7° THE HAMILTON ASSOCIATION. 
mid-summer and least in mid-winter—that is, it is greatest when the 
sun is north of the equator and least when south. Frequently dis- 
turbances occur which cause a temporary irregular effect on all the 
needle over a considerable area. These are termed magnetic storms, 
and are often connected with manifestation of electrical phenomena, 
such as aurora borealis, violent thunder storms, and still more gen- 
-erally with those solar outbursts known as sun spots. To give some 
idea of the extent of the magnetic disturbances during the aurora 
we may mention that on May 13th, 1869, the declination at the 
Royal Observatory, Greenwich, varied one degree, twenty-five min- 
~ nutes, while the vertical force experienced four successive maxime. 
During April of the same year the declination at Stonehurst varied 
two degrees, twenty-three minutes and fourteen seconds in nine 
minutes. The electric force produced at such times in telegraphic 
wires, though transient, are often very powerful. Professor Lomis 
mentions cases where wires had been ignited, producing brilliant 
flashes, and combustible material kindles by the discharges. On 
August 13th and September rst, 1858, the electric waves noiselessly 
worked the telegraphic needles and violently rang the alarm bells in 
the city of Paris. In addition to the resemblance between the 
aurora phenomena and those of electrical discharges in rarified tubes 
which I have shown you, we have seen that the aurora display is 
accompanied by marked disturbance in the direction and force of 
terrestial magnetism, and taken in conjunction with the strong earth 
currents which are at such times produced and with manifest polari- 
zation or setting at right angles of the arches and rays with regard 
to the magnetic meridian, may be considered as conclusive that the 
aurora is some sort of electric discharge. Professor Lomis states in 
an excellent article in the American Journal of Science that he has 
registered the extent of sun spots for six days preceding and follow- 
ing each of the great magnetic disturbances at Greenwich, and has 
compared these values with that for the very day of the disturbance. 
In this manner he has treated all the days of the great magnetic dis- 
turbance at Greenwich for a period of twenty-three years. The 
cases of disturbance thus treated amounted to 135, and from the 
results he draws the following conclusions: 1—Great disturbances 
of the earth’s magnetism are accompanied by unusual disturbances 
of the sun’s surface on the very day of the magnetic storm; 2—The 
