by suggesting solar streams 

 which are ionised but. on the whole, neutral. The groups of particles are 

 assumed to be projected from the solar prominences, and the gases in 

 these are of such high velocity, 10* cm. per sec, that the journey from the 

 sun to the earth should be possible in less than two days, without serious 

 recombination taking place. Moreover, owing to its neutrality such a 

 stream will not tend to spread outwards by the mutual repulsion of its 

 constituent particles. 



In a theoretical study of the motion of atoms from the sun, Milne has 

 calculated the limiting velocity with which they can pass out of the 

 gravitational field of the sun under the influence of radiation pressure, 

 and the order of magnitude for such elements as hydrogen is 1.6 X 10* cm. 

 per sec. If this value be taken as the average velocity, the particles will 

 take about 30 hours to travel from the sun to the earth, and in this connec- 

 tion it is of interest to note that, while there is considerable difficulty in j 

 determining the maximum activity of solar eruptions and the corre- 

 sponding maximum intensity of magnetic storms, yet an interval of] 

 30 hours does roughl}' correspond with this difference. 



Lindemann suggested that the negative ions might be lighter than thej 

 positive ones and stop at the ujipermost layers of the atmosphere, whilej 



